Radiation therapy is one of the oldest modalities for cancer treatment and is currently prescribed to more than 50% of all patients. It is based on delivering high doses of ionizing radiation to well-localized tumor targets in the body. The goal is to kill all the tumor cells with acceptable toxic effects to the surrounding normal tissue, which is unavoidably exposed. Indeed, radiotherapy success is limited by the toxicity in the normal tissue.
X-rays (photons) are used in most patients treated with conventional radiotherapy. As x-rays are delivered from an external source, they deposit most of their energy upstream of the tumor in healthy tissue. This energy deposition also occurs beyond the tumor, affecting additional healthy tissue. Special beams delivered from many directions and intensity modulation are used to increase the ratio of tumor to healthy tissue dose; however, the volume of irradiated healthy tissue increases (“dose bath”). Even with these technological advantages, x-rays are relatively inexpensive and serve as the biological response baseline for comparisons with other radiation treatment modalities.