What is Cancer?

What is cancer?

Cancer is a disease characterized by uncontrolled cell growth. These diseases most often result in an abnormal mass of tissue (tumor) without a purpose, although there are types of cancer that do not cause tumors. Cancer cells replicate (divide) faster than normal cells, lack the normal control mechanism that prevents replication, are more resistant to dying, and often continue to grow if they are not treated.

There are many different types of cancer, and any cell in the body has the potential to become cancerous. Cancers are classified based on the tissues they arise from (e.g., muscle, skin, organ). Not all tumors are cancerous. Tumors may be benign (non-cancerous) or malignant (cancerous).

Benign tumors usually remain in a local area and grow in a uniform, smooth expansion. Malignant tumors can invade the nearby tissues or spread to other parts of the body. A distinctive characteristic of malignant tumors is their ability to metastasize (spread). The cancer cells leave the primary site and travel to other locations throughout the body, binding to tissues unrelated to the original site.

There is no single 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.

How does cancer develop?

Cancer begins when the genetic material (DNA) inside a cell becomes damaged. DNA contains the instructions that tell cells when to grow, divide, perform their normal functions, and die. Healthy cells follow this orderly process and are replaced when they become old or damaged.

Damage to DNA can be caused by factors such as radiation, certain chemicals, hormones, infections, or simply by normal aging. Most of the time, damaged cells are repaired or die before they can cause problems. Cancer develops when damaged or abnormal cells survive and continue to divide when they should have died. As these abnormal cells multiply without stopping, they can form a tumor. Some cancers can also spread by releasing cells into the bloodstream, allowing them to travel to other parts of the body.

The body has many natural defenses that repair damaged cells or destroy abnormal ones before they become cancerous. However, these protective mechanisms are not perfect. Some animals inherit genes that make them more likely to develop certain types of cancer, while others develop genetic damage over time because of aging or environmental factors. When enough genetic damage (mutations) accumulates, cancer can develop.

Why did my pet develop cancer?

Many factors can contribute to the development of cancer, but it is impossible to know exactly why one pet develops cancer while another does not.

• Exposure to carcinogens (cancer-causing agents). In some cases, exposure to known carcinogens may play a role (e.g., sunlight, chemicals, and cigarette smoke).
• Genetics. Some pets inherit genes that make them more likely to develop certain types of cancer. Certain breeds appear to have a higher risk of developing some cancers (e.g., golden retrievers, Dobermans, German shepherds, Bernese mountain dogs, and boxers).
• Obesity, infection, and inflammation. These factors have been linked to the development of some types of cancers.
• Hormones. Some cancers depend on hormones to develop and persist.
• Age. The risk of cancer increases with age because cells divide many times throughout a pet's life. Each time a cell divides, there is the possibility that a mistake (mutation) will occur in its DNA.

Is cancer transmissible to other pets?

In most cases, no, cancer cannot be passed directly from one pet to another. The most notable exception is canine transmissible venereal tumor (CTVT), a rare cancer that can spread between dogs during mating.

Some viruses and other infectious organisms can also increase the risk of cancer in animals. These infectious agents may be passed from mother to offspring before or during birth, through direct contact with other animals of the same species, or by vectors (carriers) such as fleas or ticks. Although the infection can spread and potentially cause cancer, the original cancer doesn’t spread from one pet to another.

If your pet has a transmissible cancer or an infection that can lead to cancer, your veterinarian will advise you about the precautions needed to help prevent transmission to other animals.

What are the clinical signs of cancer?

The most obvious sign of cancer 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.

Other signs of cancer may include:

• Unexplained, chronic weight loss
• Abnormal bleeding
• Unexplained vomiting or diarrhea
• Swollen lymph nodes
• Coughing
• Lameness
• Bad breath or changes in eating habits can be signs of oral cancer

How is cancer diagnosed?

Your veterinarian may suspect that your pet has cancer based on their clinical signs. Radiographs (X-rays) or ultrasound may help detect internal tumors and determine whether the cancer has spread (metastasized). Blood tests can also provide clues that suggest certain types of cancer.

Collecting a sample. To identify most tumors, it is necessary to obtain a sample. Depending on the type of tumor suspected, your veterinarian may collect this sample using a fine needle aspiration (FNA), a tissue biopsy, or by removing the entire tumor. In some cases, exploratory surgery or guided ultrasound may be needed.

Fine-needle aspiration (FNA). An FNA uses a needle and syringe to collect cells from the tumor.  These cells are examined under a microscope (cytology). An FNA does not require general anesthesia or surgery. Some tumors can be accurately diagnosed this way.

Biopsy and histopathology. Many tumors require a biopsy, in which a small piece of tissue is removed. The sample is sent to a veterinary pathologist for microscopic examination (histopathology), which helps determine whether the tumor is benign or malignant.

Tumor type, grade, and stage. Malignant tumors are often classified by their type (such as carcinoma or sarcoma), grade (how aggressive the cells appear under the microscope), and stage (how far the cancer has spread). Together, this information helps predict how the cancer is likely to behave and guides treatment decisions.

If your pet is diagnosed with cancer, your veterinarian may refer you to a veterinary oncologist (a veterinarian who specializes in cancer diagnosis and treatment).

What types of treatment are available?

Surgical removal of the tumor is the most common treatment for cancer and is often the most effective. Other treatments may be considered under certain circumstances:

• If tumors are too large or too numerous to remove
• If tumors are located where surgery is not possible
• If the cancer does not form a solid tumor (e.g., leukemia)

Other treatments include chemotherapy, immunotherapy, radiation, and targeted therapies. The type of treatment recommended depends on its success with the type of cancer diagnosed. Some of these treatments need referral to a specialty center, while others may be provided by your veterinarian.

Side effects may occur with any cancer treatment, but they are generally not as severe in pets as they are in people. Many pets have no noticeable side effects from these treatments. When side effects occur, they are usually short-lived and can be managed with medication. Many pets that feel sick because of their cancer actually feel better after chemotherapy begins, especially those with lymphoma. Your veterinarian or veterinary oncologist will discuss the treatment options and help you decide which approach is best for your pet. 

Can cancer be cured?

There are instances when surgery provides a cure, with malignant as well as benign tumors. In most cases of malignant cancer, however, a permanent cure is not necessarily possible. Instead, the goal is often 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 periods of remission (when no detectable cancer can be found) with treatment, but the cancer eventually comes back. As the understanding of cancer in animals improves, the options available to control or cure it will also improve. If your pet is diagnosed with cancer, learn as much as you can about it. Although there may not be a cure, there are many options for care.