Hematologic and serum biochemical features

A variety of nonspecific hematologic and serum biochemical abnormalities can be detected in cats and dogs with lymphoma. The hematologic abnormalities result from the infiltration of bone marrow with neoplastic cells, splenic hypofunction or hyperfunction (caused by neoplastic infiltrates), chronic disease, or paraneoplastic immune-mediated abnormalities (i.e., immune hemolytic anemia or thrombocytopenia, both of which are extremely rare). Certain hematologic abnormalities (i.e., monocytosis, leukemoid reactions) may result from the local or systemic production of bioactive substances by the tumor cells (e.g., hematopoietic growth factors, interleukins). The serum biochemical abnormalities result either from the production of bioactive substances by the tumor cells (i.e., paraneoplasia) or from organ failure secondary to neoplastic infiltration. In general, the complete blood count (CBC) and biochemical profile are not diagnostic in cats and dogs with lymphoma.

Common hematologic abnormalities include nonregenerative anemia, leukocytosis, neutrophilia (with or without a left shift), monocytosis, abnormal lymphoid cells in peripheral blood (i.e., lymphosarcoma cell leukemia), thrombocytopenia, isolated or combined cytopenias, and leukoerythroblastic reactions, among others. Lymphocytosis is rare in dogs and cats with lymphoma; when present, it is usually of low magnitude (i.e., <10,000 to 12,000/μl).

Serum biochemical abnormalities are more common in dogs than in cats with lymphoma and consist mainly of hypercalcemia and gammopathies. Hypercalcemia is one of the most common paraneoplastic abnormalities in dogs with lymphoma, occurring in approximately 20% to 40% of the patients; it is extremely rare in cats, and it is more prevalent in dogs with mediastinal lymphoma than in those with the multicentric, alimentary, or extranodal forms. In most dogs with lymphoma and hypercalcemia, the tumor is of T-cell origin.

There are numerous molecular mechanism underlying hypercalcemia in dogs with lymphoma, but in most cases hypercalcemia is thought to occur as a result of the production of a parathormone-like protein, called PTHrp (PTH-related protein), by the neoplastic cells. Markedly increased serum concentrations of 1,25-vitamin D have been documented in human patients with lymphoma and hypercalcemia. We have recently recognized a similar condition in dogs with lymphoma and hypercalcemia (most of the dogs were Boxers with mediastinal T-cell lymphoma).

Hyperproteinemia is another paraneoplastic abnormality that rarely occurs in cats and dogs with lymphoma. It may be secondary to the production of a monoclonal protein by the lymphoma cells and can result in the development of hyperviscosity syndromes. Polyclonal gammopathies may also be present in cats and dogs with lymphoma.

Imaging

Radiographic abnormalities in cats and dogs with lymphoma vary with the different anatomic forms but in general are secondary to lymphadenopathy or organomegaly (i.e., hepatomegaly, splenomegaly, renomegaly); occasionally, the infiltration of other organs (e.g., lungs) may lead to the appearance of additional radiographic abnormalities.

Radiographic changes in cats and dogs with multicentric lymphoma include sternal or tracheobronchial lymphadenopathy or both; interstitial, bronchoalveolar, or mixed pulmonary infiltrates; pleural effusion (rare); intraabdominal lymphadenopathy (e.g., mesenteric or iliac); hepatomegaly; splenomegaly; renomegaly; or intraabdominal masses. Rarely, lytic or proliferative bone lesions are identified on plain abdominal or thoracic radiographs.

In cats and dogs with mediastinal lymphoma, radiographic changes are usually limited to the finding of an anterior (or, more rarely, posterior) mediastinal mass, with or without pleural effusion. In cats and dogs with alimentary lymphoma, abnormalities are rarely detected on plain abdominal radiographs (<50%). When present, they vary in nature but include mainly hepatomegaly, splenomegaly, and midabdominal masses. Positive contrast–enhanced radiography of the upper gastrointestinal tract usually reveals abnormalities in most animals. In a series of dogs with alimentary lymphoma evaluated at our clinic, abnormalities were found in all dogs that underwent positive contrast–enhanced radiography of the upper gastrointestinal tract and included mucosal irregularities, luminal filling defects, and irregular thickening of the wall, suggestive of infiltrative mural disease.

