External beam radiation therapy, also called external radiation therapy, is the most common form of radiation therapy. The patient sits or lies on a couch and an external source of radiation generated by a particle accelerator is pointed at a particular part of the body, directing the radiation beams directly at the tumor.
Rays of different energy are used depending on the tumor depth. Lower energy x-rays are used to treat skin cancer and superficial structures, whereas higher energy x-rays are used to treat deep-seated tumors (e.g. bladder, bowel, prostate, lung, or brain).
While X-ray and electron beams are by far the most widely used sources for external beam radiotherapy, a growing number of centers use heavier particle beams, particularly proton sources), which show considerable advantages in comparison with x-ray therapy.