Nomenclature used in reporting

Reporting of thyroid FNA specimens should follow a standard format that is clinically relevant in order to direct management. At the National Cancer Institute sponsored thyroid state of the science conference in Bethesda in October, 2007, consensus was reached regarding indications, pre-FNA requirements, FNA techniques, diagnostic terminology, etc.²⁵ The Bethesda System reporting terminology includes six categories: non-diagnostic, benign, atypia of undetermined origin, FN/suspicious of FN, suspicious for malignancy and malignant.²⁶ Every category carries with it the implied risk for malignancy. Each category should be further qualified as to the possible pathological entity.

If an indeterminate diagnosis is being made due to features suspicious but not diagnostic of a neoplasm, it should be so qualified, since repeat FNA may enable definitive diagnosis. If, on the other hand, it is being made for FN, qualifying it as such will clarify that distinction of benign from malignant cannot be achieved by repeat FNA, and either ancillary techniques or histological study are required. To simplify the issue, we suggest that in the former, a diagnosis of ‘indeterminate (suspicious)’ be given and in the latter ‘indeterminate (FN)’. We suggest the revised Papanicolaou system of reporting¹⁰ which is simple and easily reproducible with the following six categories that are useful in triaging patients for either clinical follow-up or surgery:

Accuracy and limitations of cytodiagnosis

In experienced hands, and in situations where the pathologist performs the needling, cytology can be a very sensitive tool with sensitivity and specificity of up to 94% and 98% for the diagnosis of malignant lesions¹⁰ and nearly 90% accuracy rates for the identification of malignancy if follicular lesions are excluded.^27,28 Cytologic diagnosis is generally accurate in thyroiditis, usual type of PC, medullary carcinoma (MC), anaplastic carcinoma (AC) and high-grade lymphoma. False negatives generally occur in cystic lesions harboring malignancy, in low-grade or intermediate-grade lymphomas occurring in a background of Hashimoto’s thyroiditis (HT), in AC with necrosis, in focal involvement of the gland by thyroiditis and in cases with dual pathology where the dominant non-neoplastic lesion overlies or obscures a small carcinoma.^29-31 False negatives have been shown to be minimized by using US-guided FNA. The false-positive rate can be reduced further by excluding indeterminate follicular lesions.

A satisfactory sample

While abundant colloid without altered blood or debris usually indicates a benign lesion, the presence of intact and well-fixed follicular cells is obligatory for a smear to be considered as satisfactory. Relaxing the criteria for a satisfactory sample often leads to higher rates of false-negative diagnoses. Many false-negative diagnoses in thyroid cytology are related to poor-quality specimens being reported as nonmalignant.⁶⁵ A cytological sample from a benign thyroid nodule can be considered satisfactory if six clusters of benign cells are seen in at least two slides prepared from two needle passes.⁶⁶ This criterion can be softened in situations where the pathologist performs the procedure. Two to three needle passes, performed gently and with small-caliber needles, are usually accepted easily by the patient. Increased numbers of needle passes improve the diagnostic rate but often at the cost of patient discomfort. US-guided procedures can greatly diminish non-diagnostic rates, especially when radiologist and pathologist work together.

Normal structures

Follicular epithelial cells and colloid are regular features in normal thyroids and in colloid goiter. Follicular cells show fragile gray-blue or pale-blue cytoplasm with indistinct or fuzzy cell borders. Coarse blue (paravacuolar) cytoplasmic granules may be seen (Fig. 6.1). Bare nuclei, similar in shape and size to normal lymphocytes, are common. Some cells may show small nucleoli.

Fig. 6.1 Follicular epithelium

Uniform cells with fragile, partially disrupted cytoplasm; bare lymphocyte-like nuclei in a background of thin colloid (MGG, HP).

In non-bloody specimens, thin colloid stains blue, violet or pink and forms a thin membrane-like coating or film, with folds and cracks due to drying of colloid on the slide (Fig. 6.2A,B). Colloid may wash off from the slide while staining but the parched-earth or crazy-pavement artifact of colloid remains and follicular cells are often seen at the smear margins. Thick colloid appears as round, dense clumps of deep blue, violet or magenta-colored acellular material, or as globular masses with superimposed follicular cells, especially in samples from NG. Colloid can be mistaken for hyalinized collagenous stroma (collagenous spherules) or amyloid. Skeletal muscle fragments appear as straps of dark-blue material with pale ovoid nuclei and cross-striations visible in higher magnification. In PAP-stained smears, thin colloid stains pale green or orange, with cracking artifacts seen. Thick colloid appears as clumps of dark green or orange material (Fig. 6.2C,D). The blue violet color and hyaline texture of colloid appear to advantage in MGG-stained smears and distinction from fibrillary collagen and deep magenta staining amyloid is easier (Fig. 6.3). In bloody smears, colloid resembles other protein-rich fluids, including serum.

Fig. 6.2 Colloid

(A, B) Thin colloid forms a varnish-like coat of relatively homogenous material, characteristic ‘crazy pavement’ and cracking artifacts (A, MGG IP; B, Pap, IP); (C, D) Thick colloid forms irregular dense clumps of material; homogeneous and violet in MGG; variable density and staining in Pap. Compare with Figure 6.3 A and B (C, MGG, IP; D, Pap, IP).

Fig. 6.3 Collagen and amyloid

(A) Fragment of collagenous stroma; fibrillar texture; red staining; a few nuclei included (MGG, HP); (B) Clump of amyloid in medullary carcinoma; structureless but not homogeneously hyaline; magenta color (MGG, HP oil).

C-cells resemble medullary thyroid carcinoma cells and need immunocytochemical stains for identification, except in C-cell hyperplasia, where they are present in large numbers. Accidental puncturing of the trachea or larynx during FNA can be suspected if the patient coughs and air enters the syringe. Smears show mucus, respiratory cells and carbon-laden macrophages. Cartilage may be seen, appearing as brilliant magenta flecks with fibrillary edges.

Simple colloid goiter

A diffusely enlarged gland with smears showing a normal cytological appearance (or abundant or very thick colloid) indicate a simple colloid goiter which is an early stage in the evolution of NG.

Nodular goiter (Figs 6.4-6.7)

Fig. 6.4 nodular goiter

Follicular epithelial cells, some of hyperplastic type with abundant fragile cytoplasm, some of involutional type with small, dark, mainly naked nuclei, background of thin colloid (A, MGG, HP; B, Pap, HP).

Fig. 6.5 nodular goiter

Cystic degeneration: involutional follicular epithelium, foamy macrophages, background of blood and thin colloid (A, MGG, HP; B, Pap, HP).

Fig. 6.6 nodular goiter

(A) Flat monolayered sheet of epithelial cells representing a flattened macrofollicle; sheet has frayed edges, honeycomb structure (Pap IP); (B) Intermediate-size follicle seen as a three-dimensional ball of cells confined by a basement membrane (MGG, HP).

Smears show abundant thick or thin colloid, follicular cells in monolayered sheets, poorly cohesive groups and as single cells, globular colloid masses with superimposed follicular cells, bare nuclei and pigment-laden histiocytes (foam cells) in varying proportions. Involutional follicular cells with small round dark nuclei and fragile, feathery cytoplasm as well as larger, hyperplastic cells with abundant vacuolated cytoplasm or with marginal vacuoles (fire-flares) are seen (see Fig 6.9). The latter may show anisonucleosis. Oxyphilic (Hurthle) cells may be seen.

Macrofollicles disrupted by needling flatten on the slide to form monolayered sheets of epithelial cells. These have a honeycomb structure due to distinct cell membranes (Fig. 6.6A)³ and frayed edges. Focally, the cytoplasm is indistinct, forming a web-like background to the nuclei. Smaller follicles may be removed intact by the needle. They appear as spherical cell clusters resembling multinucleate giant cells (Fig. 6.6B), that may be enveloped by a basement membrane. Macrofollicles are evidence of benignity and are of diagnostic significance. Hyperplastic papillae containing follicles and intact dilated follicles in cell-block preparations are supportive evidence of a benign nodule. Foam cells, often hemosiderin-laden, suggest degeneration, commonly seen in NG (Fig. 6.5A). Distinction between degenerate epithelial cells and true macrophages is not always possible as transitional forms occur that show epithelioid as well as histiocytoid features and focal atypia. Hyalinized stroma presents as irregular pink/red frayed fragments of vaguely fibrillar material, some with adherent epithelial cells (see Fig. 6.3A).

Inadequate samples may be obtained from colloid nodules due to low cellularity and degenerative change. If smears contain few or no well-preserved follicular cells, the specimen should be reported as non-diagnostic, unsatisfactory, and repeated.

The cytological appearances of NG can overlap with FN and cytological criteria alone cannot always reliably distinguish between the two. Selective sampling of a microfollicular focus in NG leads to a repetitive pattern of microfollicles or rosettes with no colloid, and distinction from FN may be impossible.⁶⁷ However, since this is a focal phenomenon, samples from other areas are likely to show macrofollicles, abundant colloid and degenerative changes recognizable as colloid goiter. In the cytological spectrum of follicular nodules, a large sheet pattern of follicular cells with thin colloid in the background indicates a macrofollicular pattern suggestive of benignity while syncytial clusters with nuclear crowding and overlapping suggest a neoplasm.⁶⁸

Cystic PCs often contain abundant colloid. This can cause diagnostic difficulties if smears are poor in cells, but a close look at the nuclear features should allow a correct diagnosis in most cases, as detailed below. Smears in FV-PC may show well-formed follicles containing colloid.

Groups of large cells with irregular nuclei, not infrequently found in NG, are probably related to degenerative change. Their origin is uncertain; they may be histiocytes or regenerating epithelial cells consistent with repair (Fig. 6.7). Prominent aggregates of histiocytes can in some cases mimic cells of PC due to similar nuclear features.⁶⁹

Fig. 6.7 Histiocytes in nodular goiter

Histiocytes with abundant cytoplasm and enlarged, irregular nuclei with distinct nucleoli; these could be mistaken for atypical epithelial cells; bare nuclei of degenerate follicular cells in the background (MGG, HP).

The value of cytology in hyperfunctioning nodules has been disputed but recent reports suggest that there may be an increased incidence of malignancy in hyperthyroidism and that FNA is a reliable diagnostic method in these cases. It appears reasonable, therefore, to evaluate these lesions cytologically prior to radioactive iodine treatment or surgical intervention.⁷⁰

Cystic nodules

Thyroid cysts are most commonly due to retrogressive changes in NG where they may be small, yielding a few drops of fluid or larger cysts yielding substantial quantities. FNA yields brownish colloid-like fluid with altered blood. Smears show foam cells that may contain hemosiderin and sparse degenerating follicular epithelium (see Fig 6.5).^31,71

Cystic change and/or hemorrhage occur in thyroid tumors (25% of PCs, and 20% of FNs in one series).⁷² Prevalence of malignancy in resected cystic nodules is 10–15%.^4,31 Partly solid and cystic nodules may also harbor malignancy.

