ESTRO 2025 - Abstract Book

S4104

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2025

2866

Digital Poster Evaluation of Positional Accuracy in Stereotactic Radiotherapy Using Four Different Immobilization Masks Mattia Nicoletto, Marco Fusella, Francesca Dusi, Davide Fiorentin, Martina Ometto, Marco Ciro Schifano, Erick Maffi, Alessandro Testolin Radiotherapy, Policlinico Abano Terme, Padova, Italy Purpose/Objective: Thermoplastic masks are commonly used for immobilizing patients in stereotactic radiotherapy for brain lesions, yet there is a need for enhanced accuracy to ensure reproducible patient positioning across treatment fractions. This study compares four immobilization masks, evaluating their effectiveness in minimizing positional shifts. The analysis of translational (Vrt, Lng, Lat) and rotational (Pitch, Roll, Rtn) shifts between fractions serves as a surrogate for immobilization accuracy, with lower standard deviations (SD) indicating more stable setups. Material/Methods: A total of 12-20 patients per mask type were analyzed, with each undergoing five fractions. Daily cone-beam computed tomography (CBCT) images were used to measure translational and rotational shifts across six axes for each mask group. Statistical analyses included ANOVA and post-hoc t-tests to assess the significance of differences in positioning accuracy among the masks. Results: Significant differences were observed in rotational shifts, with Pitch and Roll exhibiting statistically significant variations across mask groups (p < 0.05). The DS mask demonstrated the lowest average shifts in both Pitch and Roll, indicating superior rotational stability. In contrast, the CI mask showed the highest variability in these parameters. Overall, calculating the 3D shift vector the DS mask had the lowest average dispersion of the data , followed by the SO, CS, and CI masks, confirming its status as the most stable option. However, translational shifts (Vrt, Lng, Lat) did not differ significantly between masks, and an overall stability comparison across all parameters did not reach statistical significance (p > 0.05), except for rotational shift like Pitch and Roll (p = 0.035 and P = 0.029 respectively).