ESTRO 2025 - Abstract Book

S3610

Physics - Quality assurance and auditing

ESTRO 2025

Denmark. 4 Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany. 5 Department of Radiation Oncology (Maastro), GROW School for Oncology and Reproduction, Maastricht University Medical Centre, Maastricht, Netherlands. 6 KU Leuven, Department of Oncology, Laboratory of Experimental Radiotherapy, Leuven, Belgium. 7 Department of Radiation Oncology, University Hospitals Leuven, Leuven, Belgium. 8 Center for Proton Therapy, Paul Scherrer Institut, Villigen, Switzerland. 9 Department of Radiation Oncology, University Medical Center Groningen, Groningen, Netherlands. 10 IRCCS Ospedale, San Raffaele, Milano, Italy. 11 UO Fisica Sanitaria, APSS, Trento, Italy. 12 Ion Beam Applications (IBA), Dosimetry Business Unit, Louvain-la-Neuve, Belgium. 13 IBA Dosimetry, SSDL Laboratory, Schwarzenbruck, Germany Purpose/Objective: This study presents the results of an on-site reference dosimetry audit conducted in nine centeres participating in the PROTECT trial a,b . The audit, organized by the IBA Secondary Standard Dosimetry Laboratory (SSDL,Germany), aimed to verify dose consistency as a quality assurance (QA) measure across four proton and five conventional radiotherapy centeres involved in the trial. Material/Methods: Measurements were conducted under reference conditions for the determination of absorbed dose to water, as specified by the IAEA TRS-398,Rev.1, code of practice (CoP). For proton beams, three monoenergetic fields (100, 170, and 220MeV) with a measurement depth of 2cm, and three modulated spread-out Bragg peak (SOBP) beams with measurements taken at the midpoint of the SOBP, were selected. For photon beams, each centeres selected a beam energy, and reference fields were characterized by a 10x10cm² field size at the phantom surface and a 100cm source-to-surface distance. Measurements were initially performed by the local team at each centre using their dosimety equipment and were subsequently repeated by the auditing team from IBA SSDL. IBA used an IBA Blue Phantom-PT with an IBA PPC05 ionization chamber for proton beams, an IBA WP1D water phantom with an FC65-G Farmer chamber for photon beams, and an IBA Dose-1 or IBA Dose-X electrometer for both. To minimize the influence of potential beam instabilities, results are reported as the average dose per monitor unit, based on a minimum of three repetitions. Results: The study demonstrates a high level of agreement in absorbed dose-to-water measurements between each centre and IBA for both proton and photon beams, with discrepancies generally within 2 % (Figure 1, Table 1). However, one proton centre (C2) exhibited a 3.4 % deviation, primarily due to differences in setup (using a measurement depth of 2.3cm instead of the recommended 2cm) and beam quality correction factor values (using values from TRS398 instead of TRS398,Rev.1). Adjusting for these variations, the discrepancy was reduced to 2.3 % (Figure 1). Similarly, one photon centre (C4) deviated from the IAEA recommendations by using a local dosimetry protocol. A closer agreement was achieved after accounting for these protocol differences (Figure 1). The estimated combined uncertainty for the study was 3.2 % for proton and 2.3 % for photon measurements.