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Radiation therapy as an addition to the available treatment options allows a greater scope and choice of therapy in many instances. Examples of tumors commonly treated with radiation include certain oral tumors, nasal tumors, brain tumors, mast cell tumors, and soft tissue sarcomas. Radiation can be used with palliative (e.g. to palliate bone pain with primary appendicular osteosarcoma) or curative intent and be used on its own or with surgery. Sources of radiation therapy include megavoltage (>1 million electron volts of photon energy = high energy; maximum dose to tumor rather than skin) and orthovoltage (150–500 kVp = low energy; maximum dose to skin surface) external beam radiation, brachytherapy (interstitial placement of radioactive isotopes), or systemic or cavitary injection of radioisotopes (e.g. iodine131). Megavoltage irradiation has advanced with 3D imaging and planning, multi‐leaf collimators, and custom‐made blocks (Figures 2.1–2.3).