Ultrasonography constitutes an invaluable tool for evaluating cats or dogs with suspected or confirmed intraabdominal lymphoma. The technique is also helpful in the evaluation of mediastinal masses in both species (see Chapter 79). Changes in the echogenicity of parenchymal organs (i.e., liver, spleen, kidneys) detected by this technique usually reflect changes in organ texture secondary to neoplastic infiltration. In addition, enlarged lymphoid structures or organs can easily be identified using this technique. Several abnormalities are commonly detected ultrasonographically in cats and dogs with intraabdominal lymphoma; these include hepatomegaly, splenomegaly, changes in the echogenicity of liver or spleen (mixed echogenicity or multiple hypoechoic areas), intestinal thickening, lymphadenopathy (Fig. 80-3), splenic masses, and effusion. In a study of 11 cats with alimentary lymphoma evaluated ultrasonographically at our clinic, we found hypoechoic masses of the gastric or intestinal wall, focal or diffuse gastric wall thickening, a symmetrical thickening of the intestinal wall, loss of the normal layered appearance of the gastrointestinal wall, and abdominal lymphadenopathy (Grooters et al., 1994). Fine-needle aspiration (FNA) and needle biopsy can also be easily performed using this technique to guide the placement of the needle.

FIG 80-3 Mesenteric lymphadenopathy in a 12-year-old cat with diarrhea associated with an intestinal small cell lymphoma. Note the marked enlargement of the affected lymph node (3 × 5 cm).

Induction of remission

As previously discussed, my protocol of choice for the induction of remission is COAP. The agents in this protocol consist of cyclophosphamide, vincristine, cytosine arabinoside, and prednisone; these four drugs are currently available as generic products. The dosages are specified in Box 80-1. These drugs belong to four different categories, have different mechanisms of action, and do not have superimposed toxicities (with the exception of cyclophosphamide and cytosine arabinoside, both of which are myelosuppressive; however, the latter is used only for a short period); thus they fulfill the basic criteria of multiple-agent chemotherapy described in Chapter 77. The cytosine arabinoside is usually administered by the subcutaneous (SC) route because, given its short half-life and S-phase–specific mechanism of action, an IV bolus injection results in minimal cell kill; SC administration of this drug is painful in cats (and in some dogs). IV infusion of the agent is also associated with myelosuppression. The induction phase lasts 6 to 8 weeks, and weekly visits to the veterinarian are necessary during this time.

During the induction phase toxicity is minimal (<15%) and client compliance is high because most of the toxic signs are hematologic (i.e., cytopenias) and usually do not result in clinical signs that can be detected by the owners. The dose-limiting toxicity of this induction protocol is hematologic (i.e., myelosuppression leading to neutropenia); the neutrophil nadir usually occurs around day 7 or 8 because two myelosuppressive agents (i.e., cyclophosphamide and cytosine arabinoside) are given during the initial 2 to 4 days of treatment. In most cases the neutropenia is mild (2000 to 3500 cells/μl). The neutropenia is severe if the animals have neoplastic bone marrow infiltration before the initiation of treatment, have FeLV- or FIV-associated myelodysplasia or other retrovirus-associated bone marrow disorders, or receive the cytosine arabinoside by constant-rate IV infusion rather than by the SC route. Also, anecdotally, neutropenia appears to be common in Cocker Spaniels receiving this protocol. Dosage adjustments in cats and dogs that develop neutropenia are described in Chapter 78. Gastrointestinal toxicity is minimal to nonexistent; however, cats receiving cyclophosphamide occasionally become anorectic. Consequently, this drug should be administered once every 3 weeks in cats (as opposed to every other day in dogs; see Box 80-1). If anorexia develops, treatment with cyproheptadine (Periactin; Merck Sharp & Dohme, West Point, Pa.), an antiserotonin compound, at a dosage of 1 to 2 mg per cat PO q8-12 hours is indicated. Hair loss is also minimal, and it occurs primarily in woolly-haired dogs (e.g., Poodle, Bichon Frise); cats (and some dogs) may shed their tactile hairs during treatment.

