Top 5 Questions from Owners of Pets with Cancer

1. What caused my pet’s cancer?

The short answer to this question in many cases is "We don’t know." I recognize this is a heated question in veterinary medicine and that owners are inundated with theoretical causes of cancer (in people and an animals) in the media, in print, and on the Internet.

In general, the best answer I can give is that cancer results from a combination of genetic and environmental factors. Evidence for a genetic cause of cancer in animals is supported by examples of breed predispositions to certain tumor types. There are also heritable forms of cancers that result from mutations in sperm and egg cells.

The majority of genetic alterations leading to cancer occur because of spontaneous mutations. These mutations may occur as a result of chronic exposure to known cancer-causing substances (e.g., sunlight or chemicals).

Environmental causes of cancer have been established in veterinary patients, but I think it’s very important to recognize how difficult it is to truly prove causality when it comes to tumor development and environmental factors. Although we often do not know the underlying cause of the cancer, advances in surgical, medical, and radiation oncology allow us the opportunity to provide treatment options for owners and help their pets live longer as a result.

2. Will performing an aspirate/biopsy cause the cancer to spread/become more aggressive?

Though tumor cells can disseminate into the bloodstream during surgical manipulation, the ability of these cells to actually arrest within a distant anatomical site and grow into new tumors is poor and, fortunately, most circulating tumor cells are rapidly destroyed by the host’s immune system.

Pretreatment biopsies are typically recommended in order to obtain a diagnosis prior to making more definitive treatment recommendations. Exceptions would include cases where the biopsy procedure is associated with a high degree of morbidity (e.g., biopsy of the brain/spinal cord), or when knowing the tumor type would not change the choice of therapy (e.g., biopsy of a splenic mass or primary lung tumor).

3. Will my pet become sick from chemotherapy?

The goal of veterinary oncology is to preserve quality of life for as long as possible while imparting the least amount of adverse effect to the patient. In general, approximately 25 percent of all animals receiving chemotherapy will experience some sort of side effect.

This generally entails what are considered mild and self-limiting gastrointestinal upset and/or lethargy during the first few days after treatment. If side effects should occur, they are usually well controlled using over the counter or prescription medications.

Approximately five percent of chemotherapy patients will have severe side effects that require hospitalization. With appropriate management, the risk of these side effects causing the death of a patient is less than one percent.

If a patient experiences serious side effects, the dose of chemotherapy is reduced to avoid similar complications in the future. In general, the quality of life for patients receiving chemotherapy is excellent. Studies have indicated that the majority of owners are happy with their decisions to pursue treatment for their pets and with the outcomes, and would elect to pursue treatment again after seeing how well their animals did during therapy.

4. Does my pet’s age factor in to his/her ability to withstand treatment with chemotherapy/radiation/surgery?

Cancer is a disease of older animals, and most of the information available for how pets will respond to treatment, risk of side effect, and outcome are based on studies where the average age of patients is in the geriatric (>10 years) range. Every precaution is made to be sure that patients are healthy enough to undergo treatment prior to instituting therapy, which is the rationale behind the recommendation to perform baseline staging tests and lab work.

These tests will ideally allow us to know everything about a cancer patient from nose to tail before starting treatment, and can help us better predict outcomes, side effects, and even tailor the treatment plans. The age of the patient typically does not factor in nearly as much as their overall health status does.

5. Can my pet be around family members or other animals while undergoing treatment?

In general, while a pet is receiving chemotherapy, it is considered safe for that animal to interact with all family members. Depending on the chemotherapy drug(s) that the pet is receiving, there may be certain times after a treatment that the pet would be considered at a higher risk for picking up an infection, so precautions may be necessary during a very specified time period.

For oral chemotherapy drugs administered at home, it is important that the capsules or pills are kept out of the reach of children. Individuals who are pregnant, trying to become pregnant, nursing, or considered immunocompromised should not handle chemotherapy drugs. We recommend that owners wear non-powdered latex or nitrile gloves when handling chemotherapy drugs and that the person handling the drugs washes their hands afterwards. It is very important never to split or crush drugs, or open the capsules, as this can increase the risk of exposure.

Metabolites of chemotherapy drugs are present in urine and/or feces for up to 72 hours after an animal has been treated. Dogs should be walked away from public areas during this time period. Gloves should be worn when handling an animal’s feces, litter, vomit, etc. Hands should be washed thoroughly after handling any potentially contaminated fluids/waste.



This entry is paraphrased from a lecture designed for veterinary students. It was originally written by one of my mentors, who probably would rather remain anonymous, but who somehow manages to come through in my voice more often than would be expected.

Dr. Joanne Intile

Image: foto ARts / via Shutterstock

Help us make PetMD better

Was this article helpful?

Get Instant Vet Help Via Chat or Video. Connect with a Vet. Chewy Health