ESTRO 2025 - Abstract Book

S2600

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

Results: Maximum attenuation of TBI Step was 15% (static field) and 6% (full arc) at position C. Optimal HU values for the best agreement between measurements and calculations are -960HU and -300HU for Varian couch surface and inner foam, respectively and -960HU, 0HU and 600HU for rotational couch surface, interior and rotational adapter, respectively. Absolute deviations between measured and calculated attenuation were within 1%. Conclusion: Radiation attenuation by treatment couches is significant when in the beam path. TBI Step and Varian Exact © IGRT couches were successfully modeled in the Eclipse ® TPS, which currently supports virtual incorporation of support structures when planning. For accurate VMAT TBI planning, it is recommended that the two couches be included in dose distribution calculations. References: Savini, A., Bartolucci, F., Fidanza, C., Rosica, F., & Orlandi, G. (2016). Modeling of couch transmission in the RayStation treatment planning system. Physica Medica , 32 (5), 735–740. https://doi.org/10.1016/j.ejmp.2016.04.012 Teke, T., Gill, B., Duzenli, C., & Popescu, I. A. (2011). A Monte Carlo model of the Varian IGRT couch top for RapidArc QA. Physics in Medicine and Biology , 56 (24), N295–N305. https://doi.org/10.1088/0031-9155/56/24/N01 Tuğrul, T. (2018). Absorption ratio of treatment couch and effect on surface and build-up region doses. Reports of Practical Oncology & Radiotherapy , 23 (1), 1–5. https://doi.org/10.1016/j.rpor.2017.10.004 Keywords: Couch modeling; TBI Step; Varian IGRT couch Digital Poster Assessing the accuracy of EBT4 and EBT-XD films for in-vivo minibeam radiotherapy (MBRT) dosimetry Darwin Garcia, Chrystian Quintero Mateus, Maryam Mashayekhi, Doug Moseley, Scott Lester, Robert Mutter, Sean Park, Michael Grams Radiation Oncology, Mayo Clinic, Rochester, USA Purpose/Objective: MBRT uses an array of sub-millimeter wide, planar beams of radiation to deliver an alternating pattern of high dose “peaks” and low dose “valleys” to treat cancer. Following our recent first-in-human application of MBRT (1), subsequent clinical trials will require accurate in-vivo dosimetry of both the high (~10 3 cGy) and low (~10 2 cGy) doses that are the hallmark of MBRT (1). Gafchromic film is an attractive dosimeter providing 2-D dose measurements with high spatial resolution and near tissue equivalence. We evaluated the accuracy of dosimetric measurements spanning 0-7000cGy using the commercially available Gafchromic film models, EBT4 and EBT-XD (Ashland, NJ, USA). Material/Methods: Films were placed in a stack of 30x30x20cm 3 kV-rated plastic water and exposed to known radiation doses using an Xstrahl 300 orthovoltage unit (180 kVp) calibrated using the AAPM TG 61 in-air protocol (2). Calibration curves were generated for each film model using eight doses between 0-7000cGy. The accuracy of each calibration curve was tested using eight additional films exposed to known doses encompassing typical MBRT peak and valley dose ranges, between 150-6900cGy. Film calibration and analysis was performed using single-channel dosimetry with FilmQA Pro software following a previously published protocol (3). A water-equivalent phantom was 3-D printed to replicate a clinical patient treated with MBRT (Figure 1) and peak and valley doses were measured using both film models. EBT4 and EBT-XD films were exposed to 2475 monitor units, corresponding to a clinically delivered peak/valley dose of 1800/180cGy at the surface. Furthermore, to evaluate the robustness to MBRT dose-escalation, both film models were exposed to 4500 monitor units. 2172