Radiation physics and chemistry in heavy-ion cancer therapy

Heavy ions, such as carbon and oxygen ions, are classified as high-LET radiations, and produce a characteristic dose-depth distribution different from that of low-LET radiations such as gamma ray, x-ray and electrons. Heavy ions loose less energy at the entrance to an irradiated biological system up to some depth than the low-LET radiations, while they dump a large amount of dose within a very narrow range at a certain depth, producing the characteristic sharp peak, and this peak domain of the dose is called the Bragg peak. Therefore, by controlling the Bragg peak, it becomes possible to irradiate only the tumor region in a pin-point manner, while avoiding irradiation of the normal tissue, thus making heavy-ion therapy ideal for deep-seated tumor treatment. However, very little is known about what is going on in terms of physics and chemistry inside the Bragg peak. In this paper the current status of our understanding of heavy-ion interactions and remaining problems of physics and chemistry for the heavy-ion treatment are explored, particularly in the Bragg peak region. Specially, the survey of the basic physical quantity, the mean energy required to form an ion pair (W value) for heavy ions of interest for radiotherapy is presented. Finally, the current clinical status of heavy-ion therapy is presented