As our understanding of this disease increases and additional diagnostic tests and treatment options become available, a current understanding of available diagnostics and treatment options is crucial to best support pet owners during what is often a stressful time.

Who Is at Risk?

Dogs of any age can develop lymphoma, though it classically affects middle-aged to older dogs. Dogs with T-cell lymphoma tend to be younger. Several breeds appear to have higher incidences of lymphoma, including boxers, bullmastiffs, basset hounds, Scottish terriers, St. Bernards, Briards, Irish Setters, Rottweilers, bulldogs, and pit bulls.¹

The cause of lymphoma in dogs is not well understood; however, some risk is likely heritable. Environmental exposures have also been implicated as potential risk factors—including exposure to commercially applied pesticides, herbicides, and household chemicals or living in industrial areas or near industrial sites. One study specifically focusing on boxers identified proximity to nuclear power plants, chemical suppliers, and crematoria as risk factors for lymphoma.³ Impaired or altered immune function may also increase the risk for lymphoma.¹

Clinical Presentation of Lymphoma

Most veterinarians have seen the classic presentation of a dog with enlarged lymph nodes and owners may even notice this at home as "new lumps." Most dogs with multicentric lymphoma do not initially show overt signs of illness. However, when dogs do feel sick, they may exhibit clinical signs such as decreased appetite, weight loss, vomiting, diarrhea, polyuria, polydipsia, or fever.

Other clinical signs will depend on the organs affected by the disease. Greater than 80% of dogs develop the multicentric form with peripheral lymph node involvement. Other forms of lymphoma include primary gastrointestinal, mediastinal, or cutaneous lymphoma, with other extranodal sites being an even less common origin.¹

Lymphoma Diagnostics

Evaluation of an affected lymph node with biopsy or cytology is essential to confirm a suspicion of lymphoma. A biopsy is required for definitive classification and diagnosis. However, cytology is considered a sensitive and specific first-line diagnostic test for lymphoma. A fine needle aspirate and cytology is often preferred over biopsy as a lower cost and less technical and invasive option. However, it's important to note that certain lymphoma subtypes, including the less aggressive indolent lymphomas, cannot be definitively identified by cytology alone.²

Whether your patient is well or sick, additional diagnostic testing—including a complete blood count (CBC) and chemistry panel—to evaluate for signs of bone marrow involvement and assess overall organ function is warranted, as well as to screen for paraneoplastic syndromes such as anemia or hypercalcemia. Urinalysis is also recommended to characterize renal function.¹ It's possible that the CBC or chemistry may be normal even with a confirmed diagnosis.

Imaging, such as thoracic radiographs and abdominal ultrasound, can also be considered to evaluate the involvement of internal organs. The process of looking for what organs are affected by cancer is called "staging." These imaging tests may also be necessary if lymphoma is suspected with only mild or minimal peripheral lymph node enlargement or if there are clinical signs attributable to internal organs.¹

Immunophenotype describes whether the cancer cell of origin was a B- or a T-lymphocyte. You may have heard that B-cell lymphomas have a better prognosis than T-cell lymphomas. While this is true for high-grade lymphoma, remember that lymphoma is a varied disease. In fact, some T-cell lymphoma subtypes can have an excellent prognosis compared to some B-cell cell lymphomas.^3,4 Among the high-grade lymphomas, B-cell lymphomas are more common than T-cell. Some breeds have genetic immunophenotype predisposition; boxers, for example, are predisposed to T-cell lymphoma.⁵ The most common tests for immunophenotype are flow cytometry (preferred) or clonality testing, such as PARR.

Finally, there has been a lot of innovation in the diagnostics space, such as a recently announced test that detects lymphoma biomarkers through multimodal bioanalytical techniques that can even distinguish between B- and T-cell disease.

Treatment for Lymphoma

Chemotherapy is the mainstay treatment of multicentric lymphoma, and luckily, most dogs tolerate chemotherapy well. The goal when treating dogs for lymphoma is to achieve a remission where their disease is clinically undetectable while maintaining a good quality of life.

Lymphoma can be a rewarding disease to treat, with 85–95% of patients experiencing complete remission of their cancer for a median of 5–9 months using multiagent protocols like CHOP (cyclophosphamide, hydroxydaunorubicin [doxorubicin hydrochloride], Oncovin [vincristine], prednisone). Such protocols range from 15–25 weeks in duration and have been associated with median survival times of about a year (in the most commonly seen form of lymphoma).⁶ Single-agent treatment protocols can also be considered if a CHOP-based protocol is not feasible.

Radiation therapy is not commonly utilized to treat multicentric lymphoma, with the exception of half-body irradiation. In one recent study of dogs diagnosed with B-cell lymphoma, dogs receiving half-body radiation in addition to chemotherapy had improved long-term outcomes compared to dogs undergoing CHOP alone.⁷

With any chemotherapy option, supportive care medications for side effects, like nausea or decreased appetite, are routinely prescribed to help keep dogs feeling well. The steroid prednisone is a component of most chemotherapy protocols used to treat lymphoma. Prednisone alone is also a critical component of palliative care when clients decline chemotherapy or chemotherapy has failed.

A diagnosis of lymphoma cancer in dogs can be stressful for families to navigate. By understanding the disease and available diagnostics and treatment options, veterinarians can guide owners toward timely diagnostics and appropriate care.

References:

1. Vail DM, Thamm DH, Liptak JM. Hematopoietic Tumors. Withrow and MacEwen's Small Animal Clinical Oncology. Elsevier, pp. 688–772.
2. Seelig DM, Avery AC, Ehrhart EJ, et al. The Comparative Diagnostic Features of Canine and Human Lymphoma. Veterinary Sciences. 2016; 3: 11.

3. Seelig DM, Avery P, Webb T, et al. Canine T‐Zone Lymphoma: Unique Immunophenotypic Features, Outcome, and Population Characteristics. Veterinary Internal Medicine 2014; 28: 878–886.

4. Hughes KL, Ehrhart EJ, Rout ED, et al. Diffuse Small B-Cell Lymphoma: A High-Grade Malignancy. Vet Pathol2021; 58: 912–922.

5. Craun K, Ekena J, Sacco J, et al. Genetic and environmental risk for lymphoma in boxer dogs. Journal of Veterinary Internal Medicine. 2020; 34: 2068–2077.
6. Thamm DH. Novel Treatments for Lymphoma. Veterinary Clinics of North America: Small Animal Practice.2024; 54: 477–490.
7. Best MP, Straw RC, Gumpel E, et al. Long‐term remission and survival in dogs with high‐grade, B cell lymphoma treated with chemotherapy with or without sequential low‐dose rate half‐body irradiation. Journal of Veterinary Internal Medicine. 2023; 37: 2368–2374.

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