Pile-up correction techniques for real-time dosimetry in photon radiotherapy

In radiotherapy, accurate in vivo dose monitoring can improve overall success of treatment. For hadronic therapy, the approach is to measure residual radioactivity after the treatment. In the more widely used, gamma ray radiotherapy under 20 MV, dose monitoring in vivo is far more complicated, since the only way to measure irradiation accuracy is while tinac is operating at extremely high dose rates. Linac´s leakage outside the nominal beam can be as high as 10¹¹ photons per cm²s at 1 m, leading to a 2 Gcps event rate even on a small detector crystal (2 × 2 × 20 mm³ LYSO). One of the prerequisites for real time dose monitoring under such extreme conditions is the ability to handle high count rates as well as measuring the energy of incident photons in highly piled-up regimes. The same techniques can be applied elsewhere, ego to improve results in moderately saturated systems. The scope of the first part of our study was to determine the count rate dependance of energy resolution, photo peak efficiency and background characteristics for scintillation crystals at high count rates. Pile-up correction of the digitized PMT´s output led to both higher efficiency and accuracy than classical methods at high count rates. Thus, we were able to measure energy spectra of incident photons at rates above 10⁷ events per second. Fast asynchronous digitization and the application of real-time digital pile-up correction techniques enable PET setups to operate at photon fluence rates almost three orders of magnitude higher than those assumed by [1] and could make dosimetry in gamma ray radiotherapy feasible with significantly less lead shielding. The engineering problem of handling high data rates and the fundamental question of random coincidences under such regimes remains a subject of further work.