Veterinary Oncology M. Jankowski DVM and C. Khanna DVM, PhD, Dipl ACVIM (Oncology) Friendship Hospital for Animals, Washington, D.C What is Cancer? Cancer is an uncontrolled growth of cells that can occur in any tissue in the body. All cancers occur as a result of damage or changes in the genes of a cell. Most often several changes in different genes contribute to cause a cancer to develop and progress. Cancers can be described as either benign or malignant. Benign cancers tend to stay in a single part of the body without spreading. Malignant cancers, however, leave the site of the original (primary) tumor, invade into close-by tissues, enter the blood stream, and start to grow at new locations in the body. This process of spreading from an original tumor to another part of the body is known as metastasis. A tumor can metastasize through the blood stream or through the lymphatic vessels of the body. Cancers can develop from any normal part of the body. The tissue type from which a cancer develops as well as the underlying genetic changes associated with the cancer largely determines the behavior of cancer. Considerable progress has been made in our understanding of the causes of cancer. A current explanation for the uncontrolled growth of cells that characterizes cancer is the loss of a cancer cell's ability to die. Normal cells in our bodies, and in the bodies of our pets, must be able to die if they become damaged. It has been recognized that cancer cells actually lose the ability to die and for this reason begin to grow uncontrollably. If a cell loses the ability to undergo natural cell death, then the cell will continue to divide with the damage unrepaired. Cancer can be thought of as a group of damaged cells that are unable to naturally die. Cancer in Veterinary Medicine The problem with cancer in the general human population is well recognized. The following statistics emphasize the scope of this problem: One in four Americans (56 million Americans now living) will develop cancer. Over one million new cancer diagnoses will be made this year. This does not include benign skin cancer. Between 50 and 60% of those diagnosed with cancer will die of their disease. Cancer is one of the major causes of death in pet animals. Twenty-three percent of all dogs-and 45% of dogs older than 10 years of age-die of cancer. The statistics for cats are not much different. The incidence of cancer in pet animals is increasing. We are providing better health care for pet animals, which allows them to live longer and then develop diseases associated with age like cancer. There is also an increase in public's awareness of cancer treatments. The public is rightfully demanding that this advanced care be provided for their pets with cancer. The antiquated notion that there is no hope for pets with cancer has been abandoned by most of the veterinary community. Not all cancers seen in pet animals can be effectively treated; however, the number of cancers for which there are no options is becoming smaller each day. At this time there are many options for pet owners to consider for the cancer care of their pet. Cancer Treatment Methods The four primary methods to treat cancer are surgery, radiation therapy, chemotherapy, and immunotherapy [Below the fourth method is "biotherapy"]. The goal of these cancer treatments is to destroy abnormal cancer cells while sparing normal cells. The method for treatment of most cancers is similar in human and veterinary cancer patients. An important difference exists. In human oncology the ultimate goal of therapy is cure. To achieve this goal it is necessary to treat people very aggressively. This aggressive approach to treatment results in cure in a certain percentage of patients, but also results in the side effects that are commonly associate with cancer therapy. In veterinary oncology, the goal of therapy is to maximize and extend the quality of life of the pet. This approach is taken because we are unable to provide the type of supportive care that aggressively treated patients require, and because it is so important for our veterinary patients to enjoy a good quality of life during and after treatment. The result of this approach is well-tolerated chemotherapy and radiation therapy protocols with good treatment responses for several cancers. The disadvantage is a smaller percentage of "cured" patients. Surgery Surgery is an extremely effective treatment method. It is a highly effective form of cancer treatment if the cancer exists in a localized area of the body. The success of a surgical treatment for cancer depends on the following: A tumor localized to a single site without spread to other parts of the body A tumor type that is slow to spread from the primary tumor site The experience of the surgeon The location of the cancer Radiation Therapy Radiation treatment can be considered for tumors that are not amenable to surgical treatment (referred to as primary radiation therapy) and/or for tumors that have been removed by surgery when there is a concern for the tumor returning (adjuvant radiation therapy). Success with radiation therapy is determined by the following: A cancer type sensitive to radiation therapy A tumor localized to a single site without spread to other parts of the body The location of the cancer Radiation damages and kills rapidly growing cells more effectively than normal slower growing cells. Most cancer cells are rapidly growing and therefore are more sensitive to damage than normal cells. Radiation therapy is given in several small doses, called fractions, over a period of 3 to 4 weeks. These small daily fractions are given to maximize the damage to cancer cells and limit the damage of normal cells. Normal cells that are damaged most easily are the more rapidly dividing cells of the body (e.g., skin, hair, the lining of the mouth, and the eye). Fortunately, the damage to normal cells occurs only in the parts of the body that are exposed to the radiation. Recovery from this damage is usually self-limiting (usually 5 to 7 days) but can require supportive care. Chemotherapy Chemotherapy is used in both humans and animals when the spread of a cancer has gone beyond a local region or if the cancer exists in multiple locations. Chemotherapy is most effective in the following situations: If the cancer is highly sensitive to chemotherapy If there is a small amount, preferably only microscopic, of cancer present When the patient is healthy and feeling well Chemotherapy kills cancer cells preferentially over normal cells because cancer cells are more rapidly dividing, have decreased ability to repair the damage exerted by chemotherapy, and are unable to eliminate the chemotherapy from within the cell as well as normal cells can. There are many different classes of chemotherapy agents, and each class has its own distinct mechanism of action and toxicity. Cancers can become resistant to chemotherapeutic agents. For this reason it is important to use a number of different drugs in chemotherapy protocols, each at the highest dose that is tolerated by the patient. The goal of chemotherapy is to give enough drug to kill all the cancer cells while limiting damage to normal cells. Side effects of chemotherapy occur when normal cells are damaged by the treatments. As with radiation therapy, the cells most commonly affected are the most rapidly dividing cells in the body (e.g., bone marrow cells, cells lining the intestine, and occasionally hair cells). The most common side effects associated with this chemotherapy in animals are vomiting, diarrhea, and a decrease in appetite. Most veterinary chemotherapy protocols are well tolerated. If there is vomiting it is usually mild and limited to 1 or 2 episodes. In approximately 10% of patients the side effects may require the evaluation of a veterinarian. Biotherapy The current and future breakthrough(s) in cancer treatment will come from biotherapy. Biotherapy refers to treatments that are directed against specific parts of a tumor. Biotherapy also includes strategies to stimulate the body's immune system to detect and eliminate the cancer. A biotherapy strategy that has received considerable press recently is called anti-angiogenic therapy. It is known that for a cancer to be able to metastasize to other parts of the body, it must be able to create its own blood supply (a process called angiogenesis). Novel forms of biotherapy have been generated to prevent angiogenesis and therefore prevent cancer metastasis. Current clinical trials are underway in human and veterinary oncology to evaluate this promising biotherapy strategy. Treatment options for pet animals with cancer may include surgery, radiation therapy, chemotherapy, and biotherapy. Combinations of these treatment approaches have been used to provide even more favorable responses against cancer. The management of cancer in pet animals is based on maximizing quality of life. With this as a goal, the treatment of pets with cancer is becoming more and more common and more and more effective.