During this phase, owners are instructed to monitor their pet’s appetite and activity level, measure their lymph nodes (if superficial lymphadenopathy was present initially), and take their pet’s rectal temperature daily (pyrexia is usually secondary to neutropenia and bacteremia or sepsis). If pyrexia develops, owners are instructed to contact their veterinarian immediately so that their pet can undergo a complete physical examination and CBC (for additional information, see Chapter 78). Treatment with COAP results in CR within 1 to 14 days of the start of therapy in most animals (>85% in dogs, >70% in cats). This remission is usually maintained throughout the induction phase.

In dogs with diffuse alimentary lymphoma we use a more aggressive doxorubicin-containing protocol (CHOP; see Box 80-1) because, in my experience, the response rate to COAP is low. This protocol is more expensive and more likely to cause adverse effects than the COAP protocol. We typically use lomustine (CCNU) in dogs with epidermotropic T-cell lymphoma (see Box 80-1).

In dogs and cats with multicentric (or any other anatomic form of) lymphoma coexisting with neurologic signs, we usually use the COAP protocol but administer the cytosine arabinoside as a continuous IV infusion (200-400 mg/m² as an IV infusion over 24 hours for 1 to 4 days) in order to attain high concentrations of this drug in the CNS. This protocol tends to cause marked myelosuppression in cats, so we typically administer cytosine arabinoside as a 12- to 24-hour infusion (200 mg/m²) in this species. More information on the treatment of dogs and cats with suspected or confirmed CNS lymphoma is given later in this chapter.

Maintenance

The protocol recommended for the maintenance phase of treatment is LMP (“lump”), which consists of chlorambucil, methotrexate, and prednisone (see Box 80-1). These three drugs also act by three different mechanisms of action and have different toxicities. The advantages of this protocol include its reduced cost compared with the cost of the induction phase; its ease of administration (all the drugs are administered orally by the owners); its minimal toxicity; and the fact that intensive monitoring by a veterinarian is not necessary.

The toxicities associated with LMP maintenance chemotherapy are minimal. Of the three drugs in this protocol, methotrexate is the only one that is associated with moderate to severe toxicity. In approximately 25% of dogs and cats receiving methotrexate, gastrointestinal tract signs consisting of anorexia, vomiting, or diarrhea develop. Anorexia and vomiting are more common than diarrhea and usually occur after the patient has been receiving the drug for more than 2 weeks. In these cases treatment with an antiemetic, such as metoclopramide, on the days the animal receives the methotrexate, at a dosage of 0.1 to 0.3 mg/kg PO every 8 hours, alleviates or eliminates the upper gastrointestinal tract signs. We have recently used maropitant (Cerenia, Pfizer Animal Health, Kalamazoo, Mich.) at a dosage of 2 mg/kg PO every 24 hours to prevent chemotherapy-associated nausea and vomiting. Gastroprotectants, such as famotidine (0.5 mg/kg PO q24h) may also be effective in preventing or minimizing this adverse effect. In cases of methotrexate-associated diarrhea, treatment with a bismuth subsalicylate–containing product (Pepto-Bismol) may also alleviate or eliminate the signs; however, it may be necessary to discontinue the drug. Hematologic toxicity associated with LMP therapy is minimal to nonexistent. In a very small proportion of cats (i.e., <5%) receiving chlorambucil for weeks to months, serum biochemical abnormalities consistent with cholestasis that resolve on discontinuation of the drug may develop. Recently, tonic or tonic-clonic convulsions have been decribed in cats receiving chlorambucil.

During this phase the patient is examined every 6 to 8 weeks, at which time a complete physical examination and a CBC are performed. As with the induction protocols, owners are instructed to monitor their pet’s activity, appetite, behavior, rectal temperature, and lymph node size. As previously discussed, over the past few years we have been instructing the owners of pets with multicentric lymphoma to closely monitor the size of the lymph nodes in their pets; when the nodes start enlarging (i.e., relapse), a fourth drug is added to the LMP protocol (usually vincristine, at a dosage of 0.5-0.75 mg/m², IV, q1-2 weeks). This usually suffices to reinduce remission and maintain it for several weeks or months.

Most animals treated with this protocol remain in remission for approximately 3 to 6 months. If a relapse occurs, reinduction of remission (as discussed next) is instituted. After remission is reinduced, animals can be treated with a modified maintenance protocol, as described in previous paragraphs.

