Sarcoma Cancer in Dogs

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Two vets standing beside large dog
Two vets standing beside large dog (Image: Tashi-Delek/iStock/Getty Images)

To be told that your pet has cancer is devastating -- but not necessarily a death sentence. Since sarcoma cancers are five times more common in dogs than people, doctors have learned a lot from our best friends, and every time medical knowledge increases, so does the efficacy of treatments available. “Fighting sarcoma is like slogging through mud," says veterinary oncologist Stephen Withrow, founding director of the Flint Animal Cancer Center at Colorado State University, the largest facility of its kind in the world. "It’s not a sprint. There are no home-runs but we’ve made progress.”

Sarcoma vs. Carcinoma: Differences

The clinical terminology applied to categories of cancer is the same for dogs and humans. The primary difference between the groups of cancers known as carcinomas and sarcomas is the nature of the cells and tissues where disease originates. Cancers that affect "epithelial" or membrane tissues, such as the lining of the colon, breast, lung or prostate, are carcinomas. Cancers affecting "mesenchymal" or stem cells, which start out the same but differentiate into the different kinds of muscle, fat, bone and connective tissue found throughout the body, are sarcomas. The Latin prefix applied to sarcoma identifies the tissue of origin. For example, osteosarcoma is bone cancer, while lymphosarcoma involves lymphocytes, cells generated by the immune system.

Soft Tissue Sarcomas

Sarcomas originating in soft tissue such as skin, muscles, organs and fat can be difficult for vets to diagnose because different types have similar characteristics, says the National Canine Cancer Foundation. Though these cancers most commonly affect skin and subcutaneous tissues, they may originate anywhere on a dog's body. Typically, the first sign is a soft mass or lump that may look and feel self-contained but isn't. In reality, sarcomas are aggressive and can spread quickly to affect other mesenchymal cells in the original area and beyond. Larger breeds, especially golden and Labrador retrievers, are at higher risk for developing sarcomas.

Symptoms, Treatment and Outlook

In addition to a lump, other symptoms of soft tissue sarcomas vary depending upon the location of tumors. Hypoglycemia -- abnormally low blood sugar -- and weight loss are common, says the NCCF. With tumors originating in the abdomen, urogenital and gastrointestinal tracts, vomiting, diarrhea and black feces caused by internal bleeding are also often noted. With treatment, the prognosis for dogs with sarcomas is encouraging. Though surgical removal of the tumors is always recommended, affected limbs don't necessarily need to be amputated. With surgery alone, the median survival time is almost four years. With a combination of surgery and radiation therapy, dogs can survive longer than six years. However, a third of sarcoma-affected dogs will eventually die from causes related to the cancer.

Osteosarcoma: Bone Cancer

Osteosarcoma, or OSA, is the most common bone cancer in dogs, notes the National Veterinary Cancer Registry, accounting for about 85 percent of tumors originating in the skeletal system. While it can affect dogs of any size and breed, OSA occurs more often in middle-aged large and giant breeds. Lameness in the affected leg is typically the first sign, frequently accompanied by painful swelling. These aggressive tumors spread so fast that according to Washington State University's College of Veterinary Medicine, at time of diagnosis, 90 to 95 percent of dogs will have more than one OSA tumor, even though these metastases -- new tumor sites -- might not yet be evident.

OSA Treatment and Prognosis

Surgical amputation of OSA-affected limbs combined with chemotherapy to treat the metastatic disease produces "fair to good" results, says WSU. Although amputation is a wrenching decision for most pet owners, "dogs can function on three legs much better than most owners think," WSU assures. About half the dogs treated with surgery and chemo will still be alive a year after diagnosis; a quarter for two years; and some will be cured. Without chemo, amputation may relieve pain but owing to metastases, the dog's life expectancy will only be about six months. For dog owners who opt for palliative care to keep their pets as comfortable as possible for the time they have left, radiation helps relieve pain in 70 to 80 percent of cases. When OSA is found in other bone locations, vets can treat it with a combination of surgery, chemo and radiation.

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