Lymphoma (lymphosarcoma) in animals is a type of cancer defined by a proliferation of malignant lymphocytes within solid organs such as the lymph nodes, bone marrow, liver and spleen. The disease also may occur in the eye, skin, and gastrointestinal tract. Lymphoma in dogs Lymphoma is one of the most common malignant tumors to occur in dogs. The cause is genetic, but there are also suspected environmental factors involved, including in one study an increased risk with the use of the herbicide 2,4-D. This risk was not confirmed in another study. Breeds that are commonly affected include Boxer, Scottish Terrier, Basset Hound, Airedale Terrier, Chow Chow, German Shepherd, Poodle, St. Bernard, Bulldog, Beagle, Rottweiler and Golden Retriever. The Golden Retriever is especially susceptible to developing lymphoma, with a lifetime risk of 1:8. Classification The cancer is classified into low and high grade types. Classification is also based on location. The four location types are multicentric, mediastinal, gastrointestinal, and extranodal (involving the kidney, central nervous system, skin, heart, or eye). Multicentric lymphoma, the most common type (by greater than 80 percent), is found in the lymph nodes, with or without involvement in the liver, spleen, or bone marrow. Mediastinal lymphoma occurs in the lymph nodes in the thorax and possibly the thymus. Gastrointestinal lymphoma occurs as either a solitary tumor or diffuse invasion of the stomach or intestines, with or without involvement in the surrounding lymph nodes, liver or spleen. Classification is further based on involvement of B-lymphocytes or T-lymphocytes. Approximately 70 percent are B-cell lymphoma. Cutaneous lymphoma can be classified as epitheliotropic (closely conforming to the epidermis) or non-epitheliotropic. The epitheliotropic form is typically of T-cell origin and is also called mycosis fungoides. The non-epitheliotropic form is typically of B-cell origin. Signs and symptoms General signs and symptoms include depression, fever, weight loss, loss of appetite, loss of hair or fur and vomiting. Lymphoma is the most common cancerous cause of hypercalcemia (high blood calcium levels) in dogs. It can lead to the above signs and symptoms plus increased water drinking, increased urination, and cardiac arrhythmias. Hypercalcemia in these cases is caused by secretion of parathyroid hormone-related protein. Multicentric lymphoma presents as painless enlargement of the peripheral lymph nodes. This is seen in areas such as under the jaw, the armpits, the groin, and behind the knees. Enlargement of the liver and spleen causes the abdomen to distend. Mediastinal lymphoma