Yellow lab laying on exam table

Cancer Treatments for Cats and Dogs


Cancer is just as prevalent a disease in our pets, as it is in us. In fact, it is the leading cause of disease related deaths in both dogs and cats. The cause of many forms of cancer remains as illusive today as ever, but environment (in the broadest sense) and genetics are the two key factors. While we have little control over genetics, we do have some control over the environmental factors. This being said, the genetic predisposition to cancer cannot be overstated, as demonstrated by the high incidence of cancer in specific breeds, such as the Golden Retriever and Boxer which exceeds 60%.

Although new ongoing research is uncovering novel treatment options with exciting prospects, not all treatment methods are successful. With many of the contributing factors being beyond our control, the focus on cancer is usually aimed towards early diagnosis.

When cancer is detected in a cat or dog, multiple methods of treatment are often used in an effort to have the most successful outcome. The mainstays of cancer treatment in pets are surgery, chemotherapy, radiation therapy and gene therapy.


The goal of surgery is ideally to remove all of the cancer. However, this often isn’t achievable either due to the accessibility of the cancer to the surgeon or because it’s already spread to other parts of the body. Even if the surgery was unable to remove all of the cancer, the decreased size or number of the tumour(s) might make additional treatment methods more successful. In pets, even if the tumour seemed isolated, such as when a large haemangiosarcoma in the spleen is removed along with the entire spleen, additional treatment is likely to be recommended.

After surgical removal, what is used next depends upon the actual tumour type and location. Some cancers are more open to radiation, like mast cells, and others are better treated with chemotherapy, like certain forms of lymphoma.


Chemotherapy essentially is a process whereby very toxic drugs are introduced (usually injected) into the body with the intent of targeting and killing the rapidly growing cancer cells. The challenge is that these drugs aren’t as specific as we would like and there can be significant collateral damage when healthy cells are killed as well. Consequently, chemotherapy can be quite hard on some patients, depending upon the patient and the drugs used. Ancillary drugs, like prednisone, are often used in conjunction with the chemotherapy drug to offset some of the side effects, such as loss of appetite or inflammation secondary to cell death.

In both dogs and cats, multiple chemotherapy sessions are usually required over many weeks and most veterinarians will refer these patients to a veterinary oncologist. The main reason is due to the very strict protocols that need to be followed due to the toxicity of the drugs themselves and to minimize the risks to the veterinarians and staff who are handling and administering these drugs. 

Radiation Therapy:

The next line of attack, again depending upon cancer type, is radiation therapy. All such cases require a referral, usually to a tertiary veterinary facility, such as a teaching hospital. Radiation also requires multiple treatments over weeks and can be very effective. The challenge, as with chemotherapy, is collateral damage to surrounding healthy tissue. However, there are a small number of facilities that have access to a linear accelerator. This high tech machine takes a 3D image of the entire tumour and then emits a radiation beam that accurately follows the 3D image and only destroys the cancer cells, with very little damage to healthy tissue. Fortunately, this technology is available in Canada at the Ontario Veterinary College’s Animal Cancer Centre in Guelph.

Gene Therapy:

The broad field of gene therapy is in its relative infancy in veterinary medicine, but is showing some exciting promise and involves immunotherapy, oncolytic (cancer destroying) virotherapy and gene transfer. Because of the numerous similarities between cancers in people and pets, there is a great deal of collaborative efforts being conducted in this area of “translation” medicine.

  • Immunotherapy -

  • Immunotherapy uses genetically modified cells and viral particles to stimulate the immune system through the mobilization of protective antibodies to specifically destroy cancer cells. Encouraging results have been seen with a canine vaccine against melanoma. An additional application of the antibody approach is in diagnostics. They can be combined with radioactive isotopes, so that when they attach to the tiniest cancer cells, otherwise too small to be detected, they can now be readily targeted.
  • Oncolytic Virotherapy - 

  • Oncolytic virotherapy introduces viral particles into the body, which target cancer cells and replicate within them causing their destruction. Great promise has been shown in treating cats with feline adenocarcinoma with this method.
  • Gene Transfer - 

  • Gene transfer essentially introduces a new genetic code into the cancer cell in order to destroy it. The concept seems simple and straightforward, but there are some obstacles to overcome before this can become a widely practiced clinical application. Recent research does show promise, though.

Cancer treatment in veterinary medicine has come a long way and exciting inroads are made every year. The challenge with any cancer treatment though, is that it is expensive which is why pet insurance should be a serious consideration for all pet owners. The long-term outcome of any treatment approach will vary with the type of cancer, how soon it was diagnosed, and the overall health of your pet. Which treatment option(s), if any, are best for your pet can only be determined by a frank and open discussion with your veterinarian. Such discussions can take their emotional toll on the whole family, but the overriding consideration has to be what is in the best interests of your pet.