External Photon Beams: Physical Aspects

Radiotherapy procedures fall into two main categories: external beam radiotherapy and brachytherapy. In external beam radiotherapy the radiation source is at a certain distance from the patient and the target within the patient is irradiated with an external radiation beam. In brachytherapy (see Chapter 13) radiation sources are placed directly into the target volume (intracavitary or interstitial brachytherapy) or onto a target (surface mold or intraoperative radiotherapy). Most of the external beam radiotherapy is carried out with photon beams, some with electron beams, and a very small fraction with more exotic particles, such as protons, heavier ions or neutrons.