ESTRO 2025 - Abstract Book

S2599

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

reproducible patient rotation, but they can also impact dose distribution. This study focuses on modeling and validating the rotational TBI Step (IT-V, Innsbruck, Austria) used alongside a Varian Exact © IGRT couch.

Material/Methods: A. Couches modeling in Eclipse TPS

Both couch and over-couch consist of a carbon fiber shell sandwiching low-density foam. The Varian couch transitions linearly from a thin top to a thicker end, while the TBI Step has constant thickness and includes a rotatable adapter that indexes to the Varian couch. Surface and inner material of both couches and rotative adapter were contoured and modeled manually using CT images. B. Attenuation measurements on LINAC Measurements were performed using 6MV photons and a 10x10cm 2 field and a Farmer ionization chamber placed in a solid water phantom for a static gantry angle (180°) and in a Quasar ® phantom for a full arc (179°-181°). Attenuation measurements were acquired at three longitudinal positions (Figure 1). The phantoms were positioned such that the center of the Farmer ionization chamber coincided with the isocenter, maintaining a source-to chamber distance of 100 cm for all gantry angles. Relative attenuation was calculated with gantry 0° as a reference, where no couch attenuation occurred. C. Validation of dose calculation using couches model Virtual phantoms replicating the experimental setup were created in the Eclipse ® TPS. The modeled couches were placed beneath the phantoms, and attenuation values were calculated using the AAA algorithm. HUs values were tuned for each modeled component to optimize the agreement between measured and calculated attenuations.