A definitive diagnosis of cystic NG requires adequate sampling of any solid component. Cystic fluid containing only macrophages and no epithelial cells does not rule out a cystic neoplasm. The gross appearance of cyst fluid is not helpful.³¹ Presence of atypical cells in a ‘cyst fluid only’ sample should lead to a ‘suspicious’ diagnosis (not non-diagnostic).⁷³ A cystic lesion that can be completely evacuated with no palpable nodule remaining and no epithelial atypia indicates benignity.⁷¹ While 4–40% of cystic lesions can be permanently cured by FNA, others need repeated evacuations. False-negative cytological diagnosis is most common in cystic carcinomas, especially PC, where only up to 60% can be correctly diagnosed.^71,74 Re-biopsy of the cyst bed and of any residual or recurrent swelling, preferably with US-guidance, is advisable. Recurrent cysts, lesions greater than 3–4 cm in diameter and lesions in young males are indications for surgical excision.

Thyroglossal cysts yield clear or mucoid fluid and smears may contain squamous or respiratory epithelium associated with colloid.⁷⁵ Foregut cyst of thyroid yields yellowish fluid and smears show detached ciliary tufts and macrophages.⁷⁶ Rare epidermoid cysts of thyroid have been described.⁷⁷

Clear fluid aspirated from a lateral cystic lesion suggests a parathyroid cyst and parathormone estimation of the fluid is advised.⁷⁸ Thymic cysts yield clear fluid containing lymphocytes.

Acute suppurative thyroiditis

This is an uncommon but potentially life-threatening condition⁷⁹ occurring mostly in debilitated or immunosuppressed individuals.⁴² Patients present with extremely tender thyroid enlargement, fever and high ESR. Smears show neutrophils, necrotic cells and debris. Intracellular bacteria, (usually Gram-positive cocci), may be present. Less commonly, mycobacteria, viruses, aspergillus, actinomycosis, cryptococcosis and pneumocystis have been observed.⁴² Cytologic material can also be sent for culture and sensitivity.

Autoimmune thyroid disease

The syndromes comprising autoimmune thyroid disease are many intimately related illnesses, the two most common being Graves’ disease (GD) (with goiter, hyperthyroidism and, in many patients, associated ophthalmopathy) and Hashimoto’s thyroiditis (HT) (with goiter and euthyroidism or hypothyroidism). Immunological mechanisms in these diseases are closely related and the syndromes are connected together by similar thyroid pathology, co-occurrence in family groups, and transition from one clinical picture to another within the same individual over time. Antibodies to thyroid peroxidase (TPO-Ab), produced mainly by intrathyroidal lymphocytes, are the hallmark of autoimmune thyroid disease and are present in most patients with HT and in 75% of patients with Graves’ hyperthyroidism.⁸⁰

Graves’ disease (primary hyperplasia) (Figs 6.8 and 6.9)^81,82

Diagnostic criteria

♦ Colloid-free bloody background,

♦ Moderate to high cellularity,

♦ Follicular cells in monolayered sheets, follicular or ring structures,

♦ Moderate amounts of pale, cobweb-like, delicately vacuolated cytoplasm,

♦ Marginal vacuoles (fire flares).

Cytological study is not usually sought in GD as the clinical and biochemical profile are characteristic in most cases. However, cytology aids distinction from other conditions that present with thyrotoxicity, such as toxic NG, de Quervain’s thyroiditis, HT presenting in toxic phase and rarely in thyroid carcinomas.⁷⁰

Smears are bloody with scant colloid. Cellularity is moderate to high with follicular epithelium present as monolayered sheets, rings or follicular structures with suggestion of columnar shape (Fig. 6.8A). Cytoplasm is abundant and cobweb-like and delicately vacuolated, with larger marginal vacuoles giving a characteristic ‘fire-flare’ appearance.⁸² ‘Fire-flares’ are pale pink/red clumps of material measuring 1–7 µm in diameter, often with a pale center, mainly seen at the rim of the cytoplasm around the edges of aggregates of follicular cells (Fig. 6.9). They may correspond to colloid droplets seen ultrastructurally or to dilated cisternae of endoplasmic reticulum, and are indicative of cellular hyperactivity. They are present in toxic NG, and may be seen in a smaller percentage of cells in HT, diffuse or nodular goiter, and occasionally in neoplasms, including carcinoma.^83,84 In untreated GD, however, up to 100% of cells may show fire-flares. Paravacuolar granules may be seen. There may be moderate to marked nuclear atypia, especially in cases treated with radioactive iodine or neomercazole (Fig 6.8B).⁸⁵ Papillary structures may be seen, occasionally resembling PC. Hurthle cells and lymphocytes and rarely multinucleate giant cells or epithelioid cells may be seen.⁸²

Fig. 6.8 Hyperplasia

(A) Clusters of hyperplastic epithelial cells with a follicular arrangement; abundant pale vacuolated cytoplasm and suggestion of columnar shape; (B) Striking anisokaryosis/nuclear atypia in a case of medically treated GD of long duration (MGG, HP).

Fig. 6.9 Marginal vacuoles/’fire flares’

Loose sheet of hyperplastic follicular cells with abundant cytoplasm and relatively large nuclei; note clumps of homogeneous pale pink material, so-called ‘fire flares’ around the periphery of the sheet; no clinical evidence of hyperthyreosis in this case (MGG, HP).

Problems and differential diagnoses

Dilution of cells by blood may result in paucicellular, non-diagnostic smears. The bloody, colloid-free, highly cellular cytological appearance in GD with nuclear atypia may be mistaken for a neoplastic lesion, especially in the absence of proper clinical details. The cytoplasm in cells of GD is very delicate and shows a cobwebby, lace-like appearance, unlike the denser cytoplasm and better-defined cell margins seen in neoplastic lesions, especially PC.

Autoimmune thyroiditis (Hashimoto’s thyroiditis/lymphocytic thyroiditis) (Figs 6.10-6.13)^81,86-89

Fig. 6.10 Hashimoto’s thyroiditis

Aggregates of oxyphil cells; background of blood and lymphocytes; note abundant cytoplasm, anisokaryosis and prominent nucleoli in A, more numerous lymphoid cells in B (A, MGG, HP; B, Pap, HP).

Fig. 6.11 Hashimoto’s thyroiditis

(A) Syncytial cluster of oxyphil cells; abundant cytoplasm; prominent anisokaryosis; (Pap, HP oil); (B) Poorly cohesive hyperplastic cells showing oxyphil transformation; some resemblance to histiocytes; adherent lymphocytes (Pap, HP).

Fig. 6.12 Florid lymphocytic pattern of HT, 2 cases

(A) Polymorphous population of lymphoid cells with high lymphoid : epithelial cell ratio (follicular cells in top center, MGG, IP); (B) Cellular smears, mainly reactive lymphoid cells, cluster of follicular cells in centre (MGG, HP).

Fig. 6.13 Hashimoto’s thyroiditis

Plump epithelioid-like oxyphil or histiocytic cells forming a granuloma-like cluster (MGG, HP).

Diagnostic criteria

♦ Lymphoid and plasma cells in the background,

♦ Lymphoid cells impinging on follicular cells,

♦ Lympho-histiocytic collections,

♦ Hurthle cell change (variable),

♦ Multinucleate giant cells and epithelioid cells (variable).

Lymphocytic thyroiditis and HT represent different phases or manifestations of an organ-specific immunologically mediated inflammatory disease. A defect in suppressor T cells makes the gland vulnerable to cytotoxic T cells and stimulates T-helper cells to induce autoantibody production.⁹⁰ Patients usually present with diffuse or nodular thyroid enlargement and altered thyroid function caused by gradual immunologically mediated destruction of the gland. Antithyroid antibodies (especially microsomal/TPO-Ab) are significantly elevated in most cases. HT is one of the major manifestations of autoimmune thyroid disease, the other being GD.⁹¹ It is more common in Asians.⁹² In multiethnic Malaysia, a study of 88 cases of HT showed it to be more common in Indians than Chinese or Malays, and nodular presentation was seen in about one-third of cases.⁹³

  Page 126 

A bloody background with lymphoid cells, degenerative changes in follicular cells and infiltration of follicular cells by lymphoid cells are characteristic features of HT. Variable features include oxyphilic cells (Hurthle cells), plasma cells, epithelioid cell granulomas and multinucleated giant cells.

The ‘lymphocytic’ pattern of HT occurs in children and young adults with a shorter history of the disease and absent or low antibody titers.⁸⁸ Smears are dominated by a mixed population of lymphoid cells including centroblasts, immunoblasts and dendritic reticulum cells from germinal centers characteristic of a reactive lymphoid proliferation (Fig. 6.12). Germinal center histiocytes have plentiful pale cytoplasm and oval or indented histiocytoid nuclei with granular chromatin. They are often clustered and associated with lymphoid cells, some of which lie within their cytoplasm. Histiocyte-lymphocyte rosettes or lympho-histiocytic clusters may be seen. Lymphoid : follicular cell ratios are often as high as 10 : 1 with epithelial cells so inconspicuous that smears resemble reactive lymphoid hyperplasia (Fig 6.12 A).

‘Classic’ or ‘florid’ HT occurs in older patients (usually women), who are more often hypothyroid and have raised TPO-Ab. The smear background shows lymphocytes with a variable number of plasma cells. There is prominent oxyphilic (Hurthle cell/Askanazy cell) change with single and syncytial aggregates of cells showing abundant, dense, finely granular, gray-blue cytoplasm (MGG), and well-defined cell borders. Nuclei are 2–4 times the size of normal follicular cell nuclei (Figs 6.10 and 6.11) and may show atypia and prominent nucleoli. Normal-appearing follicular cells may be present, showing features of hyperactivity. A characteristic feature is that of lymphocytes (and occasionally plasma cells) seeming to adhere to or infiltrate follicular cells, supporting the theory of direct epithelial damage by lymphocytes. Multinucleated giant cells and epithelioid cells can be seen in up to 40% of cases.^87,93 Neutrophils and eosinophils may be seen adhering to or infiltrating follicular cells in early stages.^42,93

  Page 127 

Seven to thirty-three percent of cases are antibody negative.^87,90,93 In a study of 150 cases of HT, overall Ab positivity was found to be 88.67%.⁹⁴ TPO-Ab showed significant correlation with high lymphoid : epithelial ratios but not with cases showing follicular hyperplasia or hashitoxicosis (see below). It appears that antibody positivity may depend on the phase of the disease. Absence of serum antibodies can also be explained on the basis of local antibody production by intrathyroidal lymphocytes.⁹³

Problems and differential diagnoses

♦ Distinguishing bare thyroid nuclei from lymphocytes,

♦ Lymphoid populations in other lesions,

♦ Hyperthyroiditis/hashitoxicosis,

♦ Distinction from lymphoma,

♦ Distinction from follicular and Hurthle cell neoplasms,

♦ Distinction of Hurthle cells from other large nonlymphoid cells,

♦ Granulomatous thyroiditis (GT),

♦ Distinction from fibrosing variant of HT,

♦ Psammoma bodies.

