In particle therapy, beams of energetic protons or certain ions are used to irradiate tumors or other types of diseased tissue. The interaction of therapeutic particle beams in matter is characterized by a very well-defined range, a high-dose deposition localized in a small region (the Bragg peak), and the steep dose gradient at the distal edge of the Bragg peak. On the other hand, these features imply that possible uncertainties in the determination of the particle range might have severe consequences, such as tumor underdosage, or the irradiation of healthy tissue.

To minimize the risks, it is necessary to quantify the actual dose deposition with high precision. For this purpose, verification methods aimed to determine the range of the particle beam are being investigated worldwide. Some of these methods are based in the detection of secondary radiation. For example, positron emitting nuclei are created along the beam path; their decay and subsequent annihilation of the emitted positrons give rise to two annihilation photons which can be detected using the same technology as in Positron Emission Tomography (PET). Another important effect is the excitation of nuclei along the particle path; these nuclei return to their ground state by emitting single gamma rays ("prompt-gammas"). Imaging techniques are being developed to use prompt-gammas to indirectly determine the particle range. One proposed technology is based on Compton cameras.

• SiFi-CC collaboration
• Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Torino, Italy