Cancer is a disease from which no creature is immune. It can develop in any part of the body and is as common in dogs and cats as it is in humans. Broken down to its most basic definition, cancer is a group of diseases caused by abnormal cells. These cells replicate and grow uncontrollably, whereas normal cells only reproduce when needed to replace dead cells or during the growth phases of young creatures. The most common cancers found in animals are mammary cancer, skin cancer, bone cancer, oral cancer, sarcomas and lymphomas. Given its prevalence, most people have heard words like tumor, malignant and remission, but may not know what they mean. The following are definitions of some common words associated with cancer:

Carcinogen: substance that encourages normal cells to transform into cancer cells
Malignant: group of abnormal cells that will invade other cells; cancer
Benign: not cancerous
Tumor: abnormal tissue mass; can be malignant or benign
Cyst: a fluid-filled growth; usually benign
Margin: the edge of a tumor; when the margin is negative for cancer cells it’s assumed the entire tumor was removed
Biopsy: test of tissue from tumor or cyst to determine if cancer cells are present
Chemotherapy: treatment with drugs designed to destroy cancer cells
Radiation Therapy/Treatment: treatment with x-rays designed to shrink size of tumor
Transformation: the process of normal cells changing into abnormal cells
Remission: disappearance or reduction in the signs and symptoms; may not be cured
Metastasize: the spread of cancer from one body area to another usually through the bloodstream or lymphatic system
Regression: decrease in size of tumor or extent of cancer
Relapse/Recurrence: reappearance of disease symptoms after remission

Risk Factors

Most of the information regarding cancer in animals comes from the research on the disease in humans. Age, genetics, and environment all play a role in a pet’s likelihood of developing cancer. Older animals, like with many other diseases, have a higher rate of cancer. It is estimated that cancer is responsible for one half of the deaths of dogs over the age of ten. Genetics is also a factor. Breeds like boxers and golden retrievers have been shown to have a higher incidence of cancer. Second-hand smoke, excessive exposure to ultraviolet light, viruses, and chronic inflammation are all environmental issues which have been shown to cause some types of cancer in both humans and animals. Many of these environmental factors can be controlled, thereby reducing the risk of cancer.

Symptoms and Detection

Cancer cannot be avoided. The best that can be done is to reduce the risk factors that we can and to diagnose it early. When treatment is started during the early stages, the prognosis for a return to a healthy life is much better. A thorough annual examination by your veterinarian is your pet’s best chance. Your pet may not always show symptoms of illness until the cancer has advanced, but through tests such as blood work, x-rays and urinalysis your veterinarian may be able to detect disease. Owners should also perform at home evaluations to note lumps, open sores, and changes in appetite and energy levels. The ten most common signs of cancer, according to the American Veterinary Medicine Association, are:

abnormal, persistent swellings
sores that do not heal
persistent lameness or stiffness
loss of appetite
bleeding or discharge from any orifice
offensive odor
difficulty eating or swallowing
weight loss
hesitation to exercise or loss of stamina
difficulty urinating, defecating or breathing

Have your pet examined immediately if you note any unusual changes in behavior, personality or physical difficulties.

Treatment Options

While not all cancers can be cured, there are treatment options: surgery, chemotherapy and radiation therapy. These can be used separately in combination. Your veterinarian will decide on the preferable course of treatment based on the type and severity of the cancer, the cancer’s location and your pet’s overall health. There are advantages and disadvantages to each treatment which your veterinarian will discuss with you. The ultimate goal of any treatment is to eliminate the cancer. In cases where that is impossible, therapies are used to improve the pet’s quality of life as much as possible.

Surgery: This is an effective option for localized tumors. Removing the mass removes the cancer, in some cases, offering a cure. In more progressive cases, the surgery can relieve pain or restrictions to mobility even though cancerous cells remain behind. Surgery can be followed by radiation or chemotherapy.
Radiation Therapy: Radiation, or x-ray, kills or severely damages cells which divide rapidly and is effective on many types of cancers. Although it may not totally destroy a tumor, it can reduce its size. This can alleviate symptoms and improve quality of life. Radiation can damage normal cells in the area surrounding the tumor.
Chemotherapy: This treatment is often used to control or prevent the spread of cancer through the use of drugs. Although these drugs target rapidly dividing cells, they can impact normal cells. Chemotherapy is most frequently used in cancers that affect the whole body, such as lymphomas, rather than localized tumors.

Regardless of the treatment options available, the main consideration must be the animal’s quality of life. Be sure to discuss this with your veterinarian before making any decisions. It is natural to want to utilize any means available to save your pet’s life, but you must think about the type of life that pet will be able to live after treatment. Again, your veterinarian can advise you on your pet’s diagnosis and prognosis. If no treatment has a reasonable projection of success, you may choose to offer your pet hospice care. This means keeping them comfortable for as long you can. When their quality of life is affected, euthanasia may be the most humane option.

American Veterinary Medical Association
National Canine Cancer Foundation
The Merck/Merial Manual for Pet Health 2007

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