Stripped follicular cell nuclei resemble lymphocyte nuclei in size and shape. However, they have more homogenous chromatin and denser nuclear rim and lack the basophilic rim of cytoplasm seen in lymphocytes. Smears in HT show a polymorphous population of lymphoid cells (including mature and transformed lymphocytes) with lymphoglandular bodies in the backgound.

Lymphoid cells are often seen in smears from PC, especially the diffuse sclerosing and Warthin-like variants.^95,96 Multiple sampling is important to ensure representative sampling so that neoplastic cells are not missed. Histological sections from thyroids resected for NG often show focal collections of lymphoid cells or lymphoid follicles. This does not imply a diagnosis of thyroiditis and review of cytologic smears from such cases rarely show lymphoid cells.⁸¹

Some cases of HT may present in a hyperthyroid state with increased T3 and T4 levels (hashitoxicosis).⁸⁷ In these cases, TPO-Ab are often negative.⁹⁴ Smears are highly cellular with hyperplastic follicular cells showing fire-flares. Lymphoid and Hurthle cells may be few or focal, and distinction from GD is often difficult or impossible. Six monthly follow-up of these cases with cytological monitoring and thyroid function tests have shown the patients becoming euthyroid within a few months to 2 years, with concurrently changing smear pattern to reflect the usual features of HT.⁴² Awareness of the close kinship between various forms of autoimmune thyroid disease (GD, HT and postpartum thyroiditis – described later) facilitates diagnosis and management.

While high-grade non-Hodgkin’s lymphoma is easily identified on cytological preparations, low-grade and MALT lymphomas are often difficult to distinguish from reactive lymphoid populations seen in thyroiditis. Approximately 75% of lymphomas arise in a background of HT. Focal involvement can cause sampling problems. The smear pattern may be of HT in one part of the gland, that of obvious lymphoma in another (see Fig. 6.65). The ‘florid lymphocytic’ type of thyroiditis with scant epithelial cells, common in young patients, should be viewed with suspicion if found in elderly individuals, and efforts made to rule out lymphoma. Flow cytometry and molecular assessment of lymphoid infiltrates in fine needle samples are being increasingly used for the distinction of lymphoma from thyroiditis.⁹⁷

Follicular and Hurthle cells in HT may show atypia significant enough to cause concern to the inexperienced observer (Fig. 6.11A). Sometimes, and more often in younger patients with florid lymphocytic thyroiditis, abundant active-looking epithelium may be aspirated, leading to a suspicion of neoplasia. While Hurthle cell atypia in a background of lymphoid cells is recognized as part of the diagnostic spectrum of the disease, selective sampling of lesions of focal nodular hyperplasia constituted by atypical Hurthle or follicular cells (seen in early phase of some cases of HT) with scanty or no lymphoid populations can be easily mistaken for neoplasia.^42,87 In so-called ‘burnt-out’ HT only oxyphilic cells may be present in smears.^89,98,99 This problem is more pronounced when such a case has a nodular presentation. Multiple sampling offers the best chance of finding evidence of lymphoid infiltration. Disorganized, poorly cohesive masses of oxyphilic cells with prominent nucleoli are more indicative of neoplasia, sheet-like structures infiltrated by lymphocytes of hyperplasia.¹⁰⁰ However, this distinction is not always obvious. Some authors advise against making a diagnosis of follicular/oxyphilic neoplasm in the presence of pleomorphism and abundant lymphoid cells.^82,101,102 The possibility of a neoplasm with surrounding HT should be considered if abundant epithelium and lymphocytes are aspirated.^98,101,103 Coexistence of HT with differentiated thyroid carcinoma (4%) or lymphoma (1%)⁴² may have to be considered, especially in cases with nodular presentation, observed in 22–80% cases in various series.^104,105

Many ‘hypertrophic epithelial cells’ with abundant cytoplasm are present in early stages of thyroiditis. These lack the dense cytoplasm and well-defined cytoplasmic borders of Hurthle cells. Some may even have ‘fire flares’ and presumably represent TSH-stimulated cells. Epithelioid-like cells with elongated, spindle-shaped cytoplasm and elongated or bean-shaped nuclei may be seen. These may resemble oxyphilic cells and forms morphologically intermediate between these two types occur (Fig. 6.13). Histiocytes or dendritic reticulum cells from germinal centers could be confused with oxyphilic cells, but lack the characteristic cytoplasmic density.

Cases of HT showing giant cells and/or epithleioid cells⁹³ may be confused with granulomaotus thyroiditis (GT).⁸¹ The inflammatory infiltrate in GT is not uniformly lymphocytic but is mixed, with evidence of more severe tissue destruction. The smear pattern is dominated by multinucleated giant cells surrounding extruded colloid and epithelioid cell collections. A few cases have showed overlap in cytological features with HT,¹⁰⁶ and thryoid function tests and antibodies are required to clarify the diagnosis.

  Page 128 

Fibrosing variant of HT may be confused with Riedel’s struma,⁴² a rare thyroid manifestation of systemic collagenitis occurring in euthyroid middle-aged women. Smears yield paucicellular material containing collagenous fragments, bland or plump spindle cells and myofibroblasts.¹⁰⁷

Psammoma bodies have been described in association with HT¹⁰⁸ and in other benign processes. Their presence in smears probably warrants surgical biopsy because of close association with PC. Association of HT per se with PC is also reported.¹⁰⁹

de Quervain’s thyroiditis (subacute thyroiditis; granulomatous thyroiditis) (Figs 6.14-6.17)^42,81,87

This is a spontaneously remitting granulomatous inflammation of the thyroid, of possible viral etiology, occurring predominantly in females from the second to the fifth decades. Patients usually present with chills, fever, fatigue and a painful tender goiter that may be unilateral or spread from one lobe to the other.

Smears in GT are dominated by the presence of large multinucleate giant cells with numerous nuclei, granulomatous aggregates of epithelioid cells, degenerating follicular cells, neutrophils, macrophages and lymphocytes in a dirty smear background that shows debris and colloid.

The follicular cells often show degenerative features and contain dark-blue (golden in PAP-stained smears) cytoplasmic ‘paravacuolar’ granules representing lipofuscin or lysosomal debris. These granules are not a specific feature of this condition and may be seen in involutional follicular cells of NG, in GD and occasionally in PC and FNs.¹¹⁰ Stripped or crushed nuclei may be present. The giant cells are the hallmark of the disease and are characteristically very large, containing up to 200 nuclei (Figs 6.14 and 6.15), but even without them the diagnosis may be suggested if the other features of the disease are present (Fig. 6.16). Hurthle cells may be seen in some cases.

Fig. 6.14 de Quervain’s thyroiditis

Large multinucleated giant cells; clumps of thick, engulfed colloid (MGG, IP).

Fig. 6.15 de Quervain’s thyroiditis

Huge multinucleate histiocytic giant cells, dirty background of clumps of colloid, inflammatory cells and degenerate epithelial cells (Pap, IP).

Fig. 6.16 de Quervain’s thyroiditis

Mixed cell reaction; histiocytes, degenerating epithelial cells and lymphocytes in a ‘dirty’ background with thin colloid (MGG, HP).

The overlap with cases of HT showing multinucleate giant cells and granulomas has already been discussed. Epithelioid and giant cells may rarely be seen in GD. Giant cells are often seen in PC in association with lymphocytes and therefore PC must be considered in the differential diagnosis.¹¹¹ Rarely, the thyroid may be the site of mycobacterial infection, sarcoidosis or other infectious granulomatous processes.⁴²

Cases showing Hurthle cells may be confused with HT.¹⁰⁶

A small follicle withdrawn intact may simulate a multinucleate giant cell. The spherical nature and distinct outline of the structure in contrast with the irregular, flat form of true histiocytic giant cells should prevent confusion (Fig. 6.17A).

Fig. 6.17 de Quervain’s thyroiditis

(A) ‘Pseudogiant cell’; a small follicle in a smear from NG with a smooth well-defined border and a small amount of colloid in the center; superficial resemblance to a multinucleated giant cell (MGG, HP oil); (B) Degenerating epithelial cells with paravacuolar granules (left) and epithelioid histiocytes (right) in de Quervain’s thyroiditis (MGG, HP).

Cases with nodular presentation may simulate a lymph node or a neoplasm. Conversely, carcinomatous involvement of thyroid can mimic GT^112,113 or simulate silent thyrotoxic thyroiditis (hyperthyroiditis).

Characteristic cytological features of GT may be absent in the resolving phase of the disease.

Vigorous palpation of the gland may cause extrusion of follicular colloid, inciting a giant cell reaction. These granulomas are scanty, minute and clinical and functional profiles of GT are absent.¹¹⁴

Silent thyrotoxic thyroiditis (painless thyroiditis, subacute lymphocytic thyroiditis)^88,115,116 usually occurs in women as a sporadic or post-partum condition (post-partum thyroiditis), presenting with a small, diffuse, painless goiter. The disease may go through hyperthyroid, euthyroid, hypothyroid and recovery phases similar to GT. Low titers of TPO Ab are transiently present in two-thirds of cases. Smears show scattered lymphoid cells and giant cells in occasional cases. Unlike GT, there is no pain or any evidence of preceding viral infection and granulomas are uncommon. Rare cases have shown cytologic similarity to HT.¹¹⁶

Follicular neoplasms (Figs 6.18-6.24)^{23,29,68,117-120}

Fig. 6.18 Follicular neoplasm

Cellular smears of single cells, microfollicles or rosettes in a repetitive manner; benign adenoma by histology (A, MGG, HP; B, Pap, HP).

Fig. 6.19 Follicular neoplasm

Smears very similar to Figure 6.18; follicular carcinoma with vascular invasion by histology (A, MGG, HP; B, Pap, HP).

Fig. 6.20 Follicular neoplasm

Microfollicular cell clusters/rosettes; some nuclear hyperchromasia and coarseness in both (A) follicular adenoma and (B) follicular carcinoma (MGG, HP oil).

Fig. 6.21 Follicular neoplasm

(A) Small intact follicles with basement membrane; small uniform nuclei; follicular adenoma. (B) Microfollicular groups; enlarged nuclei; drop of abnormal colloid in small central lumen; follicular carcinoma (MGG, HP).

Fig. 6.22 Follicular neoplasms

Rationale for cytological diagnosis.

Fig. 6.23 Follicular carcinoma

Aspirate of distant lymph node metastasis of disseminated follicular carcinoma; syncytial cluster with some rosetting; bland nuclear chromatin; mild nuclear enlargement and anisokaryosis (MGG, HP).

Fig. 6.24 Atypical adenoma and well-differentiated follicular carcinoma

(A) Smear from atypical adenoma with bizarre cells; histology showed similar epithelial atypia but no capsular or vascular invasion; uneventful follow-up; (B) Well-differentiated follicular carcinoma metastatic to bone – cellular smear with prominent microfollicular pattern, relatively uniform nuclei and fire-flares (MGG, HP).

