Chemotherapy is drug therapy designed to kill rapidly dividing cancer cells. Many chemotherapeutics are made from plants, trees and bacteria and many are the same drugs used to treat human cancer patients. Chemotherapy is often employed to treat systemic cancer, cancer that has spread, to treat cancer that cannot be treated with surgery or radiation therapy alone or to increase the effectives of other treatment modalities.
As mentioned, chemotherapy is designed to kill rapidly dividing cancer cells but in so doing also kills normal cells that naturally reproduce quickly. This includes the lining of the intestines, bone marrow and for some dog breeds, hair follicles. This is where most chemotherapy side effects come from.
While most dogs and cats tolerate chemotherapy without incident, roughly 20% will experience mild lethargy, loss of appetite, an episode or two of vomiting and/or diarrhea. These effects rarely last more than 48 hours. Approximately 5% of our patients will experience severe versions of these same side effects. Recovery from severe side effects is very common but often requires supportive care in a hospital setting. Fatal side effects from chemotherapy are reported and occur in less than 1% of patients.
Although we often use the same drugs used in people under similar circumstances, our goals are a bit different. While, longevity is important, our primary focus in veterinary oncology is to enhance quality of life. Therefore, we minimize toxicity in our pets by using lower doses and sometimes prescribing longer treatment-free intervals. In so doing, cure is rare, but long-term remissions are common.