What does cancer look like in dogs?

Updated February 21, 2017

Cancer is common in dogs. Because cancerous tumours can grow on virtually any part of the dog--including bones, blood, skin and organs--it can look like many other conditions or illnesses. Understanding the types of cancer that may be common to your dog's breed can help you know what to look for. Although symptoms of cancer can be ambiguous, your vet should know the clinical signs.


There are dozens of canine cancers. Some of the more common include leukaemia, which is a cancer of the bone marrow and white blood cells, and lymphoma, which can spread rapidly through the dog's lymphatic system. Bone cancer, common in some large and giant-breed dogs, attacks bones and joints and sometimes originates in connective tissue such as ligaments and cartilage. Dogs are much more susceptible to skin cancer than humans. There are also cancers specific to all the organs, including the thyroid gland, kidneys, lungs and reproductive organs. Symptoms are different for each type of cancer.


Although most cancer symptoms can easily be confused for other conditions, Dr. Lisa Carlson writes in the "Dog Owner's Home Veterinary Handbook" of warning signs that should not be ignored: sudden changes in urinating or defecating, and changes in eating or drinking habits; rapid weight loss, especially if the dog is still eating normally; lumps and swellings that don't go away or are growing, and sores that don't heal; and changes in energy level, such as sleeping too much, unwillingness to play or exercise and lethargy. Don't ignore signs of pain, such as limping or whining. Dogs in pain may also become aggressive. Brain cancer can also change temperament, or affect eyesight and hearing.

Understand Breed Risks

Mixed-breed and purebred dogs are at risk for cancer. Some breeds are very susceptible to particular cancers, and knowing your dog's breed (or predominant breed, if mixed) risk can alert you to symptoms to heed. Research the breed online; most breed club websites have information on breed health (see Resources below).

Size, Age and Reproductive Status

Generally, middle-aged and older dogs are more likely to get cancer. Intact females are prone to mammary cancer. The first symptoms will be lumps along her teats. Bone cancer affects almost exclusively large and giant-breed dogs. Look for sudden limping or swelling on the legs, jaw or skull. Lymphoma and organ cancers usually make a dog unwilling to eat and lethargic. Sores that don't heal may indicate skin cancer.


There is no single test that generally detects cancer. A basic blood chemistry test showing high levels of white blood cells, excess calcium in the blood or organ malfunction may indicate the possibility of cancer, but this isn't a definitive test.

Some benign tumours (called fatty lymphomas) are common in older dogs. These are soft lumps and can be pea-sized to golf ball-sized. They look the same as some malignant tumours but usually are easy to move under the skin, whereas malignant tumours feel more firmly attached. Your veterinarian can do a simple needle biopsy and examine the cells under a microscope to determine if it's cancerous.

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About the Author

Stevie Donald has been an online writer since 2004, producing articles for numerous websites and magazines. Her writing chops include three books on dog care and training, one of which won a prestigious national award in 2003. Donald has also been a painting contractor since 1979, painting interiors and exteriors.