FNs are classified as benign (FA) and malignant (FC). FAs and most FCs are encapsulated tumors, occurring in one of the lobes. Histological diagnosis of a well-differentiated FC requires demonstration of capsular and/or vascular permeation. Most FNs, especially adenomas, have a uniform internal structure that is reflected in the cytological smears. FAs are more common in women and microscopically show a variety of histological patterns such as microfollicular (fetal), normofollicular, macrofollicular, trabecular, solid (embryonal), Hurthle cell and atypical adenomas.⁴² Cytologically, follicular lesions include FA, FC, cellular NG and FV-PC.¹²¹

Smears in FN are cellular in a bloody background that is usually devoid of colloid. Many uniform-sized follicular cell clusters, microfollicles and rosette formations are present. Syncytial aggregates, nuclear crowding and overlapping are also often seen.

The repetitive smear pattern with uniform cell population is in contrast to the variable pattern of different cell types seen in colloid and hyperplastic nodules. Microacinar clusters with a central lumen (that may contain a drop of colloid) represent microfollicles (Figs 6.18, 6.19 and 6.21B). These are characteristic of FN but may be found focally in NG. Rosette-like groupings without a lumen (Fig. 6.20) suggest a more solid growth pattern. A trabecular pattern is represented by rows and elongated aggregates of epithelial cells that resemble papillary structures when they adhere to strands of vascular stroma (see Fig. 6.33B,C). Small blood vessels with adherent epithelial cells can be found in any type of follicular neoplasm (see Fig. 6.25A).

The distinction between FN and NG is the most common differential diagnostic problem in solitary nodules as cytological appearances overlap (Fig. 6.22). A microfollicular focus in a colloid nodule cytologically resembles a microfollicular neoplasm, while smears from a macrofollicular (colloid) adenoma resemble a dominant nodule in multinodular goiter. Jaffar¹²² indicated that the presence of hemosiderin within macrophages and follicular cells excludes FN. The false-negative rate of cytology in FN may be 30% or more because of the inability to recognize normofollicular neoplasms.¹²³ However, these distinctions are of little clinical importance as long as the nodule is recognized as benign and spared from unnecessary surgery.

Most FCs are microfollicular, trabecular or solid, contain little colloid, and will be reported as ‘FN’ by cytology. Although the failure to recognize FC as neoplastic has been surprisingly high in some series,⁷ other studies show high diagnostic sensitivity, with false-negative rates as low as 0–2%.^11,13,28

Cytological features in FA and FC are similar, with cellular smears composed of syncytial clusters of crowded cells. There is a tendency for uniform nuclear enlargement in FC, whereas FA may show small or large nuclei.¹²⁴ These differences are often subtle, with much overlapping (Fig. 6.20).^3,68,125 Cells from a well-differentiated but clinically aggressive FC may not appear obviously atypical or enlarged in smears (Figs 6.23, 6.24B). Anisokaryosis per se is more a feature of non-neoplastic lesions such as NG and thyroiditis.⁴²

Most authors are content to use cytology to select cellular FNs for follow-up or surgical excision, and to leave a diagnosis of malignancy to histological assessment of capsular and vascular invasion. Reporting cytological atypia and any suspicion of malignancy, although not diagnostic, may be of some use in making the choice between follow-up and immediate surgical excision. The ultimate prognosis of microfollicular, solid or trabecular adenomas is uncertain and these should probably be excised anyway.

FC has been considered the second most common thyroid cancer, accounting for 10–20% of all thyroid malignancies.¹²⁶ The proportion of carcinoma in lesions designated as FN has been reported as ranging from 14% to 44% in previous series.^3,119 In a large series of operated cases from 1994 to 2002 from Bethesda,¹²⁷ an 18.2% rate of malignancy was found in cases in which a cytological diagnosis of ‘possible FN’ was given and 20.9% in cases with a definitive cytological categorization of ‘FN’.

Clinicians (and cytopathologists) are conventionally preoccupied with the inability of cytologically distinguishing FA from FC, leading to numerous ancillary investigations to help make this distinction. Computer assisted cell morphometry, ploidy analysis and determination of AgNORs in cytological smears have been tried with variable success.^61,128,129 Results of proton magnetic resonance spectroscopy have been encouraging.¹³⁰ However, the most promising technique for the future is probably immunocytochemical demonstration of molecular markers.¹³¹ Positive immunostaining with CD44v6 or galectin 3 (with a score of G2) in combination with FNA^54,55 have been noted to be useful in cases with indeterminate cytology.⁵⁶ Telomerase activity⁵⁸ and microarray analysis of cytologic samples⁵⁹ are other ancillary tests that may aid in distinguishing benign from malignant follicular lesions.

Interestingly, there are studies indicating that FC is gradually becoming a rare entity.^{18,22,24,132,133} Prevalence of malignancy in operated cases of FN¹¹⁸ was 31%, and 9% of these were follicular or Hurthle cell carcinomas. The incidence of FC was only 2% of thyroid cancers in LiVolsi’s series²⁴ and 1% at the University of Chicago Medical Center.²² Discussing the gradual demise of FC, De May²² opined that independent of the quality of any diagnostic test employed, statistically speaking, its predictive value would be affected by the prevalence of the disease in question. It stood to reason therefore that, if the prevalence of FC is low, the predictive value of cytology for FC would also tend to be low, although cytology is otherwise an excellent diagnostic modality.

Atypical adenomas show foci of extremely pleomorphic cells that cytologically simulate malignancy (Fig. 6.24A). These are designated as carcinoma only if there is capsular or vascular invasion. Nuclear pleomorphism is not a common feature of well-differentiated FC and is not considered a cytological criterion of malignancy. On the other hand, it is frequent in dyshormonogenetic goiter, treated GD, and following chemotherapy and radiotherapy.^42,85

In recent years, FV-PC has attracted much attention. Due to the presence of follicular groupings and colloid in these tumors (see Fig. 6.43), it constitutes a generous proportion of malignancies reported as FN.^21,22,118 However, a proportion of nuclei display typical cytological features of PC (described below in section on PC). In some series, high cytologic accuracy rates for FV-PC¹³⁴ with 93–94% sensitivity, specificity, positive and negative predictive values have been demonstrated using ultrafast PAP stain.¹³⁵

No cytological criteria clearly distinguish parathyroid from follicular thyroid neoplasms.¹³⁶ Agarwal et al.,¹³⁷performing US-guided FNA of 53 parathyroid adenomas, reported low sensitivity as a major limitation. Smears showed moderate cellularity with monomorphous, round to slightly oval cells predominantly arranged in loose two-dimensional clusters with occasional papillary fragments. The majority of them exhibited stippled nuclear chromatin and bare nuclei were seen in the background. There was no significant pleomorphism, mitotic activity, or prominent nucleoli. Parathyroid adenomas are usually not palpable but radiological examination is capable of locating most non-palpable parathyroid lesions. However, the occasional parathyroid adenoma may be intrathyroidal.¹³⁸ Cytohistological overlap in parathyroid and thyroid follicular lesions can be a problem at the time of frozen section evaluation, and intraoperative parathyroid hormone monitoring may be required. In parathyroid carcinomas, clinical, biochemical and radiological findings are usually characteristic. Cytology, combined with clinical, radiological and immunocytochemical findings can enhance diagnostic accuracy.¹³⁹ Tseleni-Balafouta¹⁴⁰ felt that due to overlapping morphological features, in order to avoid surgical mismanagement, the possibility of a parathyroid lesion should be stated clearly in cytology reports in all colloid-free cellular follicular lesions. Parathyroid incidentalomas comprise 0.4% of lesions in patients referred for suspected thyroid nodules, and parathyroid hormone analysis in FNA washouts has been found to be a diagnostic aid.¹⁴¹

FNs are often highly vascular and aspiration of blood may obscure neoplastic cells. It should be understood that excessively blood-stained smears are not necessarily related to poor technique but may signal the possibility of a neoplasm. Repeat needling using the fine needle capillary sampling technique with a 26 or 27-gauge needle after a gap of a week or two often yields diagnostic material.

Colloid in small follicles is often very dense and may be laminated, but the regular edge is unlike that of a psammoma body.

Variants of follicular neoplasms

Fig. 6.25 Oxyphil (Hurthle cell) adenoma

(A) Sheets of oxyphil cells adherent to capillary blood vessels; histology oxyphil adenoma; (B) Oxyphil cells in a trabecular arrangement; histology oxyphil adenoma (MGG, HP).

Fig. 6.26 Hurthle cell carcinoma

(A) Similar pattern as Figure 6.25B of trabecular groups of oxyphil cells (MGG, HP); (B) Large polygonal cells with well-defined cell margins, basophilic cytoplasm, vesicular nuclei and macronucleoli (MGG, HP).

HCTs, constituting 1.5–10% of thyroid tumors, represent a controversial pathological entity,¹⁴² regarding both their behavior and classification (whether they represent metaplasia in follicular or papillary neoplasms or constitute a distinct entity). Most tumors are encapsulated, and capsular and/or vascular permeation are standard criteria of malignancy. Recent reports suggest that Hurthle cell carcinoma may be a more aggressive tumor, distinct from FC.¹⁴³ Rare familial cases of aggressive, metastasising Hurthle cell carcinoma have been reported.¹⁴⁴

Smears from HCT yield abundant material consisting of large, polygonal Hurthle cells with oval nuclei and abundant, well-defined, granular cytoplasm. Tumor cells appear singly, in acinar arrangement, and in monolayered sheets of variable sizes.^145,146 Nuclei are eccentrically placed. Occasional cells may be ovoid or rounded. Nuclear pleomorphism may be present but is not as common in adenomas as in non-neoplastic lesions like HT. Occasional small syncytial tumor cell clusters and naked nuclei may be seen. Carcinomas show relatively smaller Hurthle cells with monomorphic or pleomorphic nuclei, macronucleoli and ill-defined cytoplasm. Crowded sheets, syncytial tumor cell clusters of variable sizes and naked nuclei may be present.⁹⁰ Computerized interactive morphometric analysis of nucleolar features may be helpful in distinguishing benign from malignant lesions.¹⁴⁷ However, due to frequent morphological overlap, a cytological diagnosis of HCT is preferred, with further categorization deferred for histological study. Galectin 3 immunostain has been shown to stain Hurthle cell adenomas.⁵⁵ Combination of galectin 3 and HBME has been demonstrated to show 99% sensitivity and 88% specificity in tumors composed of Hurthle cells (Hurthle cell adenomas, carcinomas and oncocytic variant of PC).¹⁴⁸

Hurthle cells are seen in a variety of non-neoplastic lesions such as NG, GD, HT and GT. In most of these conditions, they are admixed with other cells that indicate the nature of the lesion, such as lymphoplasmacytic cells in HT, epithelioid cells in GT, etc. A mixture of Hurthle cells and ‘normal’ follicular epithelial cells is more consistent with a hyperplastic nodule.^99,146 In early stages of HT, needling of proliferating Hurthle cell nodules may lead to a smear pattern dominated by Hurthle cells. The presence of a high percentage of dyshesive Hurthle cells with large nucleoli, with some cells showing significant nuclear enlargement and pleomorphism, are indicative of a neoplastic Hurthle cell lesion.^99,146 Flat sheets of Hurthle cells are more characteristic of thyroiditis, poorly organized and poorly cohesive cell clusters of neoplasia.¹⁰⁰ Neoplastic and non-neoplastic Hurthle cell nodules may develop following irradiation to the head and neck.¹⁴⁹ Smears in such cases often suggest a neoplastic lesion. These nodules should be excised for careful histological evaluation.

