ESTRO 2025 - Abstract Book

S3184

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

1 Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, USA. 2 Faculty of Medicine, OncoRay – National Center for Radiation Research in Oncology, Dresden, Germany. 3 Institute of Radiooncology, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany Purpose/Objective: Online adaptive proton therapy (OAPT) is a highly researched topic, with its first clinical implementation marking a recent milestone [1]. As more centers consider implementing OAPT, the choice of daily imaging modality becomes critical. While cone-beam CT (CBCT) has been employed for direct photon dose calculation using Varian's Ethos system [2], no CBCT-based method has yet been approved by the Food and Drug Administration (FDA) or Medical Device Regulation for proton dose calculation. In this study, we investigate the applicability of novel, FDA-approved CBCT correction methods for OAPT of prostate cancer. Material/Methods: Two-field proton plans were retrospectively created for ten prostate cancer patients previously treated with adaptive radiotherapy on Ethos (37.5 Gy to the CTV in five fractions). The proton plans were optimized to the same prescription and approved by an experienced medical physicist. For each patient, five daily CBCTs with clinically approved contours were acquired using HyperSight imaging. Overall, four different CBCT image correction methods were compared for OAPT: i) Native Ethos HyperSight CBCT (CBCT Ethos ), ii) Deformed planning CT as used in Ethos (dCT Ethos ), iii) CBCT corrected using RayStation's histogram-based algorithm (CBCT RayStation ), and iv) Deformed planning CT with an air cavity correction from RayStation (dCT RayStation ). CBCT Ethos used a dedicated, scanner-specific stopping power ratio (SPR) calibration curve [3], while all other methods used the clinical calibration of the planning CT. For all fractions, two delivery scenarios were calculated on the daily images using the same Monte Carlo dose engine: Non-adaptive (recalculated nominal plan) and OAPT (in-house workflow) [4]. Results: Dosimetrically similar results were demonstrated in all patients for each CBCT correction method except CBCT Ethos . Figure 1 shows non-adaptive dose-volume histograms (DVHs) for two example patients, one with good agreement for all correction methods and one with large discrepancies for CBCT Ethos . Closer inspection of this case revealed that this patient was severely obese, leading to inconsistencies in CT numbers (i.e., inconsistencies in SPR), thus affecting the calculated proton range.

Figure 2 summarizes dose-volume metrics, with fraction-wise differences in the bottom row confirming our findings in Figure 1. However, it is evident that changes in patient anatomy, which can be corrected with OAPT, far outweigh the uncertainties associated with the choice of CBCT correction method.