S2267
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
plans were mapped to the planning CT. The voxel-wise BED was calculated from the individual fraction doses, assuming an alpha/beta of 10 Gy for the target and 3 Gy for OARs. Additionally, all physical fraction doses were summed to obtain the voxel-wise accumulated physical dose, which was converted to BED assuming the same physical dose distribution for each fraction (pseudo-BED). The difference between accumulated BED and pseudo-BED quantified the biological impact of inter-fraction dose distribution variations. Results: Figure 1 shows boxplots of the differences between accumulated BED and pseudo-BED for various endpoints relevant to EC. The biggest differences are seen in the PTV BED1095%. Figure 2ab show two fraction dose distributions, with the largest differences highlighted. Figure 2b shows the corresponding difference between accumulated and pseudo-BED for the same case over the whole treatment. The largest differences of up to 1 Gy occurred in areas where inter-fraction dose variations were most pronounced.
to uncorrected tumor rotation during real-time adaptive prostate stereotactic body radiation therapy. Med Phys. 2023;50:20-9. https://doi.org/10.1002/mp.16094[2] Hunt MA, Sonnick M, Pham H, Regmi R, Xiong JP, Morf D, et al. Simultaneous MV-kV imaging for intrafractional motion management during volumetric-modulated arc therapy delivery. J Appl Clin Med Phys. 2016;17:473-86. https://doi.org/10.1120/jacmp.v17i2.5836[3] Klucznik KA, Ravkilde T, Skouboe S, Keall PJ, Happersett L, Pham H, et al. First Clinical Online Real-Time Motion- Including Prostate and Bladder Dose Reconstruction during Prostate Radiotherapy Delivery. AAPM. Keywords: Prostate RT, Intrafraction motion, Dose- guidance Digital Poster 4199 Biologically Effective Dose Accumulation for Online Adaptive Radiotherapy of Esophageal Cancer Björn Zobrist 1,2 , Michael Karl Fix 1 , Daniel Schmidhalter 1 , Frédéric Corminboeuf 1 , Hossein Hemmatazad 3 , Nicolas Bachmann 3 , Peter Manser 1 , Jenny Bertholet 1 1 Divison of Medical Radiation Physics and Department of Radiation Oncology, Inselspital, Bern University Hospital, and University of Bern, Bern, Switzerland. 2 Graduate School for Cellular and Biomedical Sciences, University of Bern, Bern, Switzerland. 3 Department of Radiation Oncology, Inselspital, Bern University Hospital, University of Bern, Bern, Switzerland Purpose/Objective: Esophageal cancer (EC) radiotherapy (RT) precision is affected by large day-to-day anatomical changes. In CBCT-based online adaptive radiotherapy (oART) the treatment plan is adapted daily using a synthetic CT (sCT) generated from daily CBCT and planning CT. Promising results were observed with oART for EC, demonstrating improved pathological major response rates compared to non-adaptive cohorts1. Physical dose accumulation can be used to document treatment delivery. Anatomical changes and plan adaptation lead to day-to-day variations in the dose distribution. In this work, we calculate the voxel-wise accumulated biological effective dose (BED) to account for variations in fraction dose distribution during oART. Material/Methods: BED was accumulated for 23 patients treated for EC with oART (50.40 Gy to the median PTV in 28 fractions). First, the planning CT was deformably registered to each fraction’s sCT using the NiftyReg free-form algorithm2. Using the resulting deformation vector fields and deformable voxel geometry Monte Carlo dose calculations3, dose distributions of adapted
Conclusion: Differences between accumulated BED and pseudo- BED were observed for both targets and OARs, with the largest deviations found in the PTV BED1095%. These variations indicate that changes in the fraction dose distributions during oART can influence the biological effect, compared to assuming the same
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