Atypical adenoma with bizarre cells has been discussed above. A rule applicable to all endocrine organs is that nuclear pleomorphism per se cannot be used as a basis for cytological diagnosis of malignancy.

Clear cell change in follicular epithelium may be due to accumulation of glycogen, thyroglobulin, mucin, lipid or varying combinations of these.⁹⁰ It may also represent artifacts of formalin fixation or paraffin embedding.⁴² While clear cell morphology is well appreciated in tissue sections, cytologic smears show cells with abundant, finely vacuolated, pale but not totally clear cytoplasm (Fig. 6.27). Clear cell change may be a focal or less frequently diffuse phenomenon in FA, FC, HCT⁴² and non-neoplastic lesions. Metastatic clear cell carcinoma of renal origin is an important differential diagnosis.⁴² Signet ring adenoma containing thyroglobulin as well as mucin, PAS-D and acidic Alcian blue-positive material is a rare finding and could be due to the presence of protein polysaccharide complexes derived from partial degradation of thyroglobulin.¹⁵⁰

Fig. 6.27 Clear cell change

Follicular cells that show a uniform appearance of abundant, delicately vacuolated cytoplasm (MGG, IP).

Poorly differentiated thyroid carcinoma

Until recently, thyroid carcinoma with a poorly differentiated insular pattern was considered to be a distinct entity, a thyroglobulin-producing neoplasm, intermediate in aggressiveness between well-differentiated and anaplastic thyroid carcinoma. Reports have appeared documenting cytological features in insular carcinomas such as high cellularity, dispersed and loosely aggregated cells, solid, cohesive trabecular or papillaroid structures, intact insulae, fragile, ill-defined, granular cytoplasm, oval, hyperchromatic nuclei, occasional INCIs and/or grooves.^151-155 However, as insular pattern is often admixed with trabecular and solid growth patterns, the more suitable term ‘primordial carcinoma’ was suggested for this entity.¹⁵⁶

The current concept of pure poorly differentiated thyroid carcinoma, as per the Turin proposal¹⁵⁶ is one that shows a histologically mixed solid/trabecular/insular architecture, absence of conventional nuclear features of PC and the presence of one of the following three features: cells with convoluted (raisin-like) nuclei, a mitotic index of ≥3 mitoses/10 high-power fields and tumor necrosis. Most tumors are immunohistochemically positive for thyroglobulin and thyroid transcription factor 1, and a subset is also positive for p53.¹⁵⁷ Ras mutations are common.

Smears in poorly differentiated thyroid carcinomas are hypercellular with single cells as well as cells in solid, trabecular and insular patterns. There is marked crowding of cells and tumor cells show high nuclear cytoplasmic ratios (Fig. 6.28).¹⁵⁸

Fig. 6.28 Poorly differentiated carcinoma

(A,B) Smears showing syncytial clusters of crowded small cells with hyperchromatic nuclei (A, MGG, HP; B, Pap, HP); (C) Tissue section, same case. (H&E, IP).

Distinction from metastatic carcinomas (Fig. 6.29), especially on cytologic smears, requires detailed clinical evaluation, review of sections of previous surgery, prior cytologic smears, if any, and use of ancillary stains (thyroglobulin and TTF-1) where necessary.

Fig. 6.29 Metastatic carcinoma in thyroid

Metastasis to thyroid of breast carcinoma; smear pattern similar to poorly differentiated carcinoma as in Figure 6.28 (Pap, HP).

Differentiated thyroid carcinomas, especially FC, may show poorly differentiated foci that may be sampled by the needle, missing out the well-differentiated component. MC of small cell type can be distinguished by calcitonin immunostaining.

Papillary carcinoma (Figs 6.30-6.47)^159-162

Papillary carcinoma (PC) is the most common type of thyroid cancer, occurring predominantly in females, in all age groups but most often in the third to fifth decades.⁴² It accounts for 90% of childhood thyroid malignancies. Five to ten percent of cases have had prior head and neck irradiation, usually in the first two decades of life. Most cases show clinically evident metastatic cervical lymphadenopathy at presentation, yet the prognosis is usually good, especially in children and young adults. RET/PTC rearrangements and BRAF mutation are often seen.¹⁵⁹ PC is rarely encapsulated, infiltrates the thyroid in an irregular manner and is often multifocal with variable degrees of cystic change. Histological hallmarks of the tumor are the branching papillary structures with fibrovascular cores, lined by cuboidal or columnar cells with ‘ground-glass’ nuclei (‘Orphan Annie’ eyes). These cells show nuclear grooves and/or INCIs, and psammoma bodies are seen in many cases.

Smears in PC are cellular with numerous three-dimensional and papillary fragments (Fig. 6.30) with or without vascular cores. Often, papillae not removed intact by the needle appear as flat sheets. Sheets of cells show a distinct anatomical border, formed by a row of cuboidal or columnar cells (Fig 6.31A) with focal nuclear crowding and overlapping, features that distinguish sheets of PC from those representing benign macrofollicles (Fig. 6.31B). The tip of a papilla may be seen as a finger-like aggregate of cells with a similar edge (Figs 6.32 and 6.33A). Naked true papillary connective tissue cores are sometimes found and can be diagnostically helpful. Trabecular fragments (Fig. 6.33B) (also present in FN) are represented in smears by cohesive finger-like structures and must not be mistaken for papillae. Seventeen percent of cases show concentrically organized aggregates of tumor cells or ‘swirls’ (Fig. 6.34), the most peripherally located cells appearing ovoid with nuclei oriented perpendicular to the radius of the swirl.¹⁶³ A few cases show only dispersed single cells and syncytial aggregates; diagnosis then relies on identification of nuclear features of PC.

Fig. 6.30 Papillary carcinoma

True papillary fragment with a vascular core; columnar cells with oval pale nuclei; intranuclear vacuoles, upper left (MGG, IP).

Fig. 6.31 Comparison of sheets of papillary carcinoma and of macrofollicle in nodular goiter

(A) Papillary carcinoma. Flat sheet of epithelial cells; partly monolayered, partly with nuclear overlapping; uniformly enlarged, oval nuclei with pale powdery chromatin and small nucleoli; note ‘anatomical’ edge of a row of cells along upper edge (Pap, HP); (B) Nodular goiter. Flat monolayered sheet of cells from a disrupted macrofollicle; note frayed edges and uniformly small round nuclei (Pap, HP).

Fig. 6.32 Papillary carcinoma

Finger-like papillae with ‘anatomical’ edges (Pap, IP).

Fig. 6.33 ‘Papillary’ structures

(A) Finger-like papilla of papillary carcinoma; distinct lining on three sides by a row of cuboidal/columnar cells (H&E, IP); (B) Finger-like, pseudopapillary fragment from a trabecular thyroid carcinoma (Pap, HP); (C) Tissue section from same case as B (H&E, IP).

Fig. 6.34 Papillary carcinoma, swirls

Concentrically arranged tumor cells forming swirls with peripherally located cells appearing perpendicular to radius of the swirl (MGG, HP).

Tumor cells show uniform enlargement with dense cytoplasm and well-defined cell borders. INCIs (Fig. 6.35), characteristic of PC, are seen in up to 90% of cases. They are seen in 5% of the cells (10% if examined under oil immersion under 2–3 planes of focus).^164,165 INCIs, however, are not specific to PC as they can be seen in atypical adenomas,¹⁶⁶ hyalinizing trabecular tumors,¹⁶⁷ MC (see Fig. 6.55),¹⁶⁸ AC (see Fig. 6.59) and rarely in FC,⁹⁰ HT¹⁸ and juxta-thyroidal neoplasms (parathyroid adenoma, paraganglioma, etc.).^67,90

Fig. 6.35 Papillary carcinoma, intranuclear ‘vacuoles’

Large, oval, pale nuclei; several intranuclear cytoplasmic inclusions; pale powdery chromatin most obvious in Pap; note cells with convoluted nuclei in B (A, MGG; B, Pap, HP oil).

INCIs have sharp, well-defined, membrane-like margins and are not optically clear but resemble the cytoplasmic color and texture. They probably start as trapped cytoplasm in deep nuclear folds (grooves)¹⁶⁹ that eventually invaginate into the nucleus at foci of nuclear membrane weakness. Grooves and inclusions do not usually coexist, possibly due to the pressure of the inclusion unfolding the groove and preventing further groove formation.¹⁷⁰ Artifacts such as superimposed air bubbles or fat droplets can mimic INCIs both in PAP- and MGG-stained material.^27,171 Optically clear vacuoles and poorly defined central areas of pallor (MGG) should not be accepted as inclusions. The clear or ground-glass (‘Orphan Annie’) nuclei seen in tissue sections are represented in smears by very fine, powdery nuclear chromatin,¹⁶⁴ an important diagnostic criterion and a feature best appreciated in ethanol-fixed, PAP-stained smears (Figs 6.35 and 6.36).

Irregular nuclear shapes, convolutions (Fig. 6.35B) and longitudinal nuclear grooves or creases (Fig. 6.36) are visible in cytologic smears (in 85–100% cases) and in sections.^165,170 Grooves are obvious in alcohol-fixed material but are difficult to discern in MGG preparations. Strict criteria for recognition have been suggested: continuous grooves or creases, clearly defined and running the length of the nucleus.¹⁷² The presence in ≥20% of cells, as counted in selective fields where grooves are frequent, is highly predictive of PC.¹⁷³ Grooves, however, may be found in small numbers in 70–80% of non-papillary neoplasms, in 50–60% of non-neoplastic thyroid lesions¹⁷² and in a variety of extrathyroid tumors; hence, metastatic carcinoma and melanoma are included in differential diagnoses.⁴²

Fig. 6.36 Papillary carcinoma

Sheet of cells with large, very pale (but not optically clear) crowded nuclei; powdery chromatin; many longitudinal grooves (Pap, HP oil).

