20 ANALYSIS: SOLITARY PULMONARY NODULE

A COMMON SCENARIO

Solitary pulmonary nodule (SPN):

“to qualify as a nodule or cyst, a pulmonary lesion must present a reasonably sharp outline. Hence, a focal infiltrate with ill-defined borders is excluded…. I have set no upper limit of size for a pulmonary nodule, though I usually call the big ones a mass”1.

“a focal round or oval area of increased opacity in the lung that measures less than 3.0 cm in diameter”2.

A COMMON SCENARIO

An otherwise generally well patient has a CXR (e.g. for high blood pressure). A SPN is an unexpected finding.

Frequently, the clinical history will make a particular diagnosis highly likely. Examples: a man of 65 who has smoked two packs a day for 40 years—the default diagnosis will be a primary bronchial carcinoma. A history of carcinoma of the breast/kidney/colon some years previously will make a secondary deposit likely.

On the other hand there may be no clue from the history. In this case there are three steps:

1. Remind oneself of the causes of a SPN (Table 20.1).

2. Clinical assessment.

3. CXR analysis. This includes a search for CXR features which indicate a benign lesion and makes a CT examination unnecessary (Table 20.3).

Table 20.1 Causes of a SPN.

Most common	Benign granuloma (previous infection)
Less common	Bronchial carcinoma Metastasis	Organising pneumonia Hamartoma
Much less common	Mucoid impaction Abscess Infected bulla Infarct Haematoma	Rheumatoid nodule Wegener’s granuloma Carcinoid Sarcoid granuloma
Very rare	Arterio-venous malformation Intrapulmonary lipoma	Amyloid Hydatid cyst

Table 20.3 SPN: benign characteristics.

CXR feature	Comment
Size	1. Almost all benign nodules are less than 3 cm in diameter. 2. A nodule of less than 1 cm in diameter is very difficult to visualise⁷. Apply this maxim: “this nodule is less than 1 cm in diameter but I can see it very well indeed. This means that it is almost certainly calcified and thus it will be benign.”
Shrinking	Rapid reduction in size in days or weeks on interval CXRs — the lesion is benign (often a resolving infection).
Calcification	Benign if central, shaggy, laminated, popcorn, or stippled.
Presence of Branching	Tubular branching leading up to the nodule suggests: an arterio-venous malformation, or a pulmonary venous varix, or mucoid impaction within a bronchus
Stable	No change over two years is a strong feature suggesting a benign lesion⁸. Not an absolute guarantor^5,⁶ but indicates a high probability.

Figure 20.1 A SPN is a common finding on adult CXRs. Approximately 60–70% are benign. Some 30–40% are malignant²^-⁴.

CAUSES OF A SPN

A solitary pulmonary nodule is a very common finding on a CXR in adults. The vast majority are benign lesions (Table 20.1), usually old granulomas²^-⁴.

CLINICAL ASSESSMENT

CLINICAL DETAILS ARE CRUCIAL

Is the clinical history helpful? Sometimes a recent low grade chest infection may result in a residual area of pneumonia. A history of asthma will raise the possibility of mucoid impaction. Perhaps there has been a recent injury to the chest, e.g. a haematoma.

Is the clinical examination abnormal? Is there a mole or skin tag on the chest wall? A breast lump, an abdominal mass or an enlarged liver would raise the likelihood of malignancy.

Table 20.2 Traps: simulated pulmonary nodules.

Overlap of normal pulmonary vessels

Healing rib fracture

Rib density—benign

Rib density—malignant

Cartilage calcification — first rib

Pleural plaque

Encysted pleural fluid

Electrocardiogram electrode pad

Nipple shadow

Skin excrescence/mole

Clothing artefact

STEP-BY-STEP CXR ANALYSIS

Step 1:	Exclude any of the common traps (Table 20.2). A nodule may not be in the lung. A lateral CXR will confirm (or exclude) the lesion as intrapulmonary.
Step 2:	Apply this rule: “the best next step is comparison with a previous CXR”. This will often remove the need for any further investigation. If a CXR two or more years prior shows that the lesion has not changed in size…the SPN can be assumed to be benign, usually a benign granuloma. This is not a 100% guarantee^5,⁶. Nevertheless, it is a highly reliable rule of thumb. Lack of growth implies a long doubling time (see opposite) which generally indicates a benign histology. This criterion has even greater power if earlier images (i.e. beyond two years) are also available for comparison. If the previous CXR was clear… the SPN is an active lesion. It is highly likely to be malignant or inflammatory.
Step 3:	No previous CXR available: look for benign features (Table 20.3).
Step 4:	No previous CXR available: look for malignant features (Table 20.4).
Step 5:	Is CT of the thorax necessary? Table 20.6 provides guidelines.

