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10 Most Common Types of Tumors and Cancers in Dogs
by Joanne Intile, DVM, DACVIM
Lymphoma is a blood borne cancer of a type of white blood cell called a lymphocyte. Dogs can develop several different forms of lymphoma, with the most common presentation showing enlargement of external lymph nodes. Other manifestations include cutaneous lymphoma (skin), gastrointestinal lymphoma, and nervous system lymphoma.
Lymphoma is a highly treatable cancer in dogs. The plan for treatment can vary, but usually involves dogs receiving treatment weekly to every other week. Chemotherapy is well-tolerated in dogs, with minimal side effects; the outcome with chemotherapy can be 1-2 years or more.
Mast Cell Tumors
Mast cells, or mastocytomas, are immune cells normally involved in allergic reactions. These cells are found throughout the body; therefore, tumors can develop in any organ system.
Dogs most frequently develop mast cell tumors in their skin. The biological behavior of skin mast cell tumors is variable and best predicted their grade, which is assigned by a pathologist who examines samples of tissue from the tumor under the microscope. Low-grade tumors are treated with surgery. Higher grade tumors are more aggressive and require multiple types of treatment, including surgery, radiation therapy, and chemotherapy.
Mast cell tumors can also develop in internal organs, including the digestive tract and urinary tract. Though rare, these locations usually indicate a poor long-term outcome.
Mast cells contain chemicals including histamine and serotonin. These chemicals are present in large quantities in the bloodstream of dogs with mast cell tumors and are responsible for causing clinical signs, including loss of appetite, vomiting, and diarrhea. Antihistamines can help lessen these symptoms.
Osteosarcoma is the most common primary bone cancer in dogs. Tumors tend to occur in the limbs of large and giant breed dogs. Osteosarcoma tumors can spread from the bone to distant sites in the body, including the lungs, lymph nodes, and other bones. This type of cancer is painful, and amputation of the affected limb is necessary to provide relief. Dogs handle amputation well and have improved quality of life when the source of pain is removed.
Chemotherapy following amputation is effective in slowing down metastases (spread to other areas), affording dogs 1-2 years of survival. Current research is focused on using the dog’s immune system to fight off cancer cells and initial results are promising for greater long term survival.
Image: X-ray of the distal femur
Lipomas are common benign skin tumors consisting of excess fatty tissue. They grow under the skin of the trunk, axillary regions, and groin. Surgery is reserved for lipomas arising in areas of the body where growth could lead to difficulty ambulating (e.g., along a limb).
Lipomas can arise between muscle tissue, most commonly in the hind limb. These tumors are called intramuscular lipomas. Surgery is recommended in such cases, though complete removal of the lipoma can be challenging. Radiation therapy can be used in addition to surgery or when tumors are too large to be removed.
Lipomas can also grow within the chest or abdominal cavity, compressing vital organs and causing discomfort. Dogs can develop malignant version of lipomas called liposarcomas, which are capable of metastasizing (spreading) to distant sites. The only way to know the difference is with a biopsy.
Melanoma is one of the most common oral cancers seen in dogs. Breeds with darker pigmented gums and tongues are at increased risk for developing this form of cancer. Melanoma is locally invasive into the underlying tissue and bone of the oral cavity, and complete removal of tumors is difficult. The best outcome is achieved with an aggressive initial operation, which often requires surgery by a board-certified veterinary surgeon.
Oral melanoma can spread to lymph nodes of the head and neck, and also to the lungs, so pre-surgical testing for metastases (spread) with samples from the lymph node and imaging of the chest is imperative. Oral melanoma is treated with surgery, radiation therapy, chemotherapy, and immune therapy. There is also a therapeutic vaccine available for treating dogs with oral melanoma.
Anal Gland Adenocarcinoma
Dogs, like skunks, have paired glands along the opening of their anus that release a rather smelly substance during defecation. Tumors of the anal glands occur rarely, but they can be aggressive, therefore a rectal exam is an exceptionally important part of every physical exam.
Anal sac adenocarcinoma tumors can secrete a hormone that tricks an affected patient’s body into thinking their blood calcium level is low, causing massive absorption of calcium from their diet, bones, and kidneys. This can raise the blood calcium levels out of range, leading to nausea, weakness, and cardiovascular complications.
When possible, surgical removal of the anal sac tumor is the recommendation of choice. Anal sac tumors can spread to distant location in the body, first to lymph nodes in the pelvic region, then lungs, liver, spleen, and even bone. Small tumors carry a more favorable outcome than larger ones.
Mammary Gland Carcinoma
Tumors of the mammary glands are common in dogs. Unspayed female dogs are at risk for developing mammary tumors due to hormonal influence on the mammary tissue. Half of the tumors dogs develop in their mammary glands are cancerous and half are benign. Of the 50% that are cancerous, about half of those will go on to cause the death of the patient.
Mammary masses in dogs should be removed with surgery; the masses are always submitted for a biopsy. This is the primary way for a veterinary oncologist to assess the risk the tumor poses for that patient. Many mammary tumors can be treated with surgery alone. Some should also receive chemotherapy to prevent or delay regrowth and/or spread.
Primary Lung Tumor
Lung cancer occurs more commonly in older dogs, and many are diagnosed incidentally, when x-rays of the lungs are performed as part of routine screening during an annual health exam or prior to a dental cleaning. CT scans are more effective at localizing the position of the tumors and are also better for picking up smaller lesions within other lung tissue.
If there is only one tumor present, surgery to remove the mass is recommended. Despite the invasiveness of this procedure, dogs recover well and complications are rare. Experienced surgeons working at veterinary facilities able to provide round the clock care are the best resources for this type of procedure. If metastasis (spread) is seen at the time of diagnosis, chemotherapy can be used to slow progression of disease.
Thyroid cancer is another cancer seen more frequently in older dogs. Owners may feel a lump along their dog’s neck while petting them or their veterinarian may palpate the mass during a routine exam, again stressing the need for regular veterinary visits.
Some thyroid tumors are functional, which means they actively secrete thyroid hormone, which causes dogs to become hyperthyroid and show signs of weight loss, hyperactivity, panting, and upset stomach signs. An asymptomatic dog with a high blood thyroid hormone level should be examined for the presence of a thyroid tumor. Treatment of choice, when possible, is surgery to remove the tumor. Radiation therapy can be used to treat tumors that cannot be completely removed. Chemotherapy is often recommended to delay or prevent metastases (spread) to distant sites in the body.
Hemangisarcoma is cancer that arises from the cells that line blood vessels. The most common anatomical locations where hemangiosarcoma arises includes the spleen, the skin, and the right atrium of the heart. The liver is also a common primary site of disease. When the spleen is affected, dogs typically will show no signs of illness until the tumor ruptures and they bleed internally. This is a life threatening situation requiring emergency surgery to remove the spleen.
The prognosis for dogs with hemangiosarcoma depends on the anatomical area where the primary tumor is found and can range from 3-4 months upwards of several years. Dogs are treated with a combination of surgery and chemotherapy. Radiation therapy is used for tumors that develop in the skin.
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