S1984
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
(California US) TrueBeam (v2.7) cone beam CTs (CBCTs). Treatment plans were calculated on the corrected CBCTs and the PTV D50 was compared to a recalculation using a validated bulk-density override approach [1, 2]. Results: For gynaecological cases, the PTV D50 deviation between the corrected CBCT and bulk density calculations was -0.1% with all comparisons being within 0.8% and 92% of comparisons being under 0.5%. For H&N, the median and maximum PTV D50 deviations were 0.0% and 0.3%, respectively. For rectum, the median and maximum PTV D50 deviations were all under 0.2%. For prostate bed, the median and maximum PTV D50 deviations were -0.1% and 0.7%, The corrected CBCT algorithm available within the TPS resulted in acceptable dose calculation accuracy for a wide range of body sites including H&N and male and female pelvis for images acquired on a variety of common treatment platforms. The corrected CBCT algorithm is quicker, easier to use, and less prone to user error than the bulk-density approach. Therefore, as long as the image quality is suitable for contouring the targets and OARs for the indication and protocol being treated, we conclude that the algorithm is a suitable strategy for employment within online (and offline) adaptive radiotherapy workflows. References: [1] Dunlop A. et al., Comparison of CT number respectively. Conclusion: calibration technique for CBCT-based dose calculation. Strahlentherapie und Onkologie 191: 2015, 970-978.[2] Chick J. et al., Towards rapid and efficient simulation- free radiotherapy: MR guided adaptive prostate radiotherapy on the MR-Linac using diagnostic MRI reference planning. Radiotherapy and Oncology 211: 2025. Keywords: dose calculation, online adaptive radiotherapy Probabilistic optimization for systematic and random uncertainties in radiation therapy Albin Fredriksson 1 , Erik Engwall 1 , Jenneke de Jong 2,3 , Johan Sundström 1 1 Research and Development, RaySearch Laboratories, Stockholm, Sweden. 2 Radiotherapy, Erasmus MC Cancer Institute, Rotterdam, Netherlands. 3 Medical Physics and Informatics, HollandPTC, Delft, Netherlands Purpose/Objective: Geometric uncertainties, such as patient setup and range uncertainty, can degrade treatment quality in Digital Poster 4238
reliability in patient-specific QA. The proposed workflow offers a reproducible and clinically feasible method for local commissioning of Mobius3D and enhances confidence in automated secondary dose verification for IMRT, VMAT, and stereotactic
treatments. References:
1. Bortfeld T. IMRT: a review and preview. Phys Med Biol. 2006;51(13):R363–R379