Radiation therapy has seen dramatic increases in demand and sophistication in recent years, which has led to creation of a board specialty in radiation oncology, granted by the American College of Veterinary Radiology, that is specifically limited to certification in radiation oncology. The sophistication of both veterinary imaging and radiation therapy has advanced to the point that only a few radiologists now actively practice in both the fields of imaging and therapy.

Radiation therapy practices generally use linear accelerators as the source of the ionizing radiation used to treat neoplasia and occasionally specific benign diseases. These machines produce powerful x-rays and electron beams with energies of 4–20 million electron volts. The x-rays are used to treat deep-seated tumors, whereas electron beams are generally used to treat tumors of the skin and subcutis. Linear accelerators are complex machines that require the support of a medical physicist to maintain safe and effective use. This increased support load is offset by the machine's flexibility and speed, which is necessary as treatment techniques become more sophisticated and complex.

Another significant advantage is the lack of need for long-lived radioactive materials, such as 137-cesium and 60-cobalt, which were used in older radiation therapy machines and had to be periodically replaced and disposed of by highly trained specialists. Care and use of these isotopes also requires many safety inspections as mandated by law. Currently it is virtually impossible to purchase these sources due to concerns about public safety.

Computerized treatment planning systems are used to improve the localization and distribution of the therapeutic beam within the patient. This limits the dose to normal tissues and also increases the dose to the neoplastic tissue being treated, increasing cure or control rates and reducing the severity of normal tissue complications. These programs are best used in conjunction with CT or MRI images, which determine the position and extent of the tumor within the body and its relative position to normal structures. Many hours of planning may be required to generate a treatment plan for a large, complex tumor.

Whenever possible, elimination of a tumor by surgery is preferred. However, in many instances large neoplasms, or those in critical areas such as the brain, are not amenable to complete or even partial surgical removal. Even when a tumor is grossly removed, microscopic foci of neoplastic cells may extend beyond the limits of the surgical field. In all of these instances radiation therapy, often in combination with chemotherapy, is useful in treating the remaining cancer. Radiation therapy is often the treatment of choice for brain tumors, nasal tumors, and other neoplasms of the head and neck. It may be the only treatment option for cancer of the vertebral column and pelvic canal. Radiotherapy is also used to treat tumors in the media-stinum and soft tissues of the skin and subcutis either before or after surgery. It is seldom used in the treatment of lung neoplasia or in the treatment of neoplastic disease of the abdominal cavity, due to the mobility of tumors in these areas. As the sophistication of radiotherapy techniques increases, more and more types of neoplasia are being treated at least in part by radiation therapy.

Because of the risk of serious and life-threatening complications associated with this treatment modality, radiation therapy should only be prescribed by and administered under the supervision of a veterinarian with special training and experience in the field. A radiation therapist should also be consulted any time further treatment is contemplated for neoplasia that has been treated by radiation therapy. This is particularly important if surgery is being considered.

Brachytherapy is the implantation of radioactive sources into the tumor to achieve radiation therapy. It is seldom used for treatment of cancers in animals due to the difficulties associated with maintenance of the sources and keeping the sources in place within the tumor. The notable exception to this is the use of radioiodine to treat thyroid adenomas in cats. Implantable radiation sources that are so small that they are permanently implanted within the body blur the margins between radiation therapy and nuclear medicine. Such techniques may well increase the interest in and availability of brachytherapy procedures in the future. Due to the risk of excessive radiation exposure and contamination of the patient or hospital, these procedures should only be performed by people with appropriate training, facility licensing, and support.

Last full review/revision March 2012 by Jimmy C. Latimer, DVM, MS, DACVR, DACVRO