Lymphoma

Lymphoma is a cancer of lymphocytes, which include several types of white blood cell that are important components of the immune system. The term “lymphoma” describes about three dozen different types of cancers with diverse appearance and clinical behavior; together, lymphomas are among the most common cancers diagnosed in dogs. From a practical perspective, the most common types of lymphoma can be divided into four clinically relevant categories, described below.

Generally, lymphomas affect middle-aged to older dogs, but they can affect young dogs (juvenile-onset lymphoma) and in rare cases even puppies. Potential causes for lymphoma have been investigated, but an underlying cause(s) has not been identified, either generally or for any specific subtype. Numerous studies suggest that lymphomas in general, and possibly specific types of lymphoma, affect certain breeds with greater frequency than other breeds or mix breed dogs. But any dog of any breed can be affected by any type of lymphoma.

Aggressive B-cell Lymphoma

B-cells are a type of immune cell that protect the body by producing proteins called antibodies. Aggressive B-cell lymphomas are the most common forms of lymphoma in dogs. The behavior of these tumors is variable and difficult to predict, but they tend to respond well to chemotherapy. The standard of care for aggressive B-cell lymphomas involves using a combination of different chemotherapy drugs.

The duration of response is variable, but about half of dogs with aggressive B cell lymphomas treated with the standard of care will remain free of disease for 6 to 8 months, and without further treatment can survive about 10 months or longer. Dogs can respond to treatment if or when the disease comes back, but the expectation is that the duration of response will be shorter for each subsequent relapse.

Indolent B-cell Lymphoma

Indolent B-cell lymphoma is a less aggressive form of B-cell lymphoma. Indolent B-cell lymphomas vary in their origin and clinical presentation. Depending on the subtype and clinical presentation, indolent B-cell lymphomas may require the same treatment as aggressive lymphomas or they might require less intense chemotherapy; in some specific cases where the tumor is confined to the spleen, surgery can be curative.

Aggressive T-cell Lymphoma

T-cells are a type of immune cell that protect the body by destroying unhealthy cells and helping to coordinate the body’s immune response. Aggressive T-cell lymphoma is less common than aggressive B-cell lymphoma, and it tends to be less responsive to treatment. These tumors are more likely to have spread to none-immune organs like the gut, lungs, kidneys, and brain than B-cell lymphomas.

In order to avoid incorrect treatments and even premature euthanasia, it is extremely important to distinguish aggressive T-cell lymphomas, which have a poor prognosis, from indolent T-cell lymphomas, which have a good prognosis.

Indolent T-cell Lymphoma

Indolent T-cell lymphomas are less aggressive subtypes of T-cell lymphoma. They are quite common in older dogs and one subtype, called T-zone lymphoma, might be more common in golden retrievers than it is in other dogs. Indolent T-cell lymphomas will often require no treatment, or if they are associated with clinical signs, they can be managed with conservative chemotherapy. Using aggressive chemotherapy protocols to treat dogs with indolent lymphomas has the potential to accelerate progression and precipitate early death.

Symptoms

Symptoms of lymphoma in dogs vary depending on the type of lymphoma as well as the stage, or how far the cancer has progressed. Most commonly, dogs develop enlarged, non-painful lymph nodes near the jaw and/or behind the knees of the hind legs.

Canine lymphoma and common sites (in red) of lymph node enlargement, created by BioRender.com
Images created with BioRender.com

Symptoms of lymphoma in dogs may include:

  • Enlarged, non-painful lymph nodes
  • Lethargy
  • Lack of appetite
  • Weight loss
  • Increased thirst and urination
  • Difficulty breathing
  • Vomiting and/or diarrhea
  • Skin lesions

If you think your dog has any of these symptoms, you should have him or her evaluated by your veterinarian.

Diagnosis

Lymphoma may be diagnosed with a fine needle aspirate, in which a needle is used to obtain a sample of cells from an enlarged lymph node or other organ to help make a diagnosis. Information from a fine needle aspirate does not provide sufficient information to classify the lymphoma and identify the best treatment option.

A biopsy of an enlarged lymph node or organ site affected by lymphoma can also be used to make a diagnosis. This involves taking a sample from a lymph node (or the entire lymph node) or other organ sites. Evaluation of the sample can provide phenotypic information used to predict prognosis and response to treatment.

Another diagnostic modality is flow cytometry, which can provide relevant information to determine the phenotype, a general context of the subtype, the likely tumor behavior and associated prognosis, and help formulate and prioritize options for treatment. PARR, or PCR for antigen receptor rearrangement, is another diagnostic that can be used to differentiate whether excess lymphocytes represent reactive/inflammatory populations or malignant populations. Flow cytometry and PARR are two examples on add-on diagnostics that carry added cost, but not added risk or mortality.

In addition to a diagnosis of lymphoma, staging diagnostics are used to determine the location of the tumor. Staging may include bloodwork and urinalysis, chest x-rays, abdominal ultrasound, and a bone marrow aspirate. In addition to revealing cancer progression, staging can help determine if there are other disease conditions, called comorbidities, which may influence treatment choices and potentially affect patient outcomes.

Treatments

If your dog is diagnosed with lymphoma, treatment options depend on the type of lymphoma, and also take into consideration cost, time, and quality of life. Working together with your veterinarian can help determine the best treatment option for your pet’s type of lymphoma whether it is chemotherapy, surgery, steroids, palliative care, or some combination of these modalities.

In general, dogs with lymphoma that have systemic signs and are clinically ill tend to have a worse response to treatment, while dogs that do not have clinical signs of illness tend to have better response rates.

If you haven’t done so already, you are encouraged to read through our cancer overview article. Other articles on this website address some other forms of cancer that you may find informative.

This article has been written by Courtney Labé and Jaime Modiano, VMD, PhD, Director, Animal Cancer Care and Research Program, University of Minnesota, St. Paul, MN