In-beam PET at high-energy photon beams: a feasibility study

For radiation therapy with carbon ion beams, either of the stable isotope {}^{12}C or of the radioactive one {}^{11}C, it has been demonstrated that the beta+-activity distribution created or deposited, respectively, within the irradiated volume can be visualised by means of positron emission tomography (PET). Those PET images provide valuable information for quality assurance and precision improvement of ion therapy. For this, dedicated PET scanners have been integrated into treatment sites at HIMAC (Japan) and GSI (Germany) to make feasible PET imaging during therapeutic irradiation (in-beam PET). A similar technique may be worthwhile for radiotherapy with hard bremsstrahlung. In addition to the dose delivery monitoring, in-beam PET has been primarily developed for, it may be expected that radiation response of tissue can be detected by means of in-beam PET. We investigate the applicability of PET for treatment control in case of using bremsstrahlung spectra produced by 15 - 50~MeV electrons. Target activation due to (gamma,n)~reactions at energies above 20~MeV yields moderate beta+-activity levels, which can be employed for imaging. The radiation from positrons produced by pair production turns out to be not usable at present because the detectors are overloaded due to the low duty cycle of medical electron linear accelerators, although the degradation of images caused by travelling of positrons between creation and annihilation seems to be tolerable.