Reinduction of remission or rescue

Virtually every dog and cat with lymphoma treated with induction followed by maintenance chemotherapy eventually relapses; this generally occurs 3 to 6 months after the start of induction therapy (median: approximately 4 months), but it can occur within weeks of starting the maintenance phase or years after the original diagnosis was made. At this time, reinduction of remission is indicated. In my experience, remission can be reinduced one to four additional times in most dogs with relapsing lymphoma. Reinduction of remission is usually not as successful in cats as in dogs (i.e., remission cannot be reinduced in most cats with relapsing lymphoma). Therefore the following discussion on “rescue” pertains mostly to dogs with lymphoma.

There are numerous “rescue” protocols described in the literature, and as a general rule, the practitioner may have difficulty deciding what protocol to choose. We currently use the D-MAC protocol (see Box 80-1), which consists of dexamethasone, melphalan (Alkeran; Burroughs Wellcome, Research Triangle Park, N.C.), cytosine arabinoside (Cytosar-U), and actinomycin D (Cosmegen; Merck Sharp & Dohme, West Point, Pa.) as our trump card for rescue (Alvarez et al., 2006). This protocol results in an over 70% remission rate in dogs with relapsing lymphoma; it has a relatively low toxicity compared with that of doxorubicin-containing protocols, and it is necessary for the owner to go the veterinarian only once every 2 weeks (instead of every week). The median duration of remission using the D-MAC protocol was 61 days (range 2 to 467+ days). Previous use of doxorubicin and failure to induce remission with the induction protocol were negative prognostic factors for response to this protocol. Thrombocytopenia occurred in 56% of the dogs, neutropenia in 17%, and gastrointestinal toxicity in 22%; three of the 56 dogs required hospitalization because of toxicity. Because the long-term use of melphalan is associated with severe chronic thrombocytopenia, chlorambucil (Leukeran), 20 mg/m², is substituted for melphalan after four cycles. If complete or partial remissions are achieved after the administration of four to six cycles of D-MAC, the patient can be started on a maintenance protocol again.

If the response to D-MAC is poor (i.e., the disease progresses), the CHOP protocol is recommended (see Box 80-1). Our protocol calls for two or three cycles of CHOP once the tumor has relapsed; if CR is obtained, the patient is started on maintenance chemotherapy at the end of the second or third CHOP cycle. The maintenance protocol in these animals also includes LMP, with the possible addition of vincristine (0.5 to 0.75 mg/m² IV once weekly to every other week, alternating weeks with the chlorambucil) or cytosine arabinoside (200 to 400 mg/m² subcutaneously every other week, alternating weeks with the chlorambucil).

After a second relapse occurs, D-MAC or CHOP is administered for two additional cycles, as described in the preceding paragraph. In our experience, after the second and third relapses, the percentage of animals in which remission can be easily reinduced decreases with each subsequent cycle. This likely stems from the development of multiple-drug resistance by the tumor cells. Other protocols that have been successful in reinducing remission in dogs with lymphoma are listed in Box 80-1. Although the probability of reinducing remission is considerably lower in cats than in dogs, one of the protocols listed in Box 80-1 can be used for this purpose.

In cats doxorubicin- or mitoxantrone-containing protocols have been used with some degree of success (see Box 80-1); asparaginase-containing protocols may also be used but generally are not as effective as in dogs.

Intensification

If a dog is undergoing induction therapy but only partial remission (PR) is obtained, intensification with one or two doses of l-Asparaginase (Elspar; 10,000 to 20,000 IU/m² IM, repeated once at a 2- to 3-week interval) may be indicated. This drug can rapidly induce CR in most dogs with lymphoma that have shown only PR while receiving COP-based protocols. Asparaginase should not be used in dogs with a history of pancreatitis or in those that are at high risk for acute pancreatitis (i.e., obese, middle-age female dogs). In my experience, l-Asparaginase appears to be less effective in cats than in dogs; doxorubicin (1 mg/kg IV q3 weeks) or mitoxantrone (4 to 6 mg/m² IV q3 weeks; Novantrone; Lederle, Wayne, N.J.) can be used as intensifying agents in cats. In a recent study only two of thirteen (15%) cats with lymphoma treated with l-Asparaginase underwent CR, and two of thirteen (15%) underwent PR; these response rates are quite a bit lower than those reported in dogs (i.e., >70%) (LeBlanc et al., 2007).

