<unk>  <unk>  is the most common form of radiotherapy (radiation therapy). The patient sits or lies on a couch and an<unk>  source of ionizing radiation is pointed at a particular part of the body. In contrast to brachytherapy (sealed source radiotherapy) and unsealed source radiotherapy, in which the radiation source is inside the body,<unk>  directs the radiation at the tumour from outside the body. Orthovoltage ("superficial") X-rays are used for treating skin cancer and superficial structures. Megavoltage X-rays are used to treat deep-seated tumours (e.g. bladder, bowel, prostate, lung, or brain), whereas megavoltage electron<unk> s are typically used to treat superficial lesions extending to a depth of approximately 5 cm (increasing<unk>  energy corresponds to greater penetration). X-rays and electron<unk> s are by far the most widely used sources for<unk> . A small number of centers operate experimental and pilot programs employing<unk> s of heavier particles, particularly protons, owing to the rapid dropoff in absorbed dose beneath the depth of the target.
External beam radiotherapy
Chemoradiotherapy
Radiation therapy
Plaque radiotherapy
Tomotherapy
Stereotactic radiation therapy
Irradiation
Selective internal radiation therapy