ESTRO 2025 - Abstract Book

S2673

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

4092

Digital Poster Stereotactic radiotherapy: characterization of instrumentation for pre-treatment dosimetry Marco Fortunato 1 , Paolo Colleoni 2 , Angelo Di Naro 3 , Roberta Muni 3 , Sara Molli 3 , Michele Sala 4 1 Department of Hygiene and Health Prevention, ATS Brescia, Brescia, Italy. 2 Freelance profession, Medical physicist, Bergamo, Italy. 3 Division of Radiotherapy, ASST Papa Giovanni XXIII, Bergamo, Italy. 4 Division of Radiology, ASST Papa Giovanni XXIII, Bergamo, Italy Purpose/Objective This study aims to compare two instruments for pre-treatment dosimetry in stereotactic radiotherapy: Portal Dosimetry, based on Electronic Portal Imaging Device, and SRS MapCHECK, a diode array with a planar geometry. Specifically, the study seeks to evaluate the sensitivity, short-term reproducibility, and angular dependence of SRS MapCHECK, exploring how variations in Monitor Units and dose rate affect the accuracy of dosimetric measurements in both instruments. Material/Methods To evaluate the performance of the devices, over 300 fixed 5x5cm² fields (without fluence modulation, with collimator and gantry at 0°) were delivered in order to freeze the degrees of freedom of the delivery conditions to those actually under study. The angular dependence of SRS MapCHECK was studied in two phases: in the first, 12 entries were acquired at 30° intervals across the entire rotation angle; in the second, a continuous gantry rotation (0°-360°) was performed with continuous delivery at a fixed dose rate and fixed field. The data were analyzed using the Г(1%, 1mm), Г(1%, 2mm), Г(1%, 3mm), Г(2%, 1mm), Г(2%, 2mm), Г(2%, 3mm), Г(3%, 1mm), Г(3%, 2mm), Г(3%, 3mm) tests. To compare the correspondence between the devices, the Wilcoxon test was applied to the pre-treatment dosimetry of 10 stereotactic studies in the pulmonary region. The study provided an analysis of the instrumental performance in a clinical context. Results The analyzed instrumentation converges in sensitivity and reproducibility for Г(2%, 2 mm), with a passing rate of 95% across the entire set of measurements. However, for Г values greater than 2% and 2 mm, a deterioration in reproducibility was observed, indicating that, in IMRT/VMAT plans, exceeding the Г-test does not always ensure accurate representation of dosimetric quality.