Chemotherapy in Veterinary Medicine

A diagnosis of cancer is stressful for pet owners, and chemotherapy for your pet can also be stressful. 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.

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. This means that 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.

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:

• Metastatic disease: cancer that has already spread to other areas of the body
• Multi-centric disease: tumors that occur at more than one site (multicentric disease)
• Nonresectable disease: tumors that cannot be removed surgically

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. Chemotherapy can be used after surgery or radiation therapy to help slow down the growth of cancer cells in other parts of the body.

What are the typical side effects of chemotherapy?

When you think of chemotherapy in humans, you probably think of nausea, vomiting, hair loss, and fatigue. However, in animals, these side effects can be mild. Compared to people who receive chemotherapy, 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. It is important to remember that an animal can have an unexpected reaction to any medication.

GI (Gastrointestinal)

Toxic effects on 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 of chemotherapy are not seen on the day of treatment. They often occur 3 to 5 days later.

Increased Risk of Infection

Suppression of the bone marrow by chemotherapeutic drugs may cause a drop in the white blood cell count, leading to an increased risk of 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 Loss

Hair follicle cells 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, the veterinarian will prescribe medications to help prevent these complications, such as anti-emetics (anti-nausea and vomiting medication). In addition, the veterinarian will give you instructions on what to do if and when a problem arises. It is very rare for animals to experience severe side effects. With proper management, most animals recover uneventfully within a few days.

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 the disease, and the general health of the animal help the oncologists to formulate a treatment protocol (the 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 the 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 if cancer returns.

What can I expect from chemotherapy? 

In many cases, cancer in our companion animals cannot be cured. Therefore, our goal is 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 the need for repeated visits, your pet's temperament, etc.), and financial responsibility all play a part in this decision. You should discuss your concerns with the veterinary oncologist and/or our veterinary social worker when making this decision.