Chemotherapy Chemotherapy

The use of cytotoxic (anticancer) drugs in isolation or in combination is an important, if relatively recent, addition to the armoury of treatments for neoplasia in cats and dogs. However these are potentially harmful agents and are best administered by experienced oncologists.

As with radiotherapy the clinical skill necessary for the successful use of these drugs is to deliver a therapeutic dose to the site of the cancer, and at the same time minimise side-effects due to normal tissues being affected by these non-selective drugs - which usually affect the growth or cell division phase of tissues. The most common complication is myelosuppression leading to leucopenia, increased risk of infection and sepsis.

Toxicity is the main limiting factor in the application of chemotherapy and for this reason combination therapy employing lower doses of drugs with different modes of action is usually preferred. Even with this approach multiple drug resistance can occur with some neoplasms.

In all cases the general physical condition of the animal must be good and pre-treatihent screening for renal and hepatic impairment is essential. Providing there is no evidence of major organ disease the absolute age of an animal is less important, except in as much as it might limit the period of lifespan that can be expected following chemotherapy. Usually one should aim to provide an animal with a normal quality of life for a period of at least 6 months to 1 year to justify the use of this form of treatment.

White blood cell counts should be performed every 2-6 weeks during treatment with cytotoxic drugs as myelosuppression is their main side effect. If the white cell count falls below 3 x 109/litre the drug dose should be halved. If it falls below 2 x 109/litre the drug should be withheld.

Some of the drugs commonly used for chemotherapy in small animal veterinary medicine are listed below in more detail.

Vinca alkaloids

Alkylating agents

Antitumour antibiotics

Other cytotoxic drugs

Success of chemotherapy