Cancer in Dogs & Cats

Originally published on 10/14/2022. Revised 8/25/2025. Written by Lizz Caputo. Reviewed by Dr. Sperry, DVM, Medical Director 

We don’t like to imagine our pets getting sick, but cancer is a reality that affects many dogs and cats. In fact, one in four dogs will be diagnosed with cancer during their lifetime.

Cancer happens when a pet’s cells start growing and dividing in an uncontrolled way. Instead of stopping when they should, these cells keep multiplying, forming lumps (tumours) or spreading through the blood or lymph system to other parts of the body. 

Cancer can affect nearly any part of the body, including the skin, bones, blood, or internal organs. Some cancers grow slowly, while others progress quickly and can make pets very sick. Early detection and veterinary care give pets the best chance for treatment and more time enjoying life together. 

While the topic is difficult, understanding the risks, signs, and treatment options can help you make informed decisions and support your pet through their health journey. 

What Causes Cancer in Dogs and Cats? 

Several factors can influence whether a dog or cat may develop cancer: 

Genetic and Breed-Related Risk Factors 

Just like in humans, genetics play a role. Some pets may be predisposed to certain types of cancer based on their lineage. Certain breeds, such as Golden Retrievers and Boxers, are more likely to develop cancer. 

Age and Environmental Influences 

As pets age, their cells become more vulnerable to mutations, increasing the likelihood of cancer. Pets over age 10 have the highest likelihood of developing the disease. Exposure to toxins like cigarette smoke and certain chemicals has also been linked to cancer in pets. 

Other Contributing Factors 

While some pet parents worry about food as a cause, there’s little scientific evidence to support this. Cancer is typically multi-factorial, meaning it can arise from a combination of influences. 

Common Types of Cancer in Dogs and Cats 

Cancer can affect nearly every organ, but some types are more prevalent: 

Lymphoma 

Lymphoma is a type of cancer that starts in the lymphocytes, the white blood cells that help your pet’s immune system fight infections. Because lymphocytes are found throughout the body—including the lymph nodes, spleen, liver, intestines, and bone marrow—lymphoma can show up in different ways. 

In dogs, it often appears as swollen lymph nodes that you can feel under the jaw, in front of the shoulders, or behind the knees. 

In cats, lymphoma usually affects the intestines, which can lead to vomiting, diarrhea, weight loss, or a decreased appetite. 

While lymphoma is a serious condition, treatments are available that can help pets enjoy more quality time with their families. 

Mast Cell Tumours 

Mast cell tumours are a type of cancer that develop from mast cells, which normally help your pet’s immune system respond to allergies and inflammation. 

In dogs and cats, these tumours usually show up as lumps or bumps on or under the skin, though they can also form inside the body, for example in the spleen or intestines. 

Some grow slowly and stay in one place, while others can be more aggressive and spread to other organs. With the right diagnosis and treatment, many mast cell tumours can be successfully managed—and some can even be cured—helping pets enjoy more happy, healthy time with their families. 

Osteosarcoma 

Osteosarcoma is a fast-growing bone cancer that usually affects the long bones in dogs’ legs and occasionally in cats. It can cause swelling, pain, and difficulty walking, and may spread to the lungs or other areas. 

While treatment usually involves surgery and chemotherapy, the main focus is on keeping your pet comfortable and helping them enjoy more time with you. 

Hemangiosarcoma 

Hemangiosarcoma is a fast-growing cancer that develops in blood vessel cells and most often affects a dog’s spleen, heart, or liver. It can be sneaky, sometimes only discovered after sudden internal bleeding. 

Unfortunately, internal forms of this cancer have a poor outlook, while skin tumours are less common and can often be treated successfully if fully removed. In cats, the disease is rare but just as aggressive. Early detection and prompt treatment give the best chance of helping your pet. 

Mammary Gland Tumours 

Mammary tumours can be either harmless or cancerous. Malignant mammary tumours are common in female dogs and cats, especially if they weren’t spayed before their first heat.  

In dogs, around half of these tumours are malignant, and the outlook depends on size, spread, and early detection. Small tumours caught early and removed surgically can sometimes be cured, while larger ones may return or spread. 

In cats, most mammary tumours are malignant (around 80 to 90 percent) and can grow and spread quickly, so the prognosis is more cautious even with surgery. Early spaying and prompt veterinary care are key to prevention and improving outcomes. 

Oral Melanoma 

Oral melanoma is a fast-growing, aggressive cancer that appears in a dog’s mouth, often on the gums, tongue, or lips. It can invade nearby bone and tissue and spread to lymph nodes and lungs. 

Unfortunately, the outlook is generally poor, especially for larger tumours or those already spreading. Surgery can help manage the tumour locally, and treatments like radiation or immunotherapy may extend survival, though long-term control is challenging. 

In cats, oral melanoma is rare but just as aggressive. Early detection and treatment give pets the best chance at a better outcome. 

Early Warning Signs of Cancer in Pets