Cancer in our pets

Neoplasia is the uncontrolled, abnormal growth of cells or tissues in the body, and the abnormal growth itself is called a or tumor. It can be benign or malignant. Benign tend to grow slowly; displace, but do not tend to invade, the surrounding body tissues; and do not spread throughout the body. Malignant neoplasms, on the other hand, can be unpredictable and grow at various rates (sometimes rapidly), invade the tissues around them, and spread, or metastasize, to other parts of the body. The word “tumor” or “mass” is often used to describe the actual swelling or other physical appearance of a neoplasm. The Figure 1: A chart of comparative cancer rates for humans, word “cancer” is often confused with neoplasia, but only dogs, and . malignant neoplasms are truly cancers.

What Causes cancer in Pets?

Most people are very surprised to learn that dogs and cats have a higher incidence of many tumors than do humans (fig.1). Cancer can occur at any age, but is diagnosed much more frequently as pets age. Other than euthanasia in animal shelters, cancer is the number one killer of dogs and cats.

Approximately 1 in 4 dogs will, at some stage in their life, develop neoplasia. Almost half of dogs over the age of 10 will develop cancer. Dogs get cancer at roughly the same rate as humans, while there is less information about the rate of cancer in cats. Some cancers, such as , are more common in cats than in dogs.

Through careful statistical evaluation by veterinary epidemiologists (scientists who study patterns of disease in populations), some breed predilections for cancer have been noted. Ten years ago, if a were to have been asked the breed of dog with the highest incidence of cancer, he or she would undoubtedly reply "the Boxer." When a sick, aged Boxer was presented at the Texas A&M University Veterinary Medical Teaching Hospital, we suspected a tumor almost immediately (fig. 2).

Since then, however, several other breeds have been noted to have an extremely high incidence of cancer much like the Boxer; the Golden Retriever, Rottweiler, and Bernese Mountain Dog currently have an inordinately high incidence of certain tumors, and these dogs seem to develop cancer at Figure 2: A boxer with . an earlier age than dogs of other breeds. Other breeds with a notably high incidence of cancer are the Boston Terrier, English Bulldog, Scottish Terrier, and Cocker Spaniel. Breeds with a relatively low incidence of cancer include the Beagle, Poodle, Collie, and Dachshund. Why do some breeds have a high incidence of cancer while some are rarely affected by it? If we knew the answer to that question, we would undoubtedly be closer to preventing cancer than we are. Probably the answer will come through the study of molecular biology and genetics; many of these dogs have probably inherited (or acquired through DNA mutation) abnormal genes that predispose them to cancerous transformation of cells (fig.3).

Cancer is not only a problem of purebred dogs, however; if it were, the "mutt" should have one of the lowest incidences of cancer. Yet, ironically, the dog of mixed ancestry has an average incidence of cancer, along with the Irish Setter, Schnauzer, Labrador Retriever, and dogs of many other breeds.

Some types of tumors are more prone to develop in one type of dog than in others. For example, the giant-breed dogs like the St. Bernard and the Great Dane have a much higher incidence of (a very malignant bone tumor) than does the general canine population. White cats, cats with white on their faces, and dogs with white hair and thin Figure 3: A Golden Retriever with swelling skin are prone to develop cancers in their skin due to long- of the leg due to osteosarcoma. term exposure to the ultraviolet rays of the sun. Black dogs have a comparatively high incidence of , or pigmented skin tumors. There is evidence that secondhand smoke increases the risk of some cancers in dogs and cats. Spaying reduces the risk of mammary cancer in dogs. Half of all breast neoplasms in dogs and greater than 85% of all breast neoplasms in cats are malignant, and spaying female pets before 12 months of age reduces this risk. eliminates the risk of . Conversely, there is evidence that spaying and neutering can increase the risk of certain other cancers. Genetic predisposition to some cancers in certain breeds or breeding lines has also been reported. Talk to your veterinarian about the benefits, risks, and timing of spaying or neutering your pet.

Symptoms

There are many possible symptoms of cancer, depending on the type of tumor a pet has. The most obvious sign is development of a lump that is not a normal structure. Some lumps are tumors and others are not. Some tumors are benign and others malignant (cancers). It is possible for internal cancer where the lump cannot be seen or otherwise detected to produce more generalized and non-specific symptoms, such as poor energy, poor appetite, weight loss, vomiting, diarrhea, change of bowel or bladder habits, lameness, drooling, regurgitation, difficulty swallowing, changes in breathing, cough, swelling of the abdomen, unusual bleeding or discharges, changes in behavior, seizures, poor coordination, or infections and/or wounds that do not heal. Cancer can produce just about any symptom, so see your vet soon if your pet has any problems at all.

