Canine malignant lymphoma is a common cancer in dogs. It is a progressive, deadly disease caused by the harmful growth of lymphoid cells. Lymphoma most commonly arises from organized lymphoid tissues including the bone marrow, thymus, lymph nodes, and spleen. Other common sites include the skin, eye, central nervous system, testis, and bone. Although it is common, the causes and origin of the disease are not well understood. Possible causes or contributing factors include viral infection, environmental contamination with herbicides, magnetic field exposure, genetic abnormalities, and dysfunction of the immune system.

The signs of canine lymphoma will vary somewhat depending on the part of the body involved and the extent of the disease. In dogs, 4 distinct forms of lymphoma can occur: 1) multicentric, which originates in multiple places; 2) alimentary, which occurs in the digestive system, 3) mediastinal, which occurs within the chest; and 4) extranodal, which may involve the kidneys, central nervous system, or skin.

Multicentric lymphoma is by far the most common form, accounting for about 80% of cases. An early sign of multicentric lymphoma is the rapid and nonpainful enlargement of lymph nodes, which may become 3 to 10 times their normal size (see Immune Disorders of Dogs: Immune System Tumors in Dogs). In addition to this, cancerous lymphocytes may move into internal organs including the spleen, liver, bone marrow, and other sites. Late in the course of disease, when there are multiple, large tumors, dogs may show general signs of illness, including lack of energy, weakness, fever, loss of appetite, and depression.

Alimentary lymphoma is much less common and accounts for less than 10% of all canine lymphomas. Dogs with this form of the disease may have signs related to stomach upset, such as vomiting and abdominal pain. When the disease affects most of the intestinal tract, dogs may have loss of appetite, vomiting, diarrhea, and continued weight loss because they cannot digest food properly.

Mediastinal lymphoma is also uncommon. Dogs with this form of the disease may have an enlarged thymus, lymph nodes, or both. As the disease advances, signs may include trouble breathing as fluid builds up in the chest and puts pressure on the lungs. The tumor may block the vein that routes blood from the upper part of the body into the heart. In addition to symptoms related to breathing, some dogs with mediastinal lymphoma pass large amounts of urine and drink more than normal. This can be caused by an increase in calcium in the blood, a syndrome seen in 10% to 40% of dogs with lymphoma.

The medical problems associated with extranodal lymphoma vary and depend on which organ is affected. Skin lymphoma may appear as single, raised, slow-healing sores or widespread, scaly regions. Signs of lymphoma at other extranodal sites include difficulty breathing (lungs), kidney failure (kidneys), blindness (eyes), and seizures (central nervous system).

Canine lymphoma is often relatively easy to diagnose by testing the affected organ system. In dogs with multicentric lymphoma, a needle biopsy of enlarged lymph nodes usually provides enough cells to test to confirm the diagnosis.

Treatment and Outlook

Treatment of multicentric canine lymphoma with chemotherapy using a combination of drugs is often successful, with more than 90% of all dogs improving to some degree. Individual treatment plans vary with respect to the drugs used, dosage, and frequency and duration of treatment. With chemotherapy, the expected survival time for dogs with B-cell lymphoma is about 9 to 12 months. For dogs with T-cell lymphoma, expected survival times are shorter (6 months). Dogs that do not respond to the usual drugs may improve when other treatment plans are used. These alternate plans may include other drugs or radiation.

Treating other forms of lymphoma is often more difficult. Alimentary lymphoma, if concentrated in one area, can be treated effectively with surgery to remove the tumor, together with combination chemotherapy. However, if lymphoma is spread throughout the intestinal tract, the response to treatment is less dramatic and survival times are shorter (often less than 3 months). The use of combination chemotherapy with or without radiation therapy can give dogs with mediastinal lymphoma considerable improvement in survival times and quality-of-life scores, but remission averages about 6 months for T‑cell lymphoma. Dogs with an abnormally high level of calcium in the blood, a condition often associated with mediastinal lymphoma, are also less likely to have prolonged survival times. Lymphoma involving other extranodal sites, such as the skin, can be managed with surgery, radiation, and whole system chemotherapy as appropriate; however, the disease often does not respond to medical treatment.

Last full review/revision July 2011 by Peter H. Holmes, BVMS, PhD, Dr HC, FRCVS, FRSE, OBE; Nemi C. Jain, MVSc, PhD; David J. Waltisbuhl, BASc, MSc; Michael Bernstein, DVM, DACVIM; Karen L. Campbell, MS, DVM, DACVIM, DACVD; Timothy M. Fan, DVM, PhD, DACVIM; Wayne K. Jorgensen, BSc, PhD; Sarah E. Payne, DVM, DACVIM