Management of solitary and extranodal lymphomas

The clinician faces a dilemma when confronted with a dog or cat with a solitary lymphoma, regardless of whether it is nodal (i.e., stage Ia disease) or extranodal (i.e., a solitary cutaneous or oral mass). Should the mass (or lymph node) be treated in the same manner as other solitary malignancies (i.e., by wide surgical excision)? Should the patient be treated primarily with chemotherapy? Should the patient be treated with a combination of surgery, irradiation, and chemotherapy? Unfortunately, there are no correct answers to these questions.

In my experience, seemingly solitary lymphomas become (or already are) systemic in most animals. Exceptions include some oral and some cutaneous solitary T-cell lymphomas. Although cures have been achieved through the surgical excision or irradiation of solitary lymphomas, they are extremely rare. Therefore it is important not to underestimate the malignant behavior of this neoplasm by treating the patient only with a local treatment modality, such as surgery or radiotherapy. The following guidelines can be used in this subset of patients:

Radiotherapy constitutes an excellent treatment modality for dogs and cats with solitary lymphomas because lymphoma cells are extremely radiosensitive. Marked responses (CR or PR) are seen within hours or days of the start of such treatment. Different sources and protocols have been used in cats and dogs with lymphoma, but in general 3 to 5 Gy (300 to 500 rad) per fraction is delivered daily or thrice weekly for a total of six to ten fractions (total dose, 30 to 50 Gy [3000 to 5000 rad]). We have successfully used coarse fractionation radiotherapy (7 Gy once a week for 4 treatments) followed by maintenance chemotherapy (discussed later) in dogs with solitary oral T-cell lymphomas. Special settings in which radiotherapy is beneficial include CNS lymphomas (see following paragraphs) and upper airway lymphomas that cause respiratory compromise.

Another decision the clinician must make if chemotherapy is to be used is which protocol to use and for how long. There are also no specific guidelines for this. We use a standard induction chemotherapy protocol (COAP) in most cats and dogs with solitary lymphoma after they have undergone surgical excision or irradiation. After completion of the induction phase, the animals are treated with a maintenance protocol (LMP) and remission is reinduced as necessary (as in other forms of lymphoma). In our experience, early relapses occur in most animals treated with only maintenance chemotherapy protocols after the surgical excision of solitary lymphomas.

Central nervous system lymphoma

The treatment of choice for cats and dogs with primary or secondary epidural lymphoma is radiotherapy plus multiple-agent chemotherapy. If radiotherapy facilities are not available, multiple-agent chemotherapy is an effective alternative approach. It is my clinical impression that the surgical excision of such masses does not provide a significant advantage over chemotherapy alone or radiotherapy plus chemotherapy, given the fact that the latter two forms of treatment consistently induce rapid remissions (i.e., within 12 to 36 hours of the initiation of therapy). However, because surgery may be necessary to confirm the diagnosis, surgical excision of the mass is usually attempted at that time. If radiotherapy is available, three to five doses weekly of 3 to 4 Gy, to a total of 25 to 30 Gy, are indicated. The COAP protocol alone has been effective in inducing remission in cats with epidural lymphoma.

In cats and dogs with lymphoma of the neuropil (i.e., true CNS lymphoma), chemotherapy with or without radiotherapy is the preferred protocol. In animals in which it is possible to localize the lesion (i.e., by neurologic examination, CT, or MRI), radiotherapy should be used in conjunction with chemotherapy. If this is not possible, diffuse craniospinal irradiation can be performed.

Intrathecal chemotherapy can be used in cats and dogs with confirmed or highly likely neuropil lymphoma. The drug of choice is cytosine arabinoside (Cytosar-U) because it is almost nontoxic, it is inexpensive, and it is easy to administer. However, IV administration of this drug as a constant rate infusion (CRI) at dosages of 200 to 600 mg/m² over 24 to 72 hours achieves similar results and is our preferred approach. Responses to intrathecal or IV CRI cytosine arabinoside are usually quite spectacular. Dogs and cats that are tetraparetic, demented, or comatose usually regain normal neurologic status within 6 to 48 hours of receiving the first dose of this agent. In addition, disappearance of the neoplastic cells from the CSF can be documented within hours of the injection.

We frequently induce clinical and cytologic remission (i.e., normal neurologic status and disappearance of neoplastic cells from CSF) in cats and dogs with primary or secondary CNS lymphoma treated with COAP (using cytosine arabinoside as an IV infusion). As previously discussed, an alternative drug that crosses the blood-brain barrier and is effective in eliminating lymphoma cells is lomustine (CCNU; see Box 80-1) administered at a dosage of 60 mg/m² PO every 3 weeks in dogs and at a dosage of 10 mg/cat every 3 weeks in cats; we have seen marked improvement or disappearance of neurologic signs in dogs and cats with lymphoma treated with this drug.

Despite the fact that remissions are easily attained in dogs and cats with CNS lymphoma, they are relatively short in duration compared with the duration of remissions in dogs and cats with disease in other anatomic locations. Most dogs and cats with CNS lymphoma relapse within 2 to 4 months of diagnosis; however, prolonged remissions (i.e., 6 to 12 months) are possible.

Ocular lymphoma

Ocular lymphoma can be treated using a variety of modalities. However, the eye behaves similarly to the blood-brain barrier in that adequate intraocular concentrations of chemotherapeutic agents are usually difficult to attain. If the clinician and owner want to try to preserve the animal’s eye, there are several alternatives to enucleation. As in animals with CNS lymphoma, the administration of cytosine arabinoside as a slow IV drip usually results in remission of the tumor. Lomustine is also effective in dogs and cats with intraocular lymphoma.

Cutaneous lymphoma

Cutaneous lymphoma is the most common extranodal form of lymphoma in dogs seen at the Veterinary Teaching Hospital of The Ohio State University. In dogs with cutaneous involvement secondary to multicentric lymphoma, we use a standard chemotherapy protocol (i.e., COAP). In dogs with epitheliotropic T-cell lymphomas we use either doxorubicin-containing (i.e., CHOP; see Box 80-1) or lomustine (CCNU)-containing protocols. In a recent study of 46 dogs with epidermotropic cutaneous T-cell lymphoma, 15 (33%) underwent CR and 23 (50%) underwent PR, for a response rate of 83% (Risbon et al., 2006). The median number of treatments to achieve a response was 1 (range, 1-6). The overall median duration of response was 94 days (range, 22-282). Sixteen dose reductions were required because of neutropenia (10/46), thrombocytopenia (1/46), anemia (1/46), increased liver enzyme activity (3/46), or unspecified reasons (1/46).

Alimentary lymphoma

We use standard chemotherapy protocols (i.e., COAP) in dogs and cats with solitary mural or nodal (e.g., mesenteric or ileocecocolic lymph node) involvement. Even though surgery is not necessarily indicated for these dogs and cats, a fair number are referred after exploratory surgery and an incisional or excisional biopsy has been performed. In general, the response in these animals is good. Dogs and cats with diffuse intestinal lymphoma usually respond poorly to chemotherapy. Responses to doxorubicin-containing protocols (i.e., CHOP) appear to be better than those to COAP, although survival times are short (4 to 6 months). Dogs with colorectal lymphoma and cats with gastric lymphoma tend to respond extremely well to COAP chemotherapy; we have documented remission times in excess of 3 years in these subsets of patients. In cats this may be related to the fact that Helicobacter spp. may play a role in the development of gastric lymphoma, as H. pilori does in people; we treat all cats with gastric lymphoma with combination chemotherapy and antibiotics proven effective in cats with Helicobacter infection.

In cats with epitheliotropic intestinal lymphoma, a common, small lymphocytic form of the disease in older individuals, we have used a very conservative approach with excellent results. We administer a combination of chlorambucil (20 mg/m², PO q2 weeks) plus prednisone (1-2 mg/kg, PO q24-48h) or dexamethasone (4 mg/cat, PO q1-2 weeks); if clinical signs do not improve within 3 or 4 weeks, we add vincristine (0.5 mg/m², IV, q1-2 weeks). Most cats treated with this protocol have marked improvement of the clinical signs and typically gain weight. Interestingly, some of the cats exhibit no appreciable decrease in mesenteric lymph node size, despite the remarkable clinical improvement. For these cats I use the approach of “treating the patient, not the disease” (i.e., as long as the patient feels well and is free of clinical signs, the current treatment is continued).