S2326
Physics - Quality assurance and auditing
ESTRO 2026
Physics, Radiobiology and Radiation Protection Group, IPO Porto Research Centre (CI-IPO), Portuguese Oncology Institute of Porto (IPO-Porto), Porto Comprehensive Cancer Center (Porto.CCC) & Rise@CI- IPOP (Health Research Network), Porto, Portugal Purpose/Objective: Small-field dosimetry is challenging and the accurate determination of field output factors (OFs) is crucial for rigorous dose calculation. This study aimed to develop a small-field OF dosimetry audit methodology based on the IAEA TRS-483, using remote data reporting combined with on-site measurements with a solid-state detector and film. The main goal was to verify OFs and beam profiles for small field sizes. The proposed methodology was evaluated on a pilot centre, for future replication in different radiotherapy centres. Material/Methods: The audit process involved two stages. Initially, raw data measurements were reported via a detailed questionnaire for nominal field sizes of 0.5 × 0.5, 1 × 1, and 2 × 2 cm2, for photon beams with energies 6, 6 FFF, 10 and 10 FFF MV. The correct application of TRS-483 recommendations and correction factors were also assessed. The second stage involved on-site measurements. The full width at half maximum (FWHM) of beam profiles in both in-plane and cross- plane directions were measured with a microdiamond detector (PTW) and compared with GafchromicTM EBT3 film (Ashland) measurements, for correction factors determination. The OFs were validated performing point doses measurements with the solid- state detector and compared with commissioning results. An uncertainty budget was also calculated for each OF measurement. Results: A small field dosimetry questionnaire was created, for small field dosimetry practice characterization and remote data collection prior to on-site measurements, and it was able to verify the correct application of TRS- 483 guidelines.The discrepancy between on-site measured and commissioned OFs increased progressively as the field size decreased across all tested energies. The maximum difference of 3.5 % was observed for 10 FFF MV energy, for the 0.5x0.5 cm2 field. The difference between the FWHM obtained with films and microdiamond detector increases with decreasing field size, resulting in a maximum variation of 0.40 % in the correction factor value. For each OF, an uncertainty budget was determined based on the charge readings, the applied TRS-483 correction factors, and the establishment of measurement conditions. The relative standard uncertainty increases with decresing field size (Table 1), within all photon beam energies.
Conclusion: The developed methodology for small-field dosimetry audits has proven to be effective, successfully validating the OF data from the pilot centre, as the differences obtained are accounted for in the uncertainty value. The goal of future audits is to serve as an external and independent verification of small- field OFs across various radiotherapy centres, and build a reference database grounded on this measurements. References: International Atomic Energy Agency (2018) Dosimetry of small static fields used in external beam radiotherapy: An international code of practice for reference and relative dose determination prepared jointly by the IAEA and AAPM. Vienna, Austria: IAEA.Lechner, W. et al. (2022) ‘A multi-institutional evaluation of small field output factor determination following the recommendations of IAEA/AAPM TRS- 483’, Medical Physics, 49(8), pp. 5537–5550. https://doi.org/10.1002/mp.15797.Akino, Y. et al. (2020) ‘Small-field dosimetry of TrueBeamTM flattened and flattening filter-free beams: A multi-institutional analysis’, Journal of Applied Clinical Medical Physics, 21(1), pp. 78–87. https://doi.org/10.1002/acm2.12791. Keywords: Small fields, output factors, dosimetry audit Development of an automated plan filtering and extraction application to streamline large-scale quality assurance of radiation therapy plans Wei Jie Ang, Ming Long Melvin Chew, Li Kuan Ashley Ong, Jing Hu Division of Radiation Oncology, National Cancer Centre, Singapore, Singapore Purpose/Objective: Large-scale audits and quality assurance in radiation therapy often require manual retrieval of treatment plans and extraction of plan metrics, which is tedious and prone to errors [1]. While automated extraction via scripting is an alternative, it requires significant programming expertise and scripts must be Digital Poster 339
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