ESTRO 2025 - Abstract Book

S3232

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2025

1 Comprehensive Cancer Center, Helsinki University Central Hospital, Helsinki, Finland. 2 Department of Science, University of Oulu, Oulu, Finland. 3 Radiation Therapy Ward, Clinical Research Institute HUCH Ltd., Helsinki, Finland. 4 Department of Oncology and Radiotherapy, Oulu University Hospital, Oulu, Finland Purpose/Objective: The novel MR-linac systems enable the tracking of the target volume by utilising the MR cine imaging, resulting in more precise gated radiotherapy of moving targets [1] [2]. However, the tracking performance of the treatment system needs to be verified. In this study, we show how the tracking accuracy of ViewRay MRIdian system is affected by the size of tracking margins and the velocity of a moving target. Material/Methods: We used the CIRS Zeus Motion Management QA Phantom with a house-made radiochromic film insert. The water filled cylindrical insert contains a steel ball (radiation target) with a diameter of 0.8 cm and a plastic ball (tracking target) with a diameter of 1.8 cm. The film was wrapped around the surface of the insert, which can be moved in a longitudinal direction. First, we studied how the change of the tracking margins affects the accuracy of the gating. We tested tracking margins of 2 mm, 3 mm and 4 mm using the 1.8 cm plastic ball as a tracking target. For the movement pattern of the phantom insert, we used cos 6 function with an amplitude of 2 cm and a cycle time of 6 s. Secondly, we measured how the speed of the movement affects the gating accuracy. For this test, we used tracking margin of 3 mm and changed the cycle time of the cos 6 function from 6 s to 3 s. Cine imaging was performed in a sagittal plane with a frequency of four frames per second. The films were irradiated with two perpendicular fields with a field size of 2.5 cm x 4 cm. Results: The measured dose of the static target was used as a reference. The gamma indices (3mm, 3%) for measurements with tracking margins of 2 mm, 3 mm and 4 mm were 98.8 %, 98.4 % and 98.3 % respectively. For the faster moving target, the gamma index (3mm, 3%) was 97.3 %. The amount of spreading of the dose of a moving target compared to a static target is within the tracking margin for all the test cases (figures 1 and 2).