Radiation therapy refers to the use of ionizing radiation for the local and regional treatment of patients with malignant tumors and occasionally selected benign diseases. As the name implies, teletherapy, involves the administration of ionizing radiation from an external source at a distance. Teletherapy is also called external beam radiotherapy. With teletherapy the radiation source is generally a machine designed to generate ionizing radiation in 1 of 3 forms. These forms of ionizing radiation include relatively low energy x-rays (orthovoltage radiation), relatively high energy x-rays or g-rays (megavoltage radiation), or electrons. The total radiation dose administered to the patient is divided into numerous smaller fractions. Currently the most popular schedule for administering radiation therapy in veterinary patients involves administering the radiation on consecutive weekdays (e.g. Monday through Friday) over 4 consecutive weeks. Each of these therapies involves the use of a short acting anesthesia to ensure consistent patient positioning and immobility.

Brachytherapy is a form of radiotherapy that involves applying radiation from a very short distance. Common methods of brachytherapy involve the use of radioactive surface applicators (i.e. plesiotherapy), or implants. Radioactive applicators are held directly next to the tumor for a time calculated to result in the administration of a prescribed dose. Radioactive implants (in the form of seeds, needles or suspensions) are generally surgically applied. The time over which the radiation dose is administered for implants is longer than for surface applicators. As a result the implants are generally left in place for days or even weeks.

Systemic radionuclide therapy is a form of radiotherapy that involves administering the source of the radiation into the patient. With systemic radionuclide therapy the physiology of the disease provides a major contribution to the therapy ultimately resulting in the delivery of the radionuclide to the tumor. By using a radioactive material that will be delivered to the tumor by the patient's own physiologic processes, it is possible to deliver a large dose of radiation to certain tumors with a minimal amount of patient manipulation. The most commonly utilized forms of systemic radiotherapy performed in veterinary medicine include radioiodine (¹³¹I) for thyroid tumors and radioactive samarium (¹⁵³Sm-EDTMP) for primary or metastatic bone cancer. Thyroid tissue has the unique ability to concentrate iodine. The nonspecific (by either oral or subcutaneous injection) administration of radioactive iodine to a patient with a thyroid tumor can result in the very selective delivery of a dose of radiation to that tumor. Similarly the intravascular administration of a phosphorus-like chemical (EDTMP) bound to a radioactive form of samarium (¹⁵³Sm) can result in the delivery of a dose of radiation to bone tumors.