Tumor cells often show delicate soap-bubble like cytoplasmic vacuolation (septate cytoplasmic vacuoles)¹⁷⁴or squamoid cytoplasm (metaplastic cells). Dense cytoplasm and well-defined cell margins may simulate Hurthle cells (Fig. 6.37) or squamous cells (Fig. 6.38A). True squamous metaplasia and Hurthle cell change may be present. Cells with abundant vacuolated cytoplasm are seen (Fig. 6.38B) resembling histiocytes (foam cells) but with nuclear features of PC.¹⁶² Such ‘foam cell metaplasia’ is seen in about 50% of PCs and is best appreciated at the edges of the smear.⁴² Foam cell metaplasia may also be seen in cystic NG with papillary hyperplasia⁴² and in papillary breast lesions. Macrophages and cell debris may be prominent, especially when cystic change is present. Multinucleate giant cells are frequently seen and, if numerous, have been shown to be associated with larger tumor size and greater likelihood of extrathyroidal extension.¹⁷⁵ Lymphoid cells are present in 30% of cases.¹⁷⁶

Fig. 6.37 Papillary carcinoma

Poorly cohesive cells and a micropapillary cluster; psammoma bodies; abundant dense cytoplasm with distinct cell borders; uniform large nuclei, some nuclear crowding and overlapping (MGG, HP).

Fig. 6.38 Papillary carcinoma, ‘metaplastic’ cells

(A) Poorly cohesive cells with large, pale, oval nuclei, a few with grooves; some cells with dense ‘metaplastic’ squamoid cytoplasm (Pap, HP); (B) ‘Metaplastic’ cells resembling macrophages, but with nuclear features of papillary carcinoma (Pap, HP oil).

‘Chewing gum’ colloid presents as strands or chunks of dense, dark-blue (MGG) colloid, rather unlike the colloid found in other thyroid diseases (Fig. 6.39). Concentric, lamellated, calcified (psammoma) bodies are seen in cytological smears in 0–25% of cases.¹⁷⁷ They look glassy and refractile, measure about 100 µm, stain dark blue with MGG and red with PAP stain, and show concentric lamellations (Fig. 6.40) that distinguish them from non-specific degenerative calcium granules seen in many thyroid lesions. Although not specific to PC, when present in non-neoplastic thyroid tissue or lymph nodes they are an important clue to the presence of occult PC. In smears containing psammoma bodies, laminated hyaline globules, branching hyaline cylinders and irregular hyaline deposits can be seen.¹⁷⁸ Rarely, they show varying numbers of concentric layers of calcium in between the hyaline layers, suggesting an early or precursor form of psammoma bodies (intracellular, targetoid, possible precursor substances were reported in one case).¹⁷⁹

Fig. 6.39 Papillary carcinoma

Thick, ropy ‘chewing gum’ colloid; sheets of cells with enlarged ovoid pale nuclei; some ‘metaplastic’ ’cells (MGG, IP).

Fig. 6.40 Psammoma bodies

(A) Psammoma bodies showing concentric lamellation and variable staining, partly dark blue (MGG, HP oil); (B) Follicular epithelial cells surrounding a central cluster of calcific bodies from a benign colloid nodule with cystic change (Pap, HP).

Multiple criteria must be observed before making a confident cytological diagnosis of PC.^160,162 Logistic regression analysis of the various criteria suggested that a combination of INCIs, papillary structures without adherent blood vessels and dense ‘metaplastic’ cytoplasm were the three most important variables.¹⁶¹ The presence of ≥3 of the following features – papillae, psammoma bodies, nuclear grooves, INCIs and fine granular chromatin – has been reported to facilitate cytological diagnosis of PC, with frequent grooves and INCIs being the most dependable.¹⁶⁴ Sensitivity and predictive value of cytological diagnosis in several large series ranged from 60% to over 90%.^30,160,161 Nuclear enlargement, atypia and nucleoli are reported to relate to recurrence.¹⁸⁰ BRAF oncogene mutation, associated with extrathyroidal extension, recurrence and lymph node metastasis, can be identified in cytologic material and can help in optimizing treatment.¹⁸¹

As in FC, PC cells show dual immunostaining for cytokeratin and vimentin, a feature of help in identification of distant metastases as of thyroid origin. Immunostaining for cytokeratin 19, CD44, galectin 3 and p63 have been reported to be of value in the diagnosis of PC.^182-184 Nga et al.¹⁸⁵ reported the combination of positive HBME-1 (luminal/membranous) and CK19 (cytoplasmic) staining on smears from PC.

Fluid aspirated from cystic PC is uncharacteristic, brown or resembles altered blood. The diagnosis can easily be missed if well-preserved epithelial cells are scarce (Fig. 6.41).^30,31,71 Presence of numerous macrophages, with many in cohesive clusters, should raise a suspicion of PC. Some of these cells are probably degenerating tumor cells exfoliated from the cyst lining (Fig. 6.42). They may represent foam cell metaplasia in tumor cells, and careful scrutiny will usually reveal nuclear features of PC. Large cell size, pseudoinclusions, nuclear grooves, and multiple well-defined vacuoles in atypical histiocytoid cells favor a diagnosis of PC.¹⁸⁶

Fig. 6.41 Cystic papillary carcinoma

Cystic change in metastatic papillary carcinoma in cervical lymph node; mainly foamy cells with some pigment resembling macrophages; one cluster of degenerate atypical epithelial cells (MGG, HP).

Fig. 6.42 Cystic papillary carcinoma

(A) Fluid from cystic papillary carcinoma; numerous macrophages, many with pigment, some clustered; a few nuclei with longitudinal grooves; no well-preserved epithelial cells (Pap, HP); (B) Tissue section from a cystic metastatic deposit in lymph node from same case; transition from intact neoplastic epithelium lining the cyst to exfoliated degenerating epithelial cells to ‘macrophages’ with intracytoplasmic pigment (HE, IP).

Tumor cells and tumor fragments in cyst fluid often show attenuation due to pressure of the cyst fluid and may not be recognizable as such in cytological and histological preparations. The sensitivity of FNA diagnosis in cystic neoplasms may be as low as 40%,³¹ and all cystic lesions should be managed cautiously. Combining clinical and cytological criteria and using US-guidance while needling minimize false-negative diagnoses.¹⁵ Cervical node metastases of PC are also often cystic and may not yield well-preserved diagnostic epithelial cells; the possibility of metastatic PC should be considered if samples of an abnormal cervical node contain only blood, fluid and histiocytes.

Smears from PC that show lymphocytes and multinucleated giant cells may simulate HT,⁴² especially the diffuse sclerosing variant of PC that shows heavy lymphocytic infiltration. Close scrutiny of nuclear features is essential to avoid false-negative diagnosis. Infiltration of follicular and Hurthle cells by lymphoid cells is suggestive of HT, papillary and three-dimensional clusters of cells indicate PC. HT and PC may coexist.^109,187

Hurthle cell metaplasia of tumor cells may simulate HCT. If all of the tumor cells are of Hurthle cell type, an oxyphilic (Hurthle cell) variant of PC should be considered (described below).

Papillary foci (with rare psammoma bodies) are present in hyperplastic NG and in GD.¹⁸⁸ Such hyperplastic lesions lack nuclear features of PC.

Distinction from hyalinizing trabecular tumor is discussed below.

Many features of PC, such as INCIs, grooves, papillary structures and psammoma bodies, can be seen singly or in various combinations in other neoplasms and non-neoplastic lesions. Calcific debris and inspissated colloid may mimic psammoma bodies, as can also oxalate crystals in benign thyroid lesions, but the latter are birefringent. Intraluminal colloid-associated concretions in clear cell FNs may mimic psammoma bodies. Histiocytic cells in cystic NG can mimic nuclear features of PC; immunostaining for CD68 can be useful in their distinction. Cytological diagnosis of PC should never be made from isolated cytologic characteristics but from the composite cytological picture correlated with the clinical profile. Artifacts occurring during specimen preparation and handling issues such as decalcification, frozen section, and artifacts following FNA are discussed in detail by Baloch and LiVolsi.¹⁸⁹

Distinction of FV-PC from FN is described in detail in the section on FV-PC. The value of combining CK19 and HBME 1 immunostains has already been described above.¹⁸⁵

Variants of papillary carcinoma

Follicular variant of PC^134,190-192

This has attracted great interest in recent years, probably due to difficulties in its cytological distinction from FN. The tumor shows a follicular architecture with a proportion of cells showing nuclear features of PC such as powdery pale chromatin and nuclear grooves. INCIs are less frequent. Pink colloid balls may be present. However, overall cytological features are more akin to FN such as colloid, dispersed cells, acinar and syncytial clusters (Fig. 6.43). Ninety-three to ninety-four percent cytological sensitivity and specificity have been reported in some series,¹³⁵ while other studies show that a variable proportion of nodules designated as FN proved to be FV-PC.^18,22,118 The advantage of imprint smears over frozen section was stated earlier. Recent studies indicate that FV-PC may be a heterogeneous disease composed of an infiltrative/diffuse (non-encapsulated) subvariant resembling classic PC in invasiveness and metastatic nodal pattern and an encapsulated form that behaves more like FN.¹⁹³

Fig. 6.43 Follicular variant of papillary carcinoma

Microfollicular architectural pattern of syncytial clusters and follicles containing colloid, but enlarged, pale nuclei, some with intranuclear vacuoles (right) (MGG, HP).

Oncocytic variant (Fig. 6.44A,B)

This variant shows papillary and follicular structures populated by oncocytes with abundant, coarsely granular cytoplasm and nuclear features of PC. Smears show papillae and three-dimensional clusters of oncocytes, some with vascular cores.^42,196 INCIs and grooves are present. Maconucleoli are absent, an important distinguishing feature¹⁹⁶ from papillary HCT.¹⁴⁶ Differential diagnosis includes HT that is rarely associated with this variant. As this tumor is often associated with local invasion and can involve cervical lymph nodes, it may require more extensive surgery than classic PC.

Fig. 6.44 Papillary carcinoma, oncocytic variant

(A) Smear showing sheet of oncocytic cells with one cell showing INCI; (B) Tissue section of tumor showing papillae lined by oncocytic cells with nuclear features of PC.

High-grade variants of PC

These variants of PC are clinically more aggressive than the usual type.

Tall cell variant (Fig. 6.45)¹³⁴

This tumor shows solid areas, follicles and papillae lined by oxyphilic cells twice as tall as they are wide with nuclear clearing, grooves and inclusions. Smears show oxyphilic cells with reddish or cyanophilic, granular, septated, or vacuolated cytoplasm, frequent nuclear grooves and inclusions. Lymphocytes are often present. Tadpole-like cells with basal, eccentric nuclei and high MIB-1 labeling have been reported.²⁰⁴

Fig. 6.45 Tall cell papillary carcinoma

Group of tall columnar epithelial cells adhering to basement membrane material; abundant cytoplasm, nuclei more atypical than usual papillary carcinoma, nuclear inclusions and grooves were seen in other parts of the smear (MGG, HP oil).

Columnar cell variant^205,206

The least common variant of PC, this tumor has an aggressive course if unencapsulated. It shows a mixed papillary, follicular and solid growth pattern with tall columnar, pseudostratified cells lining papilloglandular structures. Smears show papillary and follicular structures lined by tall columnar, pseudostratified cells that resemble respiratory epithelium (Fig. 6.46).²⁰⁶ Nuclear grooves and inclusions are usually not seen. Dispersed plasmacytoid cells with eccentric nuclei and focal squamous metaplasia may be present.

