Adapting PET Instrumentation for Imaging During Photon External Beam Radiotherapy Treatment

This paper gives a brief characterization of the effects of in-plane photon external beam radiotherapy on PET instrumentation, and adaptations to PET instrumentation that can be used to enable biology-guided radiotherapy (BgRT). BgRT is a novel treatment approach that uses the PET emissions from tumors (after uptake of a tracer such as FDG) as a biological fiducial to deliver radiation that tracks the tumor physiological motion in real-time. However, to implement BgRT, in-plane PET data must be acquired during treatment. This is a unique challenge due to the large amount of scattered radiation produced by the treatment beam. The paper describes a system that utilizes two 90-degree PET detector arcs that rotate with an in-plane 6MV linear accelerator (linac). The PET arcs use lutetium-based scintillation crystals coupled to SiPMs. After adaptations to the acquisition time, bias voltage power supplies, and calibration procedure, the PET system maintained <; 1ns FWHM time resolution, <; 14% 511keV energy resolution, with > 94% on time while a water-filled Jaszczak phantom was treated for 41 minutes with a 2cm primary collimator and all multi-leaf collimator leaves open. In the future, improvements such as developing PET scintillation crystals with lower afterglow levels could further mitigate the effects of the photon treatment beam on the PET system and potentially enable time-of-flight performance.