More than half of dogs over 10 years of age are likely to develop cancer in their lifetime. Cancerous tumors are masses of tissue that result when cells divide more rapidly than normal, or do not die when they should. Because tumors can develop from any tissue, there are many types of tumors that can occur in a variety of locations. Knowing more about tumor types, and their possible locations, can help you monitor your dog and catch possible cancers early, which may help treat the disease before it gets out of control. Early detection is key, so you should always contact your veterinarian if you find a new mass or swelling on your dog. Many times veterinarians are able to provide treatment in their clinic, but other times they may need to refer the patient to a veterinary oncologist—a veterinarian that specializes in the treatment of cancer.
Most treatment plans for canine tumors involve surgical removal of the tumor. Depending on the tumor type and location, your veterinarian may recommend adding other treatments such as chemotherapy or radiation therapy.
Chemotherapy drugs work by damaging rapidly dividing cancer cells while sparing normal cells. Because of this, normal tissues that also rapidly divide (such as those found in the intestine, bone marrow, and hair) can be transiently affected by chemotherapy. Many owners hesitate to pursue chemotherapy in their pets based on their knowledge of side effects in human cancer patients. It is important to remember that chemotherapy protocols are very different for dogs. Veterinary Oncologists have a different goal, which is to provide a good quality of life with minimal side effects. For this reason the doses of chemotherapy are lower in dogs than in people, and side effects are much less common. Should side effects occur, the drug doses are lowered for future treatments.
Below is some information on clinical signs, tumor behavior, breed predisposition, and treatment options for the most common cancers seen in dogs.
- Lymphoma or Lymphosarcoma
- Mast Cell Tumor
- Squamous Cell Carcinoma
- Mammary Carcinoma
- Apocrine Gland Carcinoma (Anal Sac)
- Transitional Cell Carcinoma
- Soft Tissue Sarcoma
Lymphoma is among the most common type of tumor seen in dogs, representing 20 percent of all canine cancers. Currently, dogs are 2-5 times more likely than people to develop lymphoma. This tumor can affect any breed of dog at any age. Some breeds, such as the Golden Retriever, are more likely to be affected, indicating a likely genetic component to the development of lymphoma.
Signs of lymphoma vary depending on the location of the disease:
Peripheral Lymphoma or Lymphosarcoma
The most common sign is enlargement of the peripheral (outside the body cavity) lymph nodes—particularly the nodes under the jaw, in front of the shoulders and behind the knees. Owners may bring their pet to the clinic with the complaint of swollen glands. In these situations, the dog may not have any other signs or may be showing subtle signs, such as a decreased appetite or lethargy.
Internal Lymphoma or Lymphosarcoma
Another form of lymphoma affects the internal lymph nodes, or tissues that are made of similar lymphoid material, such as the liver or spleen. Signs of internal lymphoma are often severe, consisting of vomiting, diarrhea, abdominal pain, or difficulty breathing. Left untreated, these can become fatal. Most internal lymphoma canine patients that do not receive treatment will die from the disease in as little as 3 or 4 weeks.
Treatment options for lymphoma consist of either a single drug (such as prednisone) or multi-drug chemotherapy. Owners who choose to use prednisone, a steroid, may see a quick reversal of signs or disappearance of swollen lymph nodes possibly lasting for 2-3 months before the signs return. Most dogs (75-90%) respond quickly to chemotherapy with multiple drugs and enjoy a good quality of life for an average of 12-16 months of remission, depending on which drugs are used.
Hemangiosarcoma is a tumor that develops from cells that line blood vessels (endothelial cells). This tumor most commonly affects middle-aged or older dogs of any breed. There is an increased frequency in Golden Retrievers and German Shepherds. Hemangiosarcoma is described as indolent in onset, meaning it develops slowly over time and is not painful to the dog. Signs usually do not show until late in the disease when the dog suffers from internal bleeding due to the tumor rupturing. The organ most frequently affected is the spleen, which can cause extreme blood loss, with the dog showing signs of shock such as sudden weakness, pale gums, and labored breathing. This tumor also frequently affects the heart, liver and skin.
Because the signs do not show themselves until the disease is very advanced, a complete cure is unlikely even with extensive treatment. This treatment usually involves surgery to remove the spleen and control bleeding, with recommendations for chemotherapy starting two weeks later. Even with aggressive treatments, most dogs do not survive very long once diagnosed with Hemangiosarcoma. Survival times with surgery alone are reported to be 2 to 3 months, and 5 to 7 months when combined with chemotherapy.
Osteosarcoma is the most common type of primary bone tumor in the dog. It most frequently affects the long bones (front and rear limbs) of the dog, but can be found in any bone including the skull or ribs. This tumor is usually associated with the giant dog breeds, with Great Danes being 200 times more likely than toy breeds to be affected. Most dogs are between 7 to 10 years of age at the time of diagnosis.
Osteosarcoma is a very aggressive and rapidly spreading tumor with even microscopic (not visible to the naked eye) spread assumed at the time of diagnosis. Because of this, recommended treatment is surgery usually an amputation, to remove the primary tumor, followed by chemotherapy to treat distant spread or metastasis. With surgery alone, most dogs survive 4-6 months, which can be increased to 10-12 months with additional chemotherapy.
Mast cells are immune cells found throughout the body that play an important role in allergic reactions. Most mast cell tumors are found on the skin and may be detected by a sudden swelling or growth. The mast cells contain small granules that make them fairly easy to diagnose with a simple needle aspirate in your veterinary office. Boxers and bulldogs are more frequently diagnosed with Mast Cell Tumors compared to other breeds.
