Lymphoma in Dogs

What is lymphoma?

Lymphoma is a cancer of the lymph nodes and lymphatic system. This cancer may be localized to one region or spread throughout the entire body. The lymphatic system includes the lymph nodes, specialized lymphatic organs, such as the spleen and tonsils, and the lymphatic vessels. Together, these components of the lymphatic system carry out several essential roles in the body, including the movement of fluids and other substances through the body, as well as carrying out immune functions in response to toxins or infections.

Is lymphoma common in dogs?

Lymphoma is a relatively common cancer, accounting for 15-20% of new cancer diagnoses in dogs. It is most common in middle-aged and older dogs, and several breeds are predisposed, suggesting that there may be a genetic component to lymphoma, although this has not been confirmed. Some breeds predisposed to lymphoma include:

• Chow Chow
• Basset Hound
• Scottish, Airedale, West Highland White, Yorkshire, and Bull Terriers
• Golden Retriever
• English Bulldog
• German Shepherd
• Beagle
• Rottweiler
• Saint Bernard
• Poodle

"...several breeds are predisposed, suggesting that there may be a genetic component to lymphoma..."

There are four different types of lymphoma in dogs, varying in severity and prognosis:

Multicentric (systemic) lymphoma. This is, by far, the most common type of canine lymphoma. Multicentric lymphoma accounts for approximately 80-85% of cases in dogs. In multicentric lymphoma, lymph nodes throughout the body are affected.

Alimentary lymphoma. This type of lymphoma affects the gastrointestinal tract. Alimentary lymphoma is the second most common type of lymphoma.

Mediastinal lymphoma. In this rare form of lymphoma, lymphoid organs in the chest (e.g., lymph nodes, thymus) are affected.

Extranodal lymphoma. This type of lymphoma targets a specific organ outside of the lymphatic system. Extranodal lymphoma is rare but may develop in the skin, eyes, kidney, lung, or nervous system.

What are the clinical signs of lymphoma?         

In dogs with multicentric (systemic) lymphoma, the first sign of lymphoma is swelling of the lymph nodes. The lymph nodes in the neck, chest, armpits, groin, and behind the knees are often the most visible and easily observed. The dog’s owner may note swelling of these lymph nodes, or the veterinarian may first note them on a routine physical exam. Most of these dogs do not have any clinical signs of illness at the time of diagnosis, although they will often develop signs such as weight loss and lethargy if untreated.

In the other, less common forms of lymphoma, clinical signs depend on the affected organ. Alimentary lymphoma causes gastrointestinal lesions, resulting in vomiting, diarrhea, and weight loss. Mediastinal lymphoma creates lesions within the chest that take up space in the chest cavity, commonly resulting in coughing and shortness of breath. The effects of extranodal lymphoma vary significantly, depending on the organ involved.

 How is lymphoma diagnosed?

Not all dogs with enlarged lymph nodes have lymphoma. Enlarged lymph nodes may also occur due to infections or autoimmune diseases, so your veterinarian will perform tests to determine the cause of your dog’s clinical signs.

The most common test to diagnose lymphoma is a fine needle aspirate (FNA). In this test, a veterinarian inserts a needle into an enlarged lymph node (or another organ) and removes a small number of cells. These cells are then examined under a microscope, looking for evidence of cancerous cells that indicate lymphoma.

If FNA is inconclusive or impractical due to the lesion’s location, your veterinarian may perform a biopsy. This involves the surgical removal of a tissue sample from the lymph node or lesion. This sample will be processed and examined under a microscope at a veterinary laboratory to look for the presence of lymphoma.

Your veterinarian will also likely perform baseline screening bloodwork to assess your dog’s overall health. There are two components to this bloodwork. A complete blood cell count involves examining the cell types within your dog’s blood and assessing quantities of red blood cells, white blood cells, and platelets. Serum biochemistry is used to assess the function of your dog’s internal organs.

If your dog is diagnosed with lymphoma, your veterinarian may perform additional testing to find out more information about the lymphoma and develop a treatment plan. These additional tests may include the following:

• Immunohistochemistry. This test uses specialized stains to distinguish between two types of lymphoma: B-cell lymphoma and T-cell lymphoma. Identifying whether your dog’s lymphoma is B-cell or T-cell lymphoma can provide information regarding prognosis.
• Flow cytometry. This test can be used to distinguish B-cell from T-cell lymphoma.

Your veterinarian may also recommend additional tests to determine the extent of your dog’s lymphoma. This testing commonly includes imaging such as radiographs (X-rays) or ultrasounds.

There are five stages of lymphoma. Stage I and II are rarely observed in dogs, while Stages III-V are.

• Stage I: involves only a single lymph node
• Stage II: involves lymph nodes on only one side of the diaphragm (only affects the front of the body or rear of the body)
• Stage III: generalized lymph node involvement
• Stage IV: involves the liver and/or spleen
• Stage V: involves the bone marrow, nervous system, or other unusual location
  

How is lymphoma treated?

Lymphoma is treated with chemotherapy. There are a variety of procedures used, but most consist of a variety of injections given every week. Fortunately, dogs tend to tolerate chemotherapy better than humans; they rarely lose their hair or seem to feel ill during chemotherapy. The most common side effects of chemotherapy include vomiting, diarrhea, and decreased appetite, though even these effects are not seen in all dogs. If chemotherapy is not an option due to patient factors or owner financial constraints, prednisone can be used for palliative care. Although prednisone does not treat lymphoma, it can temporarily reduce clinical signs and buy the pet some time.

"...dogs tend to tolerate chemotherapy better than humans..."

Surgery and/or radiation may be appropriate for certain types of low-grade localized lymphoma, but most cases cannot be successfully treated with surgery or radiation.

What is the prognosis for lymphoma?

The prognosis for lymphoma varies, depending on various characteristics that can only be determined by specialized testing. On average, dogs who receive no treatment (or are treated with prednisone alone) have an expected survival of four to six weeks. However, this is an average, with some dogs being euthanized or dying before the four-week point and some living past six weeks.

With chemotherapy, lymphoma can often be put into remission. While lymphoma is never truly ‘cured’, remission is a term that is used to describe the temporary resolution of all signs of lymphoma. The average remission with chemotherapy is eight to nine months, with an average survival time of approximately one year. Again, this is only an average; some dogs will die sooner, and some will live longer than one year. Your veterinarian may be able to provide more specific information on your pet’s prognosis if you pursue additional testing to characterize the lymphoma better.