ESTRO 2025 - Abstract Book

S3671

Physics - Quality assurance and auditing

ESTRO 2025

Purpose/Objective: The purpose of this study is to evaluate the dosimetric impact of an Elekta Agility MLC positional error that meets the recommended international tolerances. Material/Methods: MLC positional data was obtained during weekly QA by performing Elekta’s test “MLC Leaf and Jaw Positional Error” measured on the on-board EPID. This test measures the positional error of five nominal MLC leaf bank positions by performing a radiation isocenter detection, an EPID and collimator angular alignment, and a picket fence test [1]. This data was analyzed, passing the routine QA tolerances for more than two years. The mean positional error for each leaf and for each nominal position was calculated. RT plans were modified to account for the leaf positional errors measured, performing an interpolation between adjacent nominal positions using an in-house Python script. Modified RT plans were imported back to the TPS and recalculated on the original CT. In order to perform a finer analysis, both original and modified plans were calculated with a 2mm grid size and a 0.5% statistical uncertainty. Additionally, the total segment area change between original and modified plans was calculated to measure the plan modification. Results: It was observed that our MLC has an asymetric systematic opening with a wider open of the right leaf bank. Due to this leaf opening, all plans present general overdosage. This overdosage has no clinical relevance since the overdosage to spinal cord is still below accepted tolerances, and PTV overdosage does not exceed clinical recomendations.