Fig. 6.46 Columnar cell variant of PC

(A) Smear showing ovoid to columnar tumor cells with overlapping and pseudostratification; (B) Tissue section showing papillary structures lined by stratified columnar cells.

Diffuse sclerosing variant^95,207

Histologically, this tumor shows prominent fibrosis, a papillary pattern, squamous metaplasia and many psammoma bodies with moderate lymphoplasmacytic infiltrates. Scattered microcalcifications impart a ‘snow-storm’ appearance to the US. Clues to cytologic diagnosis are prominence of squamous metaplastic cells, psammoma bodies (Fig. 6.47) and lymphocytes intermingled with tumor cells. INCIs and grooves are present.

Fig. 6.47 Diffuse sclerosing variant of papillary carcinoma

Distorted tissue fragment of mainly ‘metaplastic’ cells; fibroblasts; many psammoma bodies (lower right) (MGG, HP).

Solid/trabecular variant

Comprising 3% of PC, this tumor has an aggressive course in adults but not in children, where it is associated with radiation exposure.¹⁵⁶ Fifty to seventy peercent of the tumor shows a trabecular and/or solid pattern. Nuclear features of PC are seen. Smears show solid masses and anastamotic, 2–10-cell thick trabecular cords, separated by endothelium-lined vascular spaces.²⁰⁸ Tumor cells show irregular nuclear contours, with some showing grooves and inclusions.

Hyalinizing trabecular tumor (Fig. 6.48)^167,209,210

This is a rare thyroid tumor of follicular cell origin with a trabecular pattern of growth and marked intratrabecular hyalinization. It has been mistaken on cytology for MC and paraganglioma with which it shares morphological and architectural similarities. Smears show medium-sized, round, oval or polyhedral cells arranged individually, in small groups or parallel arrays. Cells show low nuclear cytoplasmic ratios with moderate to abundant cytoplasm. Nuclei may be eccentrically placed with focal binucleate forms that resemble Hurthle cells or MC. Cohesive aggregates are radially oriented around hyaline material. Nuclear grooves and inclusions, seen in some cases,¹⁶⁷ may simulate PC, while dispersed cell pattern and amorphous amyloid-like or hyaline material in the smears may resemble MC. Intracytoplasmic ‘yellow bodies’ (giant lysosomes) have been described.²¹⁰

Fig. 6.48 Hyalinizing trabecular tumor

(A) Smear showing tumor cells arranged individually, in small groups and parallel arrays with a few binucleate forms; (B) Tissue section showing tumor with trabecular pattern, intervening areas of hyalinization, and focal binucleate cells.

The presence of RET/PTC1 translocations²¹¹ suggests that it may be a variant of PC, but BRAF and ras mutations are absent. Strong peripheral cytoplasmic and membranous MIB-1 staining is present.²¹² Cases of malignant hyalinizing trabecular tumor have been recorded. However, since most reported cases behaved in a benign fashion, it is considered a benign neoplasm or one of extremely low malignant potential.²¹¹

Medullary carcinoma (Figs 6.49-6.57)^90,213-222

Fig. 6.49 Medullary carcinoma

(A) Poorly cohesive cells with a ‘plasmacytoid’ appearance and moderate anisokaryosis, fragments of magenta-colored amyloid; (B) Cellular smear from another case; dispersed epithelial cells, some lined up in curved rows; anisokaryosis, many cells with two or more nuclei; stippled chromatin; a few tiny specks of amyloid (MGG, HP).

Fig. 6.50 Medullary carcinoma

Dispersed cells with a ‘plasmacytoid’ appearance and several binucleate forms; dark nuclei (Pap, HP).

Fig. 6.51 Medullary carcinoma

(A) ‘Plasmacytoid’ cells with abundant well-defined cytoplasm and eccentric nuclei; note clump of magenta-colored amyloid (MGG, HP); (B) Binucleate tumor cell with coarse, red intracytoplasmic granules (MGG, HP oil).

Fig. 6.52 Medullary carcinoma, small-cell pattern

Densely clustered cells with ovoid crowded hyperchromatic nuclei; nuclear molding (MGG, HP).

Fig. 6.53 Medullary carcinoma, amyloid

(A) Clumps of hyaline material staining a magenta color with MGG; (B) Same in Pap staining pale grayish-green to orange (HP oil).

Fig. 6.54 Medullary carcinoma, spindle cell variant

(A) Dispersed and clustered spindle cells with elongated nuclei mimicking a mesenchymal neoplasm (MGG, IP); (B) Spindle cells with speckled nuclear chromatin (Pap, HP).

Fig. 6.55 Medullary carcinoma

Smear showing intranuclear inclusion in one cell (MGG, HP).

Fig. 6.56 Medullary carcinoma, giant cell variant

Large pleomorphic cells resembling anaplastic carcinoma. The cells were strongly positive for calcitonin (MGG, HP).

Fig. 6.57 Mixed medullary and follicular carcinoma

(A) A cellular smear of dispersed cells consistent with medullary carcinoma but with some distinctly microfollicular structures (lower right) (MGG, HP); (B) Microfollicle highlighted by positive thyroglobulin staining; (C) Most cells are calcitonin positive; some groups negative (immunoperoxidase, same case).

Medullary carcinomas (MCs) constitute 5–10% of thyroid cancers²¹⁶ and are familial or sporadic with RET proto-oncogene mutations implicated in the former.²¹⁷ They are solid, firm, non-encapsulated tumors that show calcitonin-positive, round, polygonal and/or spindle cells in solid, nest-like and organoid patterns in a vascular stroma containing amyloid.⁴²

Smears in MC are cellular in a background of blood. The cell pattern is predominantly dissociated with round, oval, polygonal and spindle cells in varying combinations. Plasmacytoid cells or triangular cells with eccentric nuclei are common and they show moderate amounts of cytoplasm with well-defined cell margins (Figs 6.49 A, 6.50 and 6.51 A). The small cell type shows scanty cytoplasm, high nuclear cytoplasmic ratio and ovoid nuclei, forming dense clusters, often with nuclear molding (Fig. 6.52). The spindle cell pattern shows cells with elongated, relatively pale nuclei and indistinct attenuated cytoplasm, resembling benign or low-grade spindle cell soft tissue tumors (Fig. 6.54). Nuclear chromatin may be fine and stippled (neuroendocrine-like) or coarsely granular. INCIs have been noted in occasional single cells in up to 55% of cases (Fig. 6.55).^42,90,218

Anisocytosis is frequent with binucleate, trinucleate and multinucleate forms. Appearances may range from monomorphic (carcinoid-like) to markedly pleomorphic, mimicking AC.²¹³ Follicular or pseudopapillary patterns may be present.

Some of the cells show coarse reddish cytoplasmic granules (Fig. 6.51B) in Diff-Quik and MGG-stained smears, corresponding to neurosecretory granules seen on electron microscopy. Calcitonin is prominently present in the granular cells. Amyloid, present in 50–80% of cytological material from MC, ⁴² appears as dense amorphous clumps, staining variable shades of magenta with MGG and grayish-orange with PAP (Fig. 6.53). Cracking artifacts may be present. Congo-red staining and dichroism confirm the material as amyloid (which may otherwise be confused with hyaline connective tissue, condensed colloid or stroma of hyalinizing trabecular tumor).

Cytologic features suggestive of secretory activity such as cytoplasmic vacuoles, azurophilic granules, marginal vacuoles, and intracytoplasmic lumina with secretions have been reported.²¹⁸ Amyloid-like, intracellular and extracellular material was present that stained with calcitonin, as also the intracytoplasmic lumina. In cytologic smears, calcitonin-positive secretory material appeared to be diffusing out of cells.

Immunocalcitonin staining supports and greatly improves the accuracy of cytological diagnosis in MC.^214,215 The stain works well in air-dried, de-stained and wet-fixed smears and cell block preparations. If calcitonin staining is weak or equivocal, staining for chromogranin and/or carcinoembryonic antigen can be done. Electron microscopic study for demonstration of neurosecretory granules has been largely replaced by immunocytochemistry.

Papillary, oncocytic, giant cell, small cell, mucinous, squamoid, pigmented and clear cell variants of MC have been described. The small cell variant (Fig. 6.52) is difficult to distinguish from poorly differentiated carcinoma, metastatic small cell carcinoma and malignant lymphoma, the spindle cell variant (Fig. 6.54) from spindle cell mesenchymal lesions and melanoma and the giant cell type (Fig. 6.56) from AC.

Giant cells in MC usually show variation in cell size but not of cell type, as in anaplastic giant cell carcinoma, and bizarre mitoses are uncommon.²¹³ The oncocytic variant resembles HCT, but cytoplasmic granules in MC are reddish and macronucleoli not as common as in HCT.⁴² The papillary variant may show true papillae, INCIs and psammoma bodies. PC cells, however, show dense, non-granular cytoplasm. Definitive diagnosis requires ancillary stains/techniques. The small cell variant may be calcitonin-negative and positive for keratin and pan-endocrine markers; thus detailed clinical work-up is required to rule out metastatic small cell carcinoma.

Mixed MC and FC (Fig. 6.57)^219A can be confused with microfollicular pattern in MC, especially if amyloid has been mistaken for colloid. The dual cell population can be highlighted by staining for calcitonin and thyroglobulin (Fig. 6.57B,C) but the possibility of residual follicles being trapped by tumor may be difficult to rule out.

Cytomorphologic overlap and distinction from hyalinising trabecular tumor have been discussed.

Cytological appearance in paraganglioma resembles MC. Clinical and radiological findings may not be of use in rare cases of intrathyroidal paraganglioma.⁴² Besides, a paraganglioma-like variant of MC has been described⁴² with FNA cytological appearances.²²¹ Immunoreactivity to a panel of pan-endocrine markers as well as calcitonin and cytokeratin are required to distinguish this from paraganglioma (possible only on cell blocks).

Amyloid may be seen in smears from amyloid goiters as well as in plasmacytoma of thyroid.⁴² Attention to the morphology of the accompanying cells facilitates the diagnosis.

Anaplastic carcinoma (Figs 6.58-6.63)^{42,67,213,223-225}

Fig. 6.58 Anaplastic giant cell carcinoma

Giant mononuclear and multinucleated tumor cells (MGG, HP).

Fig. 6.59 Anaplastic spindle cell carcinoma

Cluster of pleomorphic plump spindle cells resembling malignant fibrous histiocytoma; note intranuclear vacuoles (MGG, HP).

Fig. 6.60 Anaplastic carcinoma, squamous variant

Smear showing keratinizing malignant cells in a necrotic background (PAP, HP).

Fig. 6.61 Spindle and giant cell carcinoma

Histiocyte-like giant cells; fibroblastoid spindle cells (Pap, HP).

Fig. 6.62 Anaplastic carcinoma with osteoclast-like giant cells

Malignant cells of spindle cell type and a multinucleated histiocytic osteoclast-like giant cell (HE, HP).

Fig. 6.63 Anaplastic carcinoma, inflammatory type

Many clustering polymorphs obscuring scattered single malignant epithelial cells (Pap, HP).

