November is Pet Cancer Awareness Month. To help raise awareness about veterinary oncology, I am devoting my November blogposts to the topic. Last week, my blogpost highlighted quality of life for pets receiving cancer treatment based on pet owner surveys. Today, I will focus on lymphoma, one of the more common cancers in dogs and cats.

Lymphoma is a tumor arising from the cells of the immune system. The immune system is distributed throughout the body and includes the lymph nodes, spleen and tonsils. The widespread nature of the immune system means lymphoma is usually widespread at the time of diagnosis. Therefore, treatment must also be widely distributed throughout the body. This explains why the primary treatment for lymphoma is chemotherapy, which uses drugs to treat cancer, as opposed to more targeted options like radiation or surgery.

Yet, sometimes I will see a new patient with lymphoma, and I won’t recommend chemotherapy. This can come as a surprise to some pet owners, so let me explain further below.

Indolent Lymphomas in Dogs and Cats

According to the National Cancer Institute, lymphoma is a broad term for cancer of the immune system. In reality, lymphoma has between 20 or 30 subtypes, each with their own biopsy appearance, treatment recommendation and prognosis. Some of the subtypes are considered “indolent,” meaning the malignant lymphocytes (cells of the immune system) spread slowly and do not cause illness. These cells appear much more like normal lymphocytes than malignant ones when viewed using a microscope.

In dogs, the most common form of indolent lymphoma is called T-zone lymphoma. In cats, the most common form of indolent lymphoma is gastrointestinal small cell lymphoma.

Canine T-zone Lymphoma

The T-zone refers to a specific anatomic location inside a lymph node. When veterinary oncologists find enlarged lymph nodes in certain dog breeds, including Golden retrievers and Shih tzus, it may heighten our suspicion for T-zone lymphoma. Fine needle aspiration cytology is a common test veterinarians use to diagnosis lymphoma, but T-zone lymphoma does not always appear malignant. To accurately diagnose T-zone lymphoma, we use a special test called flow cytometry. Flow cytometry identifies the cell surface markers on lymph node cells, and T-zone lymphoma has a specific pattern that is easily recognizable. Making an accurate diagnosis of T-zone lymphoma is critical, because hardly any canine T-zone lymphoma patients need chemotherapy because the cancer does not make the dog sick.

Feline Gastrointestinal Small Cell Lymphoma

Feline gastrointestinal lymphoma comes in two basic types: large cell and small cell. Cats with large cell lymphoma tend to be very ill. Despite chemotherapy, cats diagnosed with large cell lymphoma succumb to this disease in a matter of months, on average. However, cats diagnosed with small cell gastrointestinal lymphoma are much less ill.

The most common clinical sign of gastrointestinal small cell lymphoma is weight loss with or without vomiting and diarrhea. To refine this diagnosis, veterinarians will take an intestinal biopsy, but because this is an indolent lymphoma, the biopsy may not have the typical appearance of a malignancy. In these cases, pathologists will then test the DNA of the biopsy sample using a test called polymerase chain reaction for antigen receptor rearrangement or PARR. Like T-zone lymphoma in dogs, making an accurate diagnosis of feline gastrointestinal small cell lymphoma is critical, because treatment can be a simple oral medication with limited side effects.

Conclusion

No pet owner wants to hear their veterinarian say the word “lymphoma,” but I hope this blogpost will remind you to listen carefully, because lymphoma is not always a bad diagnosis.