Radiation therapy focuses a beam or field of intense energy on a certain area or organ of the body. The energy used in radiation therapy is similar to the energy used to create x-rays—except that either the radiation is many times stronger or the exposure time is much longer, so that the energy kills the cancer cells. Radiation can be applied to the cancer in 1 of 2 ways: from the outside using a machine, or from the inside using implants.

The more common method is external radiation therapy. In this technique, a linear accelerator beams rays to the site of the tumor. Normal tissue is shielded as much as possible from the destructive energy. To reduce exposure of normal tissue, multiple beam paths are often used. As a result, any negative side effects of the radiation are restricted to the area of treatment and do not affect the whole of the animal’s body.

Internal radiation therapy is also a possible method of cancer treatment, but it is rarely used for animals. Doses of radiation are delivered in and around the tumor by an implanted radioactive substance (such as cobalt). This method is known as brachytherapy. It is seldom used to treat cancers in animals because the radioactive substance must be handled carefully and kept in place within the tumor to prevent exposing humans, other animals, or healthy body parts of the pet being treated. One exception to this is the use of radioactive iodine to treat thyroid adenomas in cats. Implantable radiation sources, that are so small that they can be permanently implanted within the body, continue to be studied, and may increase the use of brachytherapy.