Principles of radiation therapy.

Radiation therapy can provide long-term control of local or locoregional cancer without removal of large volumes of tissue and with preservation of function of surrounding normal tissues. Radiation therapy is used for cancers that have extended near or around critical structures such as spinal cord, nerves, or large vessels. Normal tissue response limits the total radiation dose that can be used. The objective of radiation therapy is to provide the highest probability for local tumor control with a probability for serious complications such as bone or soft-tissue necrosis of less than 5%. Radiation therapy can be used in combination with surgery and/or chemotherapy; however, there should be a carefully coordinated treatment plan. The basic principle of cancer treatment with curative intent is to treat as early and as aggressively as possible. The first opportunity for tumor control is always the best opportunity. Radiation oncologists are using improved equipment and greater knowledge of radiation biology to maximize tumor control and to minimize normal tissue injury. Currently, most veterinary radiation oncology practices use an external beam source of radiation from either cobalt 60 teletherapy units or clinical accelerators. Many practices use daily treatments for 3 to 4 weeks. The relatively short overall treatment time prevents significant repopulation of tumor cells during the course of treatment. The total dose is divided into several smaller fractional doses that spares late responding normal tissues.