Treatment for mast cell tumors depends on the location of the tumor, and how the tumor looks when removed and biopsied (called the grade of the tumor). Sometimes the recommendation is surgery alone, and other times chemotherapy is recommended. Mast cell granules contain histamine, which can be irritating to your dog’s intestinal tract, and your veterinarian may recommend long term treatment with antihistamines such as Benadryl to counteract this problem.
This tumor has an unusual pattern of growth, therefore your veterinarian will need to remove a much larger area of skin surrounding the tumor than you expect, due to small projections or tendrils of the tumor that grow within the skin.
Melanoma, or malignant melanoma, is a tumor made of pigmented or dark skin cells that can be found anywhere on the dog’s body. Any dog can be affected, but dogs with dark skin or hair coats, such as the Scottish Terrier or Doberman Pinscher, are more frequently diagnosed. Melanomas behave differently depending on what part of the body they are found. Your veterinarian will remove the tumor and submit the sample for analysis, called histopathology, that can give information about prognosis and help to guide treatment. Tumors located in the mouth and distally on a limb, such as on the foot or toes or that have spread to lymph nodes, are usually associated with a worse outcome or prognosis.
Historically, treatment recommendations for melanoma included surgery, radiation therapy, and chemotherapy. In the past few years, a vaccine has been developed that has shown much promise in providing another strategy for control of the tumor after surgical removal. (http://www.veterinarypracticenews.com/vet-dept/canine-practice/canine-melanoma-vaccine-gets-conditional-ok.aspx)
Squamous cell carcinomas can develop on the skin and inside the mouths of dogs. Location of squamous cell carcinomas is strongly associated with survival times in the dog. Growths on the skin are often found early by owners and respond very well to surgery alone with little likelihood--less than 20%—of spread to other locations. Tumors within the mouth that affect structures like the tonsils or tongue are very difficult to remove and can grow quite large prior to being detected. For this reason, oral squamous cell carcinomas are considered very aggressive and less than 10% of dogs with tumors in these locations survive to 1 year after diagnosis.
Tumors of the mammary glands are the most common tumor seen in unspayed female dogs. They can affect any of the mammary gland and may behave locally and respond well to surgery, or more aggressively with ulceration or open wounds of the mammary gland and rapid tumor growth. Forty to fifty percent of these tumors are malignant, meaning they have spread to other locations—primarily the lungs or lymph nodes—at the time of diagnosis. Treatment usually involves surgery of the affected gland, or removal of the entire chain of mammary glands on the affected side along with the lymph nodes. Chemotherapy is recommended in cases where the tumor has spread to other areas. Factors that have been associated with poorer outcomes include large tumor size, spreading to lymph nodes, and the grade (based on histopathology results) of the tumor.
Apocrine gland carcinomas are also known as carcinomas of the anal sac in the dog. Approximately 50% of these tumors are diagnosed by your veterinarian during a routine rectal exam (part of a complete physical exam) and show no apparent clinical signs. They can also be diagnosed if your dog exhibits increases in blood levels of calcium, as the tumor secretes a hormone-like substance that causes hypercalcemia (elevated calcium).
Anal Sac Carcinomas are described as locally invasive, meaning they grow and invade tissues around the anal gland. This can make complete surgical removal difficult. If the tumor cannot be completely surgically removed, your veterinarian may recommend radiation therapy after surgery, which has been shown to be very effective in reducing the size of the tumor. If the tumor has spread to other areas (such as lymph nodes or lungs), chemotherapy may also be recommended. In some cases, the tumor has grown very large or an owner may not wish to pursue aggressive treatments. In this case, reduced doses of radiation therapy called palliative radiation therapy may be used to shrink the tumor enough to relieve signs of pain or discomfort for a period of time.
Dogs that have surgery alone for apocrine gland carcinoma usually survive 1 year, while additional treatments can increase this to 18 months.
Transitional cell carcinoma is the most common tumor of the lower urinary system (bladder and urethra) in the dog. This tumor is considered locally invasive and is moderately to highly likely to metastasize to another area. Much like the tumors of the anal gland, surgical options are dependent on the location and often these tumors cannot be completely removed. In such cases, chemotherapy or even anti-inflammatory drugs may be recommended. Occasionally, in certain cases, radiation therapy may also be recommended. Signs of Transitional cell carcinoma also depend on the location. Tumors of the bladder are often associated with blood in the urine. Tumors below the bladder, i.e., the urethra, are associated with straining or urinary obstruction.
Soft tissue sarcomas are a group of several different types of tumors that share similar characteristics. These tumors are made of connective tissue and are located either within the skin, or in tissues just below the skin. Owners often find these tumors when casually petting or grooming their dogs. Tumors that are included in this group are fibrosarcomas, hemangiopericytomas, liposarcomas, leiomyosarcomas, and nerve sheath tumors.
Soft tissue sarcomas are locally invasive, spreading by small tendrils of cancerous cells into adjacent normal tissues. Your veterinarian will surgically remove these tumors and submit them to the lab for evaluation (called histopathology). Histopathology will determine the grade of the tumor, which can predict if the tumor is likely to spread to other locations. As a group, soft tissue sarcomas have a 10-40 % chance of spreading to other areas of the body.
Treatment depends on the location of the tumor and whether there has been metastasis at the time of diagnosis. Surgery, chemotherapy, and radiation therapy have all been used to treat this tumor type.
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