By John Gilpatrick
A cancerous tumor is among the most devastating diagnoses a veterinarian will give to a dog.
That’s because cancer is both extremely common in dogs and a leading cause of death. The National Cancer Institute’s Center for Cancer Research says that about 6 million of the 65 million pet dogs in the United States will be diagnosed with cancer each year.
Additionally, in 2011, researchers at the University of Georgia College of Veterinary Medicine found that cancer was the most common cause of death in older dogs. (It’s also the leading cause of death for 71 of the 82 breeds studied.)
Erika Krick, DVM, an assistant professor of oncology at the University of Pennsylvania School of Veterinary Medicine says signs of cancerous tumors often include skin wounds that don’t heal or unexplained weight loss. That said, many dogs often have lumps and bumps that are completely benign. “If you notice something new, take your dog to the vet,” she says. “You need to know what it is, and the smaller it is when it’s diagnosed, the easier it is to treat.”
Not all tumors in dogs are cancerous, but all of them should be evaluated by your veterinarian. Continue reading for eight common and notable types of tumors in dogs, the breeds that are most susceptible, and what treatment looks like for each.
Mast Cell Tumors
Krick notes that mast cell tumors are one of the most common type of canine skin tumors. “These grow quickly and are usually red and very itchy,” she says.
That’s because the lumps contain a chemical called histamine, one of the substances responsible for itching associated with allergies. “Histamine tells the stomach to make more acid, so dogs with these tumors are also at risk for gastrointestinal ulcers,” Krick says.
Short-faced dogs—including Boxers, Pugs, and French Bulldogs—are most at risk for mast cell tumors. Typically, these breeds develop lower-grade, less aggressive tumors, while Chinese Shar-Peis are prone to very aggressive mast cell tumors. Unlike many tumors that are significantly more common in older dogs, there is a weaker correlation between age and mast cell tumor susceptibility.
Krick says treatment begins with a fine needle aspirate to retrieve a cell sample and diagnose what type of tumor you’re dealing with. Surgical removal follows. The tumor(s) should always be sent to a pathologist for grading (a measure of how aggressive the cancer is) to help determine the need for further treatment.
Christine Swanson, DVM, a veterinary medical oncologist and assistant professor at the College of Veterinary Medicine at Michigan State University, calls this benign fatty tumor very common and notes that many different breeds are prone to developing them. Lipomas usually feel like a relatively soft mass that can be moved around underneath a dog’s skin.
They vary in size, and most of the time, they’re not a serious issue. A fine needle aspirate is done to confirm the benign nature of the tumor, and the tumor is usually only removed if it’s bothersome to the dog’s normal movement or activity, Swanson says.
“Most dogs that eventually get diagnosed with this come in because they're limping,” Krick says. “It's not as common that a bone in the spine would be affected.”
An x-ray is conducted on the area in question to rule out things like arthritis. Sometimes a biopsy is necessary to differentiate osteosarcoma from other conditions that can look similar on x-rays. If cancer is diagnosed, amputation followed by chemotherapy is the treatment of choice, Krick says, though some dogs are candidates for a limb-sparing procedure. In these cases, only the affected area of the bone is removed, and either a bone graft or a metal rod replaces it.
“This is an option for tumors in the distal radius, or the lower bone in the front leg,” Krick says, although limb-sparing surgery can also be considered for osteosarcoma at other sites. “It’s an extensive procedure and lengthy recovery, but some dogs will struggle following the loss of a limb, so this represents a good alternative.”
These tumors develop through the immune system and are most prevalent in dogs three years of age or younger and in breeds including English Bulldogs, Scottish Terriers, Greyhounds, Boxers, Boston Terriers, and Chinese Shar-Peis.
“These tumors are usually benign and do not spread to other parts of the body,” Swanson says. “They will typically regress on their own within two to three months, but removal may be recommended for histiocytomas that are particularly bothersome to a pet.”
Histiocytomas are often referred to as “button” tumors, she adds, because they’re “frequently small (usually less than an inch), red, raised, and hairless.”
They can look very similar to plasma cell tumors (or plasmacytomas), though these are more common in older dogs and generally require surgery.
This cancer of blood vessels is most often found on the spleen, Krick says, because it has a big blood supply. “If and when it ruptures, the dog’s gums will get pale, its breathing will become labored, and it will have trouble getting up,” she says. Hemangiosarcomas can also develop on a dog’s heart and in the skin.
A definitive diagnosis is made by a pathologist who examines a sample of tissue from the tumor. This often occurs after surgery to take out the spleen and resolve the internal bleeding has been performed.
Chemotherapy follows surgery, Krick says, because metastasis (spread to distant sites in the body) is very common for this type of cancer. It’s most common in larger breeds like Golden Retrievers and German Shepherds.
“This is a form of cancer of the pigmented cells of the skin of dogs, and like melanoma in people, these tumors are typically black or dark brown,” Swanson says.
Many skin masses are benign, but those in the mouth and at the nailbed can be very aggressive, she adds. In the case of the latter, the toe is typically swollen and may be painful. Following an x-ray, it may be determined that the affected toe must be amputated in order to fully remove the cancerous mass.
The risks with this specific type of melanoma don’t end there. “It may metastasize to places such as the lymph nodes in the area and the lungs, liver, or other internal organs,” Swanson says. Once evidence of such metastasis has been identified, some combination of surgery, radiation therapy, and immunotherapy (a therapeutic vaccine for canine melanoma has been licensed by the USDA) is likely. Swanson says chemotherapy for canine melanoma is generally ineffective, as it is with human melanoma.
Lethargy, decreased appetite, and coughing may accompany swollen lymph nodes in dogs of all breeds with this type of cancer, although some individuals initially show little in the way of symptoms other than lymph node swelling. Krick says this swelling is most noticeable under the jaw, in front of the shoulders, and behind the knees.
A fine needle aspirate and/or tissue biopsy is done to reach a diagnosis. Then, a veterinary oncologist will conduct something called a staging test to determine where else in the body these cells might be, Krick says. The most common treatment is chemotherapy.
These benign tumors are warts in dogs, and Swanson says they can be uncomfortable and problematic. “When this infection develops, multiple hard, pale, cauliflower-like warts are noted typically on the lips, inside the mouth, and around the eyes,” she says. “The warts can be painful and severe infections can make chewing and swallowing difficult.”
Papillomas will go away after a few weeks, sometimes months—though if they’re causing major problems for the dog in question, they can and should be removed by a veterinarian, Swanson says.
These benign tumors are caused by a virus (called papillomavirus) that is transmitted by direct contact with an infected dog or contaminated objects like bedding or toys, Swanson says. While it’s best to keep affected dogs isolated from unaffected ones, the incubation period often lasts months, so by the time symptoms make themselves known, it might have already spread to other dogs in a household.
Lumps and bumps may signal cancer in pets. But there are other symptoms to watch for. Learn about 10 Signs of Cancer in Pets.