The most aggressive of all thyroid cancers, AC presents usually in women above 60 years of age as a rapidly growing, locally infiltrative, hard mass, associated with compression signs. Three major patterns or combinations of these patterns are seen, namely giant cell, spindle cell and squamoid patterns with large intervening areas of necrosis and hemorrhage.

Smears from AC may be cellular or paucicellular, depending on the amount of necrosis and the site of sampling. A malignant background diathesis of necrotic material and neutrophils is often present. Cells are extremely variable in shape and a mixture of spindle and giant cells are seen in about half of the cases. Cells show bizarre nuclei with macronucleoli, irregular nuclear membranes and coarsely clumped chromatin. INCIs may be present (Fig 6.59) in a fairly high proportion of cells, as in PC. Mitoses are numerous with abnormal forms. Keratinizing squamous cells may be present and are the dominating population in the squamous variant (Fig 6.60). Osteoclastic giant cells (Fig 6.62), chondroblastomatous, or osteoblastic differentiation may be seen.^42,226,227 Bizarre cell morphology, the presence of necrosis and mitotic activity and a clinical history of a rapidly enlarging mass clinch the diagnosis. This in turn permits palliative treatment without operative intervention. Smears may show evidence of residual differentiated papillary or follicular cancer. Previous reports of small cell type of AC are in all probability examples of small cell type of MC, lymphoma or poorly differentiated thyroid carcinoma.

Necrosis, fibrosis and inflammation may dominate the cytological picture, obscuring diagnostic cells (Fig 6.63). Diagnostic reliability in AC is limited mainly by the sampling technique. The false-negative cytologic rate can be as high as 12% and is mainly due to non-representative sampling.⁴² In order to ensure adequate sampling, large tumor masses should be subjected to at least 3–4 needle passes at different sites, concentrating on firm areas of the lesion. Paucicellular variants of AC are difficult to diagnose on cytology²²⁵ and may mimic Riedel’s thyroiditis.⁴²

The spindle cell variant of AC (Fig. 6.59) may be indistinguishable from spindle cell sarcomas. Immunostaining for cytokeratin, if positive, supports a diagnosis of AC. However, cytokeratin positivity is not always present and it may be focal (personal observation). Lack of thyroglobulin immunostaining helps distinguish it from poorly differentiated foci of differentiated thyroid carcinoma.

AC may contain remnants of pre-existing follicular or papillary neoplasms.⁴² If the needle has only sampled the differentiated component, the diagnosis of AC can be missed. Presence of INCIs in AC (Fig. 6.59) may cause confusion with high-grade PC. Grossly abnormal nuclear chromatin, tumor necrosis, mitotic activity and loss of expression of thyroglobulin suggest AC. MC of giant cell type may resemble AC cytologically; immunostaining for calcitonin and CEA are required.

Laryngeal carcinoma can occasionally extend and infiltrate the thyroid. If smears show a dominant squamoid pattern, a laryngoscopic examination is recommended to rule out a primary laryngeal carcinoma.

Giant cell carcinomas of pancreas and lung,⁹⁰ sarcomatoid renal cell carcinoma and spindle cell melanomas may metastasize to thyroid and simulate giant cell or spindle cell types of AC, respectively. The value of correlating the cytologic picture with the clinical profile and relevant investigations cannot be overestimated.

Mitotically active, atypical reactive myofibroblasts are occasionally seen in thyroiditis and NG.⁹⁰ Follicular cells undergoing repair may show nuclear aytpia with macronucleoli. Radioactive iodine and neomercazole treatment for GD leads to follicular atypia (see Fig. 6.8B). Careful history taking and thorough clinical assessment reduce diagnostic errors in most cases.

Lymphoma (Figs 6.64, 6.65)^90,228,229

Lymphoma may involve the thyroid secondarily. Primary thyroid lymphoma is rare (1–5% of thyroid malignancies).²³⁰ Most are of B-cell lineage, of MALT type and often occur in a background of HT.⁴²

Fig. 6.64 Malignant lymphoma, high grade

Highly cellular smear of predominantly large blastic lymphoid cells; note cluster of pale residual follicular epithelial cells (MGG, IP).

Fig. 6.65 Malignant lymphoma, low grade, in Hashimoto’s disease

The two figures are from the same case. (A) Mixed population of abnormal lymphoid cells, the majority are centrocyte-like; (B) Thyroiditis pattern of mixed lymphoid cells and a cluster of oxyphil epithelial cells (MGG, IP).

High-grade lymphomas show a dispersed population of predominantly large abnormal lymphoid cells of blastic type (Fig. 6.64). Patients are usually elderly females presenting with rapid thyroid enlargement and pressure symptoms, clinically mimicking AC. Low-grade lymphomas, especially with coexistent HT, present a diagnostic problem, as discussed earlier. A mixed cell population including plasma cells, suggestive of a florid reactive process, may be seen in smears of low-grade lymphoma. Diagnostic difficulties are enhanced by the presence of residual reactive follicles in low-grade MALT lymphoma. Primary thyroid involvement by Hodgkin’s disease is rare but has been documented.⁴²

Distinction of low-grade lymphomas from HT is difficult and requires flow cytometry and immunocytochemical demonstration of monoclonality.²²⁸ Situations where both diseases coexist are further complicated by sampling problems. Selective needling of the recently enlarging nodule often yields diagnostic samples. General principles guiding the distinction of non-Hodgkin’s lymphoma from reactive lymphoid infiltrates are covered in Chapter 5.

Apart from the rare small cell variant of MC, true anaplastic small cell carcinoma is extremely rare. Round fragments of pale blue cytoplasm in the background (lymphoglandular bodies) favor lymphoma, while cell clustering, nuclear molding and tear-drop cells favor small cell AC. Metastatic small cell carcinoma should be excluded. Immunostaining is indispensable.

Langerhans cell histiocytosis rarely involves the thyroid as a part of multiorgan disease²³¹ and may be mistaken cytologically for lymphoma. Admixture with eosinophils and multinucleate giant cells, indentation of nuclei, dendrite-like cytoplasmic processes and positive staining with S-100 protein are clues to the diagnosis.

Uncommon malignancies of thyroid

Mucoepidermoid carcinoma of thyroid can be rarely encountered on FNA where it is usually diagnosed as poorly differentiated carcinoma.^232,233 Brief reports of the cytology of angiosarcoma and Kaposi’s sarcoma of thyroid, hemangioendothelioma, osteosarcoma, and metastatic sarcoma are present in the literature.^42,234-236 Teratomas^237,238 occur rarely in the thyroid and are mainly seen in infants or children where they are usually benign.⁴² A few teratomas reported in adults were malignant, with the tumor showing a prominent primitive neuroepithelial component. FNA smears in an aggressive and rapidly fatal tumor in a young female²³⁸ showed features suggestive of primitive neuroectodermal tumor with frequent neuroblastoma-like rosettes. Histology of the resected tumor showed neural tubules and neurites, squamous and glandular epithelium with nodal metastasis showing a neuroepithelial pattern.

Metastatic malignancies (Figs 6.29, 6.66 and 6.67)⁴²

Metastasis to the thyroid is usually a terminal event in disseminated malignancies, questioning the rationale of conventional treatment. Cytology is very useful in determining whether the lesion in a known case of cancer is a metastasis, a thyroid primary or a non-neoplastic thyroid lesion. Rarely, thyroid metastases may be the presenting feature of an otherwise occult primary (such as renal cell carcinoma).⁴² A metastatic tumor can clinically simulate a primary neoplasm or even thyroiditis.¹¹² Lung, gastrointestinal tract (Fig. 6.66), breast (see Fig. 6.29), kidney, melanoma and lymphoma are the most frequent sites of origin. Breast carcinoma may resemble and be misinterpreted as poorly differentiated follicular carcinoma of thyroid (Fig. 6.67), and the importance of clinical notes cannot be overemphasized. When cytological features are at variance with those seen in primary thyroid cancers, it is a good policy to meticulously go through the clinical notes for any record of other previous primary cancers and to request cytology and histopathology slides of the case for review.

Fig. 6.66 Metastatic carcinoma from large bowel

Note microglandular structures, columnar cells and necrotic debris (MGG, HP).

Fig. 6.67 Metastatic breast and thyroid carcinoma, overlapping cytological features

(A) Smear from breast carcinoma metastatic to thyroid (MGG, HP); (B) Smear from follicular carcinoma metastatic to clavicle (MGG, HP).

Based on cytomorphologic pattern the following possibilities may be considered and clinical details and relevant ancillary stains used for their distinction:

Clear cell morphology: Metastasis from renal cell carcinoma, salivary gland carcinoma, clear cell melanoma, thyroid and parathyroid tumors showing clear cell change.

Papillary morphology: PC of thyroid, breast carcinoma, rare cases of melanoma.²³⁹

Oncocytic cells: HCT, oncocytic variants of PC and MC, renal cell carcinoma, salivary gland carcinoma.

Small round cells: Lymphoma, small cell variant of MC, poorly differentiated thyroid carcinoma, metastatic small cell carcinoma, malignant teratoma.

Mucin producing cells: Colonic and lung carcinomas, salivary gland carcinoma, rare mucin-producing variant of MC.

Squamous morphology: AC, laryngeal and lung carcinoma.

Spindle cell morphology: MC, AC, primary and metastatic spindle cell sarcomas, melanoma, sarcomatoid renal, pancreatic and lung carcinomas, atypical reactive myofibroblasts.

Pleomorphic multinucleate giant cells: AC, metastatic pleomorphic sarcomas and giant cell carcinomas of pancreas and lung.

Parathyroid neoplasms (Figs 6.68 and 6.69)^136,240-242

Parathyroid neoplasms and cysts have been discussed briefly in the earlier section. Cytology is of limited value in the preoperative assessment of patients with hyperparathyroidism. If a parathyroid adenoma cannot be located by surgery, cytology in conjunction with US examination of the neck or mediastinal CT can be tried. High cellularity, aggregates of cohesive cells with small nuclei 6–8 µm in diameter, coarse granular chromatin, some larger nuclei 10–30 µm, numerous bare nuclei, scattered oxyphil cells and absence of colloid or macrophages indicate parathyroid tissue.^240,242,244 Papillae and INCIs (Fig 6.69) have been noted.^243,245 Monolayered sheets and microfollicular structures suggest thyroid origin,²⁴³ but have been seen in parathyroid adenoma (Fig. 6.68); it is agreed that no single cytologic feature can definitely distinguish parathyroid from thyroid follicular nodules.

Fig. 6.68 Parathyroid adenoma

Microfollicular pattern but no colloid; small nuclei, pale fragile cytoplasm, many naked nuclei (Pap, HP).

Fig. 6.69 Parathyroid adenoma

(A) Chief cell type; dispersed cells with small round nuclei, moderate anisokaryosis, most nuclei stripped; (B) Oxyphil parathyroid adenoma; more prominent anisokaryosis, abundant but poorly defined cytoplasm; intranuclear cytoplasmic inclusions (MGG; A, IP; B, HP).