ESTRO 2025 - Abstract Book

S2643

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

Digital Poster Implementation and evaluation of the off-axis Winston-Lutz radiosurgery test in three Linacs Maria João Cardoso 1,2 , Dalila Mateus 1,2 , Sandra Vieira 1 , Joep Stroom 1 , Carlo Greco 1 1 Radiotherapy, Fundação Champalimaud, Lisboa, Portugal. 2 Radiotherapy, Mercurius Health, Lisboa, Portugal Purpose/Objective: Our department performs radiosurgery on multiple targets of reduced dimensions in a single isocenter (SRS-SIMT). This work aims to implement the off-Axis Winston-Lutz (OAWL) test on the department’s linear accelerators to measure delivery accuracy with different gantry, collimator and couch rotations (GCC). The goals are to 1) evaluate if the deviation between the radiative and mechanical isocenter for different targets (isocentric and off-axis) is less than 1mm (tolerance); 2) find out if it is necessary to establish a limit on the target-isocenter distance for clinical plans; 3) select the most suitable accelerator for SIMT plans; 4) study the sensitivity of the OAWL. Material/Methods: Tested linacs were 1) Varian Edge HD-MLC (2014), 2) Varian TrueBeam HD-MLC (2012) 3) Varian TrueBeam Millenium 120MLC (2018). The MultiMet-WL SunNuclear® phantom was used (Fig.1a). It contains an isocentric target and five off-axis targets distanced up to 7 cm from the isocenter. The phantom was irradiated, over the MV imager, using the manufacturer's OAWL test with 15 different GCC combinations. The MV images were collected (Fig.1b) and analyzed in MultiMet-WLQA-v2 software. Sensitivity tests, with translational errors in the positioning of the phantom from 0.5 mm to 2 mm and rotational errors between 0.5º and 2.0º in GCC rotations, were irradiated and the respective images analyzed using the same software. Results: In seven months, the original OAWL test was run 21 times. The average field-target coincidence deviations for all targets and GCC combinations are shown in Fig.1c.