S1636
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
patient imaging dose in CKRS. Copper filters demonstrated superior efficiency than tin, with a 0.1 mm copper filter providing the optimal trade-off between dose reduction and tube loading. Keywords: CyberKnife, Imaging dose, x-ray tube filtration Mini-Oral 1263 Validation of alanine dosimetry for small field measurements in a 1.5 T MRI-Linac Ilias Billas 1 , Yunuen Cervantes 2 , Luc Gingras 3 1 Radiotherapy and radiation dosimetry, National Physical Laboratory, Teddington, United Kingdom. 2 Département de physique, Université Laval, Québec, Canada. 3 Département de Physique, Université Laval, Québec, Canada Purpose/Objective: Alanine dosimetry is a well-established reference standard in high-energy photon beams under reference conditions in both conventional linacs and MRI-Linacs. Its performance, however, has not been evaluated for small field measurements in magnetic fields. This study investigates the feasibility and accuracy of alanine dosimetry for small field output factor measurements in an MRI-Linac, comparing it with an established small-field detector [1-3], a Medscint Hyperscint PRB-0002 plastic scintillator. Demonstrating consistent performance under these conditions is an essential step towards its potential use in end-to-end dosimetry audits for MRI-guided radiotherapy (MRIgRT) systems. Material/Methods: Alanine pellets were irradiated in an Elekta Unity 1.5 T MRI-Linac at 10 cm depth in water for field sizes ranging from 0.5×0.5 to 22×22 cm². Two pellet geometries were used: small pellets (2.5 mm Ø, 2.5 mm thick) mounted in microdiamond-shaped holders for fields ≤2×2 cm², and standard pellets (5 mm Ø, 2.5 mm thick) in Farmer-type holders for larger fields. The geometric beam centre was determined from inline and crossline scans with a microdiamond detector. Alanine holders were then positioned reproducibly at this centre. Each small-field irradiation was repeated three times; large fields once per set of five pellets. Repeat irradiations at 10×10 cm² were used to assess measurement repeatability. Alanine responses were compared with a plastic scintillator detector under identical irradiation conditions. Output factors (OF) were normalised to the 10×10 cm² field. Results: Alanine demonstrated high reproducibility, with a standard deviation of 0.35% at 10×10 cm². Agreement between alanine and scintillator OFs (figure 1) was within 1.74% for the 0.5×0.5 cm² field and within 0.5%
for all larger fields, well inside combined uncertainties (Figure 2). The results confirm excellent consistency between alanine and the scintillator detector across the full range of investigated field sizes.
Conclusion: Alanine dosimetry demonstrated excellent precision and agreement with an established scintillator detector in small fields under a 1.5 T magnetic field. These findings support its use as a reliable reference- class detector for small field dosimetry in MRI-Linac environments and highlight its potential for end-to- end dosimetry audits in MRIgRT. References: [1] Luc Gingras et al 2025 Med Phys. 2025;52:4844– 4861. Field output correction factors using a scintillation detector.[2] Yunuen Cervantes et al 2025 Phys. Med. Biol.70 105005. A systematic characterization of plastic scintillation dosimeters response in magnetic fields: I. Experimental measurements.[3] Yunuen Cervantes et al 2025 Phys. Med. Biol.70 105004. A systematic characterization of plastic scintillation dosimeters response in magnetic fields: II. Monte Carlo simulations. Keywords: alanine dosimetry, small fields, MRIgRT
Digital Poster 1287
Practical approach to 3D-printed bolus design: radiological thickness as a robust predictive parameter Aleksandra Bochyńska, Edyta Fujak, Agnieszka Walewska, Paweł Kukołowicz, Anna Zawadzka
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