Chemotherapy in Veterinary Medicine
Clinical Oncology Service
Veterinary Hospital of the University of Pennsylvania (VHUP)
Last Modified: June 5, 2013
The diagnosis of cancer is stressful for pet owners, and the prospect of chemotherapy treatments can be equally difficult. However, the fear that animals will spend most of their last days sick from chemotherapy treatments is unwarranted. Knowing how anti-cancer chemotherapy drugs work and what to expect from the treatments can help pet owners decide on whether such therapy is appropriate for their pets.
When do we use chemotherapy to treat animals with cancer?
Chemotherapy may be used as the sole treatment for certain cancers or may be used in combination with other treatment modalities, such as surgery and radiation therapy. Chemotherapy is likely to be recommended for cancer that has already spread to other areas of the body (metastatic disease), for tumors that occur at more than one site (multicentric disease), or for tumors that cannot be removed surgically (nonresectable disease). In some cases, chemotherapy can be used to try to shrink large tumors prior to surgery or to help eradicate certain types of microscopic cancer cells that cannot or have not been completely removed surgically. For cancers that are at high-risk for metastasis early in the course of disease, chemotherapy can be used after surgery or radiation therapy to help slow down the growth of cancer cells in other parts of the body.
How does chemotherapy work?
Chemotherapy drugs attack cells in the process of growth and division. Individual drugs may work through many different mechanisms, such as damaging a cell's genetic material (DNA) or preventing the cell from dividing. However, chemotherapeutic drugs cannot distinguish between malignant cancer cells and normal cells. All rapidly dividing cells are potentially sensitive to chemotherapy. Toxicity to normal, rapidly growing or self-renewing tissues in the body is the reason for most of the side effects seen with chemotherapy. Fortunately, these normal tissues continue to grow and repair themselves, so the injury caused by chemotherapy is rarely permanent.
What are the typical side effects of chemotherapy?
Compared to people who receive chemotherapy, pet animals experience fewer and less severe side effects because we use lower doses of drugs and do not combine as many drugs as in human medicine. The normal tissues that typically are most sensitive to chemotherapy are the intestinal lining, the bone marrow (which makes red and white blood cells), and hair follicles.
Toxic effects to the gastrointestinal tract are responsible for decreased appetite, vomiting, and diarrhea. These effects may be mild, moderate, or severe. In most cases, these signs are mild and usually resolve on their own or with oral medication given at home. Although infrequent, some dogs (and cats) may develop severe diarrhea requiring hospitalization and fluid therapy. In many cases, the gastrointestinal side effects from chemotherapy are not seen on the day of treatment. They often occur 3 to 5 days later.
Suppression of the bone marrow by chemotherapeutic drugs may cause a drop in the white blood cell count, leading to increased susceptibility to infection. The infection usually comes from the animal's own body (such as bacteria normally found in the intestines, mouth, skin, etc.). Severe infections may require hospitalization for intensive supportive care, including intravenous fluid and antibiotics. When a chemotherapeutic drug is used that is known to have a high potential for bone marrow suppression, a complete blood count (CBC) is checked several days after the treatment. If the white blood cell count is low but your pet is feeling well, antibiotics are prescribed as a preventative measure. Subsequent doses of chemotherapy are adjusted based on the results of the CBC. Anemia (low red blood cell count) is often a complication of cancer but is rarely caused by the chemotherapy drugs used in veterinary medicine.
Hair follicle cell in dogs (and cats) that are wire-haired or non-shedding may be particularly susceptible to chemotherapy. Certain breeds of dogs, such as terriers and poodles, will experience variable amounts of hair loss. Hair loss often is most evident on the face and tail. Whiskers and the long hairs over the eyes often fall out in cats. The hair will regrow once chemotherapy is stopped, but may initially have a modest change in color or texture.
There are many different types of chemotherapy agents and each has a different likelihood of causing side effects. If your pet is treated with drugs known to cause certain side effects, we will prescribe medications to help prevent these complications, such as antiemetics (anti-nausea and vomiting medication). In addition, we will give you instructions on what to do if and when a problem arises. We seldom see severe side effects as described above; it is estimated to be less than 5% of all pets receiving chemotherapy. With proper management, most animals recover uneventfully within a few days.
Please keep in mind that any animal can have an unexpected reaction to any medication.
How is chemotherapy given?
How a chemotherapeutic drug is administered, how often it is given and how many treatments are given varies from case to case. The type of cancer, the extent of disease, and general health of the animal help the oncologists to formulate a treatment protocol (type of drugs, dose, and schedule used) appropriate for your pet.
Some drugs are oral medications (pills) that you give at home. Others are brief injections that require an outpatient appointment. In some instances, slow infusions or repeated treatments throughout the day may require an animal to spend the day in the hospital. The treatments are typically repeated from weekly to every third week. Blood tests may be needed to monitor the effects of chemotherapy during the weeks between drug treatments.
The duration of chemotherapy depends on the type of cancer and the extent of disease. Some animals need to receive chemotherapy for the rest of their lives. In others, treatments may be spread out or discontinued after a period of weeks to months provided that the cancer is in remission, i.e., there is no detectable evidence of cancer in the body. Chemotherapy can be resumed when the cancer relapses.
We usually recommend that every patient receive at least 2 cycles of chemotherapy and then be evaluated for response before we decide to continue the treatment, change drugs or discontinue chemotherapy.
What can be expected from chemotherapy?
In many cases, we are unable to cure our veterinary cancer patients. Our goal is therefore to improve a pet's quality of life. To this end, chemotherapy can be used to minimize the discomfort caused by a tumor or to slow down the progression of the disease. For most (but not all) types of tumors, the oncologist will provide information on average life expectancy with and without treatments.
The decision of whether to pursue chemotherapy treatments can be complex. Medical information, practical concerns (such as need for repeated visits, your pet's temperament, etc.), and financial responsibility all play a part in this decision. We encourage you to discuss your concerns with the oncologist and/or our social worker when making this decision.