ESTRO 2025 - Abstract Book

S2646

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

3726

Digital Poster Patient-specific QA of proton therapy plans for Vestibular Schwannoma using EBT4 films Kinga Graczyk 1,2,3 , Małgorzata Liszka 3 , Athanasia Christou 3 , Anna Maria Flejmer 3,4,5 , Alexandru Dasu 3,4 1 Department of Medical Physics, Greater Poland Cancer Centre, Poznań, Poland. 2 Department of Electroradiology, University of Medical Sciences, Poznań, Poland. 3 -, The Skandion Clinic, Uppsala, Sweden. 4 Department of Immunology, Genetics and Pathology, Uppsala University, Uppsala, Sweden. 5 -, Uppsala University Hospital, Uppsala, Sweden Purpose/Objective: Vestibular Schwannoma (VS) is a benign tumor arising from the vestibular branch of the eighth cranial nerve. Fractionated proton beam therapy (FPRT) is considered in its management because of the potential effect of sparing the organs-at-risk (OARs) near the target. High dose gradients and the proximity of the planning target volume (PTV) to OARs present significant challenges for Patient-Specific Quality Assurance (PSQA). The aim of this study was to investigate the potential of radiochromic films for PSQA for this diagnosis in comparison to established detector array-based PSQA methods. Material/Methods: A set of five treatment plans for vestibular schwannoma, designed for the IBA Proteus PLUS System (IBA Proton Therapy, Brussels, Belgium), was included in the study. PSQA for planes corresponding to relevant depths for the target and OARs was performed using a 2D-array MatriXX PT as a clinical reference (IBA Dosimetry, Schwarzenbruck, Germany) and GAFChromic EBT4 films (Ashland Inc., Wilmington, Delaware, USA) in a solid water phantom. The GAFChromic films were calibrated at known doses using a 150 MeV proton beam using a Roos chamber (PTW, Freiburg) as a reference detector. The films were scanned with an Epson Perfection 10000 XL scanner (Seiko Epson Corporation, Japan) under the following conditions: 72 dpi, red-green-blue (RGB) mode, no color or sharpness corrections, and consistent orientation. The images were analyzed in the myQA Patients software (IBA Dosimetry, Schwarzenbruck, Germany), applying local gamma test parameters of 3%/2mm and a dose threshold of 10%. Results: Both verification methods showed comparable performances (Figure 1), with gamma passing rates (GPR) exceeding 90% for 95% of measured dose distributions. The highest evaluation result was obtained using the MatriXX PT detector for PTV, while the lowest was seen with the films for OARs. The average GPR values were 94.6% for OARs and 97.2% for PTV with the MatriXX PT detector, and 95.3% for OARs and 95.2% for PTV with the EBT4. The maximum and minimum GPR recorded were 100% and 87.7% for the MatriXX PT, and 99.3% and 82.9% for the EBT4, respectively.