Doubling time—a summary^5,⁹^-¹²

The growth rate of a lesion is often expressed in terms of doubling time (i.e. the time in which a nodule doubles in volume¹²). It has been estimated that a nodule goes through some 30 doublings before it reaches 1 cm in diameter and becomes readily detectable on a CXR¹³. Bronchial carcinomas usually take 1–18 months to double in size.

Table 20.4 SPN: malignant characteristics.

CXR feature	Comment
Size	Diameter of more than 3 cm is very suggestive of malignancy.
Margin	Ill-defined or spiculated border is a strong pointer towards malignancy.
Strands	Radiating strands at the margin — strong probability of a primary carcinoma… but not an absolute certainty.
Calcification position^9,¹⁰	Eccentric calcification raises the suspicion of a scar carcinoma.

Rule of thumb: A doubling time of less than one month, or alternatively more than 18 months, suggests benign disease. This maxim is a helpful guideline, but there are occasional exceptions.

Figure 20.2 (a) Central calcification in a SPN is usually a benign feature. (b) Eccentric calcification raises the suspicion of a carcinoma arising in an old scar.

Figure 20.3 SPN at the apex of the left lung. The margin is slightly irregular and ill-defined. These two features are suggestive of a primary bronchial carcinoma.

Figure 20.4 SPNs. In (a) the margin is irregular. In (b) a tail sign is present. A “tail” is a strand-like or linear projection of tissue extending outwards from the margin of a nodule. Either (a) or (b) should raise the strong suspicion of malignancy.

Figure 20.5 The right upper zone nodule has a cavity within it. Some benign lesions do cavitate, e.g. abscess or rheumatoid nodule. All the same, in the appropriate clinical setting, cavitation should always suggest the probability of a squamous carcinoma. Subsequently confirmed squamous carcinoma of the bronchus.

Table 20.5 SPN: unreliable characteristics… the indeterminate nodule.

CXR feature	Comment
Smooth margin	This is common with benign disease. Nevertheless, some primary carcinomas do have smooth margins. A metastasis usually has a smooth margin.
Lobulated outline	May be present in benign as well as in malignant lesions.
The tail sign	Can occur in benign and malignant lesions¹¹ (Fig. 20.4).
Cavitation	Abscesses cavitate. Squamous cell carcinomas cavitate.
Doubling time/rate of growth	Some granulomas grow slowly. Some primary carcinomas grow very slowly… others grow very rapidly.

Table 20.6 Is CT necessary?

CXR analysis	Feature	CT indicated?
Step 1	An artefact is confirmed (Table 20.2)	No
Step 2	Lesion unchanged over the previous two years	No (very occasionally, the clinical context—age, smoking history— will justify CT)
Step 2	Lesion has increased in size over the previous two years	Yes
Step 3	Classic benign features (Table 20.3)	No
Step 4	Classic malignant features (Table 20.4)	Yes
Step 4	Indeterminate features (Table 20.5)	Yes

AN INTERESTING LESION—PULMONARY HAMARTOMA^2,¹⁰

Pathology

Hamartomas are benign tumours found in various organs. All hamartomas contain cells of some of the tissues within the organ of origin. A pulmonary hamartoma invariably contains cartilage and bronchial epithelium; it often contains fat.

Clinical features

Usually an incidental, asymptomatic finding. Tuberous sclerosis is a rare association.

The CXR

90% are situated peripherally in the lung. Very occasionally it is endobronchial.

Solitary.

Most are less than 3 cm in diameter. Exceptionally—may be as large as 10 cm.

Margins sharp and well-defined.

10–20% contain calcification which may be stippled or popcorn shaped (Fig. 20.6)

Interval CXRs—slow growth does occur.

NB: fat within a SPN on CT is pathognomic of a hamartoma.

Figure 20.6 Some hamartomas contain visible calcification. It is often stippled or popcorn shaped. This figure shows the appearance of popcorn calcification. Note the similarity to a popped kernel of corn¹.

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