ESTRO 2025 - Abstract Book

S3159

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

Purpose/Objective: Brainlab Exactrac Dynamic is a system composed of a Surface Guided Radiotherapy (SGRT) 4D thermal camera (which combines structured light and thermal radiation) and a stereoscopic X-ray system. This study aims to determine the optimal threshold of Hounsfield Units (HU), smoothing parameters, and elimination of void areas to generate the BODY in Raystation Treatment Planning System (RaySearch Laboratories), thereby optimizing its SGRT system compared to the X-ray system. Material/Methods: Cranial Verification Phantom and Cranial 4Pi Immobilization Mask were used. The simulation CT was performed in a General Electric LightSpeed RT. The phantom was positioned on a Varian Truebeam with a Brainlab Exactrac Dynamic System. 32 BODY contours were generated in the Raystation TPS under Hounsfield Units (HU) between -50HU and -960HU, with smoothing parameters of 1000, 200, 60, and 0. Also the elimination of void areas of 0, 0.1, 0.4, 1cm², and any area was investigated. The phantom was positioned three times using the structured light (SGRT) and corrected with the X-ray imaging system. The displacements indicated by the X-ray images compared to the structured light positioning reflect the accuracy of the Exactrac Dynamic SGRT technique. The measured displacement vectors were normalized by the daily calibration factor to take into account the measurements on different days. To analyze the HU threshold, a cubic fit was performed, and a confidence interval was calculated, providing a range of optimal HU thresholds, while analysis of variance (ANOVA) was used to evaluate smoothing and the elimination of void areas. Results: The representation of normalized displacements against the HU threshold with a 95% confidence interval is shown in Figure 1. The influence of smoothing and the elimination of void areas are shown in Figure 2. Through cubic fitting, it was determined that the optimal HU threshold is -714±6HU with a displacement vector of 0.68±0.06mm; however, with a 95% confidence interval, the range of -999HU to -525HU can also be utilized. The ANOVA tests indicate that neither smoothing nor the elimination of void areas are statistically significant.