Online-adaptive proton therapy: Feasibility of prompt-gamma verification for CBCT-based adapted plan

Purpose/Objective Cone-beam CT (CBCT) is a promising solution for 3D in situ imaging in an online-adaptive proton therapy workflow. However, CBCT scans have an increased uncertainty in determined CT numbers, making online treatment verification essential. Our aim is to verify adapted treatment plans based on CBCTs with prompt-gamma imaging (PGI) to detect unexpected treatment deviations as well as to offer independent quality assurance of adaptation and treatment. A reliable reference simulation is a prerequisite for PGI-based online treatment verification where measured PGI data are compared with reference simulations based on the CBCT (Figure 1a). In this study, we investigated whether CBCTs are suitable for PGI reference simulations. Material/Methods For a homogeneous PMMA cylinder (∅: 18 cm) and an anthropomorphic head phantom (CIRS 731 HN), two CT scans were acquired – a conventional fan-beam CT for reference and a CBCT. For each phantom, a corrected CBCT (cCBCT) and a virtual CT (vCT) were generated using RayStation (v13.0, RaySearch Laboratories AB), yielding 3 CBCT datasets and one fan-beam CT dataset. For both phantoms, a treatment plan with one field on a cuboid target structure (30.6 cm3) was generated (Figure 1b). PGI simulations were performed on all 3 CBCT datasets as well as on the fan-beam CT, serving as reference. Spot-wise range shifts between PGI simulations on the fan-beam CT and each CBCT dataset were extracted to examine the change in reference range due to the CT number differences. According to [1], the potential deviation in CT number (CBCT vs fan-beam CT) influences both, the depth dose distribution depending on the SPR value and the material assignment as well as the spectral prompt gamma (PG) emission depending on the material assignment. Both quantities (depth dose distribution and spectral PG emission) impact the simulated PGI profile. To distinguish both effects, we additionally calculated the depth dose distribution independently by d