What is Cancer?

By Ryan Llera, BSc, DVM; Debbie Stoewen DVM, MSW, RSW, PhD; Christopher Pinard, DVM


What is cancer?

The term cancer refers to diseases characterized by uncontrolled cell growth. These diseases most often result in an abnormal mass of tissue (tumor) without a purpose, although there are forms of cancer that do not cause tumors. Cancer cells grow faster than normal cells, are uncoordinated, and often continue to grow if treatment is not pursued.

There are many different types of cancer; any cell in the body has the potential to become cancerous. Cancers are classified based on the tissues (e.g., muscle, skin, organ) from which they arise. Tumors can be either benign (non-cancerous) or malignant (cancerous). Benign tumors grow in a local area in a uniform, smooth expansion, while malignant cancers may invade the surrounding tissues or spread even farther. One of the distinctive characteristics of a malignant tumor is its ability to metastasize. This is when cancer cells leave the primary site and travel to other locations throughout the body, binding to tissues unrelated to the original cancer site.

There is no good answer as to why some pets develop cancer. About one in four dogs will develop cancer and this rate is similar for cats. Cancer is the leading cause of death in older pets.

What causes cancer?

Cancer is the result of genetic damage to cells. Cells contain genes (DNA) that tell them when to grow, divide, work, and die. Normal cells follow an orderly process and are replaced when they get old and die. This process can be interrupted by factors such as radiation, chemicals, hormones, or infections that cause cells to become abnormal. Cancer develops when abnormal, old, or damaged cells survive and replicate when they should have died and been replaced by normal cells. If abnormal cells divide and multiply without stopping, they can form tumors. Cancers can also release cells in the blood where they circulate freely through the body.

All mammals have immune safeguards to prevent or repair cellular damage. However, these protective mechanisms are not perfect. Some individuals have defects in their defenses, resulting in a higher-than-expected incidence of cancer. Some of those defects are present at birth; for example, some purebred dogs have inherited tendencies to develop specific forms of cancer. Other defects develop over time. In some instances, the protective mechanisms are unable to cope with or repair excessive damage from external (i.e., environmental) factors. No matter the defect, all result in mutations.

Why has my pet developed cancer?

There are many factors that can influence the development of cancer but, just as in humans, it is impossible to truly know why some individuals develop cancer while others do not.

In some cases, exposure to known carcinogens (cancer-causing agents) such as sunlight, chemicals, and cigarette smoke can play a factor. In other cases, especially with certain breeds (Golden Retrievers, Dobermans, German Shepherds, Bernese Mountain Dogs, and Boxers to name a few), there is a genetic susceptibility to cancer. Some cancers are causally linked to obesity, infection, and inflammation, and some need hormones to develop and persist. If cells undergo more and more divisions as the pet ages, the possibility of a mistake (mutation) occurring during these cell divisions increases. The result is that the incidence of cancer increases with age.

"Some cancers are causally linked to obesity, infection, and inflammation..."

Is cancer transmissible to other pets?

In the vast majority of cases, the answer is no. The most notable exception is a transmissible venereal tumor in dogs. However, there are some viruses and other microorganisms that can cause cancer in animals. Animals may become infected with one of these agents from their mother before or during birth, through direct contact with other animals of the same species, or through the bites of vectors (carriers) such as fleas or ticks. If your pet is infected with a specific transmissible cancer, your veterinarian will advise you about the steps you must take to prevent your pet from infecting other animals.

What are the clinical signs of cancer?

The most obvious sign of most cancers is a lump that continues to enlarge. This lump may ulcerate, bleed, or cause other physical effects (e.g., pressure and displacement of the surrounding tissues). Because cancer can develop in any cell in the body, internal cancers can be more difficult to spot. Unexplained, chronic weight loss can be an important sign of cancer. Other signs of cancer can be abnormal bleeding, unexplained vomiting or diarrhea, swollen lymph nodes, coughing, and lameness. Bad breath or a change in eating patterns can be a sign of oral cancer.

How is cancer diagnosed?

Your veterinarian may suspect that your pet has cancer-based on clinical signs. Radiographs (X-rays) may be useful in detecting internal tumors and metastases. Blood tests can help detect some tumors. To identify most tumor types, it is necessary to obtain a sample of the tumor. Depending on the type of tumor suspected, your veterinarian may obtain this sample through a fine needle aspiration (FNA), a tissue biopsy, or full removal of the tumor. In some cases, exploratory surgery or guided ultrasound may be needed.

In many cases, the simplest approach is FNA (suction removal of tumor cells with a syringe and needle) of the tumor for cytology (microscopic examination). FNA does not require general anesthesia or surgery. Some tumors can be accurately diagnosed with cytology. In most cases, however, a biopsy sample of the tissue must be examined to reach an accurate and reliable diagnosis. Your veterinarian will send the sample to a specialized laboratory for examination by a veterinary pathologist. The microscopic examination of tissue is called histopathology. The histopathology report typically indicates whether a tumor is benign or malignant.

Malignancy is often described by tumor names ending in carcinoma or sarcoma. These, together with the origin or type of tumor, the grade (how aggressive it looks under the microscope), and stage (how far it has spread), indicate how the cancer is likely to behave. If your pet is diagnosed with cancer, your veterinarian may refer you to a veterinary oncologist (a doctor who specializes in the diagnosis and treatment of cancer).

What types of treatment are available?

The most common and often most effective treatment is the surgical removal of the tumor. Other treatments may be considered for tumors that are too big or too numerous to be removed, are in inaccessible locations, and for non-tumor types of cancer (i.e., leukemia). These treatments include chemotherapy, immunotherapy, and radiation therapy and are typically performed at specialty centers.

"Approximately 85% of animals who undergo chemotherapy will not develop side effects from it."

Chemotherapy, immunotherapy, and radiation are not suitable for all types of cancer and may have significant side effects. Approximately 85% of animals who undergo chemotherapy will not develop side effects from it. Those that do, however, can usually be treated symptomatically with medications, and the symptoms are usually short-lived. Less than 5% of pets treated with chemotherapy will need to be hospitalized for adverse effects. Most pets that feel sick before chemotherapy will feel better after they receive treatment (especially pets with lymphoma).

There are many issues to be considered in the decisions around cancer treatment and your veterinarian or veterinary oncologist will discuss these with you.

Can cancer be cured?

There are instances, with malignant as well as benign tumors, when surgery provides a cure. In most cases of malignant cancer, however, a permanent cure is not necessarily possible or even the goal. Instead, the goal is to provide the best quality of life for your pet for as long as possible. The treatment should never be worse than the disease. Some pets, even those with more aggressive cancers, may experience long remissions (disappearance of detectable cancer) with treatment, but the cancer eventually comes back.

Our understanding of cancer in animals is increasing and as our understanding of the disease improves, our ability to control or cure it will also improve. If your pet is diagnosed with cancer, be sure to learn as much as you can about it. Although there might not be a cure, there may be many options for care.

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