What is Cancer?
By Debbie Stoewen DVM, MSW, RSW, PhD; Christopher Pinard, DVM

What is cancer?cancer_cell_division_2018-01

The term cancer refers to a large number of 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 cancers; 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. It is important to note that tumors can be both benign (non-cancerous and usually non-life threatening) or malignant (cancerous and life-threatening if not treated). Benign tumors grow in a local area in a uniform, smooth expansion, but 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 (where the cancer began), travel to other locations throughout the body, and bind to tissues unrelated to the original cancer.

What causes cancer? Where does cancer come from?

Cancer is the result of genetic damage to cells. The body is made up of trillions of cells that group together to form tissues and organs such as the skin, blood, muscles, bones, heart, and lungs. 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. However, sometimes this process is interrupted by factors such as radiation, chemicals, hormones, or infections that cause cells to become abnormal. Age can also be a factor, although some abnormalities may arise spontaneously.

Cancer develops when abnormal, old, or damaged cells survive and replicate when they should have died and been replaced by normal cells. When abnormal cells divide and multiply without stopping, they can form masses or growths without a purpose, called tumors. Cancers can also release cells in the blood where they circulate freely through the body.

"When abnormal cells divide and multiply without stopping, they can form masses or growths without a purpose, called tumors."

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. Others develop over time. In some instances, the protective mechanisms are unable to cope with or repair excessive damage from external (e.g., environmental) factors. No matter the defect, all result in mutations.

Why has my pet developed cancer?

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

"There are many factors that can influence the development of cancer."

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, 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.

As cells undergo more and more divisions as a 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.

Although there are many factors that can influence the development of cancer or be viewed as a cause of cancer, there is no answer as to why those factors promote the growth of cancers in some pets and not in others.

Can my pet catch cancer from another animal? Can my pet transmit cancer to others?

In the vast majority of cases the answer to both questions is NO.

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.

Feline leukemia virus (FeLV), for example, can cause cancers of the blood and lymphoid system in cats. Occasionally, an infected queen will transmit the virus to her kittens before or during birth. However, FeLV is more commonly spread by close contact with infected cats that shed the virus in their saliva, urine, and feces. If your cat is infected, it can pass the infection to other cats.

If your pet is infected with a specific transmissible cancer, your veterinarian will advise you of this and you must take steps to prevent your pet from infecting other animals.


What are the 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). However, as cancer can develop in any cell in the body, internal cancers can be more difficult to spot.

Weight loss due to loss of body fat and muscle is common with malignant cancers. 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 (mouth) cancers.

How is cancer diagnosed?

Your veterinarian may suspect that your pet has cancer based on certain clinical signs (e.g., a lump, loss of appetite and/or energy, and weight loss). X-rays may be useful in detecting internal tumors, including 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, a tissue biopsy, or full removal of the tumor. In some cases, an exploratory surgery or ultrasound guidance may be needed.

"In order to identify most tumor types, it is necessary to obtain a sample of the tumor."

The simplest approach in many cases is the aspiration (suction removal) of tumor cells with a syringe and needle (fine needle aspiration). It does not require general anesthesia or surgery. Microscopic examination of the cells obtained is called cytology. 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 preparation and 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 see 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 surgical removal of the tumor. Other treatments may be considered for tumors that are too big or too numerous to be removed, in inaccessible locations, and for non-tumor types of cancer such as leukemia. These include drugs (chemotherapy), immunotherapy (specific or nonspecific stimulation of the immune system), and radiation therapy. These treatments are typically performed at specialty centers. New approaches, such as cancer vaccines, are under research and development.

Chemotherapy, radiation, and immunotherapy 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. Those that do, however, can usually be treated symptomatically with medications, and the symptoms are usually short-lived. Less than 5% of pet treated with chemotherapy will need to be hospitalized for adverse effects. Most pets feeling sick before chemotherapy actually feel better once they have received treatment (especially pets with lymphoma).

"The most common and often most effective treatment is surgical removal of the tumor."

There are many issues to be considered in the decisions on 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 for a period of time) with treatment, but eventually the cancer comes back.

As in humans, our understanding of cancer in animals is increasing. As our understanding of the disease improves, our ability to control, and even cure, cancer 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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