ESTRO 2024 - Abstract Book

S4190

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2024

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

Stefanie Bertschi 1 , Kristin Stützer 1,2 , Jonathan Berthold 1,3,4 , Julian Pietsch 1,2 , Ulrik Elstrøm 5 , Anne Vestergaard 5 , Guillaume Janssens 6 , Stine Korreman 5,7 , Christian Richter 1,2,8 1 OncoRay – National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Helmholtz-Zentrum Dresden - Rossendorf, Dresden, Germany. 2 Helmholtz-Zentrum Dresden - Rossendorf, Institute of Radiooncology - OncoRay, Dresden, Germany. 3 Center for Advanced Systems Understanding, CASUS, Görlitz, Germany. 4 Helmholtz-Zentrum Dresden - Rossendorf, CASUS - Center for Advanced Systems Understanding, Dresden, Germany. 5 Danish Center for Particle Therapy, Aarhus University Hospital, Aarhus, Denmark. 6 Ion Beam Applications SA, Research, Louvain-la-Neuve, Belgium. 7 Department of Clinical Medicine, Aarhus University, Aarhus, Denmark. 8 Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany

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 (Figure 1a). In this study, we investigated whether CBCTs are suitable for PGI reference simulations.

Material/Methods:

For a homogeneous PMMA cylinder (Ø: 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 cm 3 ) 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 determining spot-wise integrated depth-dose (IDD) profiles. This allows for a direct spot-wise comparison between PGI range shifts and IDD range shifts.