What about chemotherapy?

Chemotherapy is frightening to most people, since many of us have had relatives that became very ill when treated with these drugs (fig.1).

Figure 1: Most cats and dogs tolerate chemotherapy well with an acceptable level of toxicity.

In dogs and cats, we generally do not expect the same severity of side effects as are often seen in humans, since veterinary oncologists usually use lower doses of chemotherapy than those used in man. We also do not generally combine drugs together to be administered in rapid succession, as is done in humans. These precautions taken in our pets prevent many of the severe side effects commonly experienced by people taking anticancer drugs.

Since chemotherapeutic drugs travel throughout the body, they can seek out tiny numbers of tumor cells which are not visible by any currently available diagnostic test, whereas surgery and conventional radiation therapy can only treat one particular area of the body. In some cases, chemotherapy alone may be able to cure a cancer; in other types of tumors, chemotherapy is used palliatively, to slow the tumor's growth so that the pet lives a longer life with quality. Anticancer drugs may also be administered when there remains a very great likelihood that there are still microscopic tumor cells in other parts of the body after a tumor has been removed surgically or treated with radiation therapy, This form of drug treatment is called adjuvant chemotherapy, because it is used to increase the number of cures achieved with surgery or radiation therapy alone. An adjuvant agent is one that makes a drug or treatment more effective; for example, adjuvants are substances used in vaccines to increase the body's antibody response to the vaccination. Adjuvant chemotherapy is used similarly, in hopes of increasing the cure rate in cancer treatment. In some cases, chemotherapy may be used before surgery or radiation therapy, in an attempt to shrink the tumor so that the subsequent surgical operation or radiotherapy may be more effective; anticancer drug treatment used in this way is called neoadjuvant chemotherapy.

Chemotherapeutic drugs kill tumor cells by several mechanisms; some drugs fragment DNA, while others stop the dividing of cells. Of course, these effects that work to kill tumor cells also cause changes in normal cells, with resultant deleterious side effects in the patient being treated. The goal of the oncologist is to achieve a drug dosage and combination of drugs that will control (or cure) the tumor without causing the patient any serious temporary or permanent harm. This is often very difficult to do, and a slight amount of nausea is common for a few days, especially at the beginning of chemotherapy. This period is called induction, and higher doses and more types of drugs may be used during this phase. The goal of induction chemotherapy is to bring the patient into remission, which is defined as tumor shrinkage until no visible tumor nodules or masses remain. After induction, some tumors may not be in remission but may have shrunk to the point that surgery can remove the rest of the tumor. In other cases, however, maintaining the state of remission will require that chemotherapy be continued. Usually, the drugs are given less frequently and at lower doses during this second phase of treatment, which is defined as the maintenance period of chemotherapy.

In most chemotherapy protocols (the doses and types of drugs given for a particular tumor, i.e., the "recipe" for treating that tumor), several drugs are given (fig.2).

Figure 2: A dog with bone cancer receiving intravenous chemotherapy.

The drugs in the protocol are chosen specifically to work on cells in different stages of cell division and by different metabolic mechanisms, in order to kill a higher number of tumor cells and decrease the chances that the pet's tumor will become resistant to a particular drug. In some tumors, chemotherapy resistance can occur quickly after treatment begins; the cancer cells develop mechanisms to prevent or heal the damage caused by the chemotherapeutic agent, and the drugs used are therefore no longer effective. Chemotherapy resistance is one of the biggest hurdles standing in the way of increasing the number of cancer cures today.

In some tumors, long-term hormone stimulation has played a part in the growth of the cancer (fig.2).

Figure 2: Most mammary cancer in the dog is hormone induced and can be prevented by early neutering.

Examples of these tumors are mammary neoplasia (breast cancer), prostatic cancer, and cancer of the perianal glands (glands that surround the rectum in the dog). For this reason, doctors may recommend therapy that prevents the cancer cells from getting or using the hormones they need. Sometimes, the pet will have surgery to remove the organs that make the hormones (such as the ovaries or testicles). In other cases, the oncologist will prescribe drugs to stop hormone production, to change the way hormones work, or to block the effects of the hormone. Like chemotherapy and immunotherapy, hormonal therapy is systemic treatment that affects cells throughout the body.