Home » Pet Health » Cancer 101 » What is Cancer and How Cancer Cells Form


Cancer is a disease characterized by the uncontrolled division of abnormal cells that can affect the functions of normal cells and tissues. While cancer is mainly known to be associated with humans, our beloved pets also develop cancer.

By improving our pets’ living environment and diet, we have increased their lifespan. However, this has also led to an increased risk of cancer development. For example, several studies from across the globe demonstrate that, as dogs age, their chances of forming cancer increase. But researchers indicate that cancer risk also depends on the dog’s breed, size, and gender. Similar risk factors exist for cats, but common cancer types may differ between dogs and cats. No matter the cancer type, the science behind the formation of cancer cells is the same in cats and dogs. 

How do cancer cells form?

Cancer cells arise due to changes in the genetic machinery within the cell, making cancer the most common of genetic diseases. The changes may occur in one cell or a group of cells and can result from one or more of the following:

    • Errors that happen while a cell divides
    • Damage to DNA because of harmful substances in the environment (like smoke or sun exposure)
    • Inherited genetic defects

Every type of cancer begins with one or more gene changes, resulting in mutations. These mutations can change the workings of the cell, preventing it from producing its normal proteins or leading to the formation of abnormal proteins. Typically, the body will eliminate damaged cells. But when the affected proteins are involved in either cell growth or death, the mutations can make the cells resistant to naturally occurring death and cause the cells to multiply uncontrollably. Unregulated cell growth is more likely as the body ages and the mechanisms to eliminate the damaged cells weaken.

Rapid and uncontrolled cell growth can lead to a mass of abnormal cells called a tumor (although this is unlikely in blood cancer). Tumors may be benign (non-cancerous) or malignant (cancerous). A benign tumor can grow but cannot spread to other body areas. In contrast, a malignant tumor can break off from the primary site and spread to other places in the body in a process called metastasis. Cancer can develop in almost any body area, including in the blood and tissues such as muscle, bones, fat, and nerves.

Despite the incidence of cancer, careful attention to your pet’s health and behaviors, and regular check-ups with your vet, will ensure an early diagnosis and proper care for your pet.

The Pet Cancer Foundation’s Website Editorial team is comprised of veterinarians, veterinary oncologists, and veterinary technicians, as well as scientific writers and editors who have attained their PhD’s in the life sciences, along with general editors and research assistants. All content found in this section goes through an extensive process with multiple review stages, to ensure this extended resource provides pet families with the most up-to-date information publicly available.

The team listing of those contributing to the information on this page is here:

Keep Your Pets Healthy Editorial Team

Last Updated: September 11, 2022

The Pet Cancer Foundation’s medical resource for pet owners is protected by copyright.

For reprint requests, please see our Content Usage Policy.

The Pet Cancer Foundation’s Medical Illustration team is comprised of medical illustration specialists and graphic designers that work in consultation with our team of experts to create the medical art found throughout our website. Though not all medical concepts require the assistance of imagery, when a page does contain a medical illustration, credit to the artist and our medical art director will be noted here.

The Pet Cancer Foundation’s medical imagery is protected by copyright and cannot be used without prior approval that includes a mutually signed licensing agreement. Please review our Content Usage Policy.

The following sources were referenced to write the content on this page: 

Blackwood L 2013, ‘Cats with cancer: where to start’, J Feline Med Surg, vol. 15, no. 5, pp. 366-377.

Bonnett, BN, Egenvall, A., Hedhammar, Å. & Olson, P 2005, ‘Mortality in over 350,000 insured Swedish dogs from 1995–2000: I. Breed-, gender-, age- and cause-specific rates’, Acta Vet Scand, vol. 46, pp. 105–120. 

Diaz-Moralli, S, Tarrado-Castellarnau, M, Miranda, A & Cascante, M 2013, ‘Targeting cell cycle regulation in cancer therapy’, Pharmacol Therap, vol. 138, no. 2, pp. 255-271.

Fleming, JM, Creevy, KE & Promislow, DEL 2001, ‘Mortality in North American dogs from 1984 to 2004: an investigation into age- size- and breed-related causes of death’, J Vet Intern Med, vol. 25, no. 2, pp. 187–198. 

Inoue, M, Hasegawa, A, Hosoi, Y & Sugiura, K 2015, ‘A current life table and causes of death for insured dogs in Japan’, Prev Vet Med, vol. 120, no. 2, 210–218.

O’Neill, DG, Church, DB, McGreevy, PD, Thomson, PC & Brodbelt, DC 2013, ‘Longevity and mortality of owned dogs in England’, Vet J, vol. 198, no. 3, 638–643. 

Stratton, MR, Campbell, PJ & Futreal, PA 2009, ‘The cancer genome’, Nature, vol. 458, no. 7239, pp. 719-724.

Tanaka, M, Yamaguchi, S & Iwasa, Y 2020, ‘Enhanced risk of cancer in companion animals as a response to the longevity’, Sci Rep, vol. 10, no. 1, pp. 19508. 

Ujvari, B Klaassen, M Raven, N Russell, T Vittecoq, M Hamede, R Thomas, F & Madsen, T 2018, ‘Genetic diversity, inbreeding and cancer’, Proc Biol Sci., vol. 285, no. 1875, pp. 1-8.

The Pet Cancer Foundation’s medical resource for pet owners is protected by copyright.

For reprint requests, please see our Content Usage Policy.