Diagnosis

In order to formulate the best treatment program for your pet, the tumor must be definitively diagnosed by looking at the cells and/or tissue that make up the tumor. Often, a fine needle aspiration cytology can be performed in the vet office, and often without sedation, and may or may not need to be sent out to a laboratory for confirmation of diagnosis. Other tumors must be submitted to the lab for to make the diagnosis. If the entire tumor can be removed, that is always preferred – the pathologist will look at the border to see if it is likely that all of it was removed. If the entire tumor cannot or should not be removed, a small piece of it can be removed and submitted to the pathologist to guide treatment.

A group of tests called “staging” is used to determine whether a tumor has spread from its primary site to other places in the body. Staging might involve blood tests, x-rays, and other tests. Advanced imaging such as computed tomography (CT) scan, magnetic resonance imaging (MRI), or positron emission tomography (PET) scan can also improve the understanding of the tumor’s location and possible treatment options. “Grading” is a process of looking at a biopsy of a tumor and determining how aggressive its behavior is likely to be. With both stage and grade, the higher the number, the more aggressive the tumor and the worse the prognosis. Treatment

There are three main methods of treating cancer: surgery; radiation therapy; and drugs (). Both surgery and radiation therapy mostly control cancer in one area of the body – they do not address the problem called metastasis, when a cancerous tumor sheds cancer cells to other part of the body through the blood stream and lymph system. If surgery is performed early enough, while the tumor is small, that is the greatest chance for cure. A few cancers can be cured with radiation therapy – mast cell tumor and hemangiopericytoma are examples.

Metastasis must be addressed by drug therapy. Most chemotherapy protocols slow down disease, and at best hope to but cancer in to remission. Remission means that the outward appearance of the animal is that they are not ill at all. Very few chemotherapy protocols are curative, though some are. When one thinks of chemotherapy in people, most people conjure up images of a very sick person who among other things often loses all of their hair. For many reasons, chemotherapy for pets is very different. For the most part, canine and feline chemo patients come and go, and get their chemo, and have a good quality of life. They almost never lose their hair, and only about 9% are sick enough at any time to need treatment in a veterinary hospital. One of the newest areas of cancer treatment in pets is , where drugs are created that are antibodies to cancer – a cancer vaccine, of sorts. There are several cancer vaccines for dogs on the market. Some of them are available to Veterinary Oncologists only (specialists), and all of them are very expensive – thousands of dollars just for the drug. For those who can afford them, they can be helpful to dogs with cancer, and carry very few side effects. Like traditional chemotherapy, immunotherapy is rarely curative, but can slow down the process of disease progression.

Blood and tissue tests for cancer in pets are being developed, similar to the many cancer marker blood tests that are available for people. Most of these blood tests are still in the stage of development, and very few are widely used by veterinary oncologists, other than a few tests for lymphoma and mast cell tumor. In the future, we hope that more of these types of tests will be available, to help us guide diagnosis and treatment of cancer in pets.

Prognosis

The response to treatment depends on the type and extent of the neoplasia, as well as the availability and effectiveness of therapy. There is no general rule regarding an individual pet’s response to therapy, but treatment can be successful for many pets with neoplasia. Benign neoplasms are usually easier to treat, and treatment of any type of neoplasia is more likely to be successful if the neoplasms are detected early. Despite a lack of metastasis, benign tumors can sometimes have damaging effects on the patient; for example, brain tumors are often benign but the pressure they create on the surrounding brain tissue can be life-threatening. Although some neoplasms (especially the more aggressive cancers) cannot be cured, treatment can prolong your pet’s life and improve their quality of life.

Referral

Oncology is a growing specialty area of veterinary care. Oncologists are who have obtained additional training beyond veterinary school and are certified by the American College of Veterinary Internal Medicine (ACVIM) in oncology. They work together with your veterinarian to provide the best care for your pet with neoplasia. If at any time you are interested in seeing a Board Certified Oncologist for expert cancer care for your pet, please ask your veterinarian for a referral.

References: AVMA Client Information Brochure – Cancer in Pets TAMU Oncology Service Wellness Handouts for Veterinarians Wendy Blount, DVM – Practical Vet Med Seminars