ESTRO 2025 - Abstract Book

S3161

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

Material/Methods: A Cranial Verification Phantom and a Cranial 4Pi Immobilization Mask (Brainlab) was used. 17 BODY contours were generated in the Elements planning system by varying the Hounsfield Units (HU) between -200HU and -950HU from a simulation CT performed in a General Electric LightSpeed RT (Figure 1). The phantom was positioned in a Varian TrueBeam using the SGRT from Exactrac Dynamic. Final position was achieved by the X-ray imaging system. These differences in positioning indicate the accuracy of the SGRT system. As long as these measures were taken during different days, the displacement vectors were normalized using the daily calibration factor. A cubic fit and a confidence interval were calculated to analyze the HU threshold and find the optimal clinical range.

Results: The smallest displacement vector obtained with the cubic fit was 0.815±0.009mm. The representation of the normalized displacements against the HU threshold with a 95% confidence interval is shown in Figure 2. Through cubic fitting, it was determined that the optimal HU threshold is -574±3HU; however, with a 95% confidence interval, the range of -787HU to -385HU can also be utilized.

Conclusion: The HU threshold to generate BODY affects the accuracy of the positioning with a SGRT. In this study, an optimal threshold of -574±3HU was found to generate BODY with Brainlab Elements.

Keywords: SGRT, Elements, Exactrac Dynamic