Lymphoma is the most common hematopoietic neoplasm affecting both dogs and cats and results from the malignant transformation
of lymphocytes. Lymphoma often arises from primary and secondary lymphoid tissues, including the thymus, spleen, lymph nodes,
and gut-associated lymphoid tissues (Figures 1A, 1B, & 2). However, because lymphocytes are capable of trafficking throughout the body, the development of lymphoma is not anatomically
restricted. Common extranodal sites for lymphoma include the skin, eye, central nervous system, testis, and bone marrow (Figures 3 & 4).1
FIGURE 1A. A dog with profound prescapular and mandibular lymphadenopathy. FIGURE 1B. A cytologic preparation obtained from
the dog in FIGURE 1A demonstrating a monomorphic population of malignant lymphoblasts, consistent with a clinical diagnosis
of lymphoma (Wright's-Giemsa; 400X). FIGURE 2. A lateral thoracic radiograph of a dog with a large cranial mediastinal (thymic)
In dogs, the incidence of lymphoma has been reported to approach 0.1% in susceptible, older individuals, with an annual incidence
rate of 84/100,000 dogs at risk.2 Although lymphoma is considered a common neoplasm, a definitive cause for its development in dogs remains to be determined.
Several hypothesized causes of canine lymphoma include retroviral infection, environmental contamination with phenoxyacetic
acid herbicides, magnetic field exposure, chromosomal abnormalities, and immune dysfunction.3-8 Because lymphoma is a heterogeneous disease process, various causes may be responsible for lymphomas with differing biologic
behaviors, as is the case in human non-Hodgkin's lymphoma patients.
FIGURE 3. Disseminated erythematous dermal plaques in an English bulldog. The histopathologic diagnosis was epitheliotropic
T-cell lymphoma (mycosis fungoides). FIGURE 4. An intact male golden retriever with painless bilateral testicular enlargement,
cytologically confirmed as a large cell lymphoma.
In cats, feline leukemia virus (FeLV) has been identified as a biologic carcinogen resulting in malignant lymphocyte transformation.9,10 Historical epidemiological investigations before the wide use of preventive FeLV vaccines estimated the annual incidence
of feline lymphoma to be 200/100,000 cats at risk.11 With the development of efficacious FeLV vaccines in conjunction with early detection and removal of viremic cats from the
general population, the incidence of FeLV-induced lymphoma has been dramatically reduced.12 In addition to FeLV as a causative factor of feline lymphoma, epidemiologic evidence exists for household environmental tobacco
smoke to act as a chemical carcinogen, which increases the risk for lymphoma development in cats.13
For most dogs and cats with suspected or confirmed lymphoma, diagnostic evaluations, referred to as clinical staging, should include a complete blood count, a serum chemistry profile, urinalysis, a thoracic radiographic examination, and an
abdominal ultrasonographic examination. For patients with anemia, thrombocytopenia, or leukopenia, a bone marrow aspirate
or a bone core biopsy should be performed to assess for neoplastic infiltration. Representative cytologic or histologic samples
collected from enlarged lymph nodes or other affected organs should be submitted to confirm the diagnosis of lymphoma. Although
cytology is a quick and acceptable means for diagnosing lymphoma, histologic evaluation of tissue samples can provide additional
prognostic information such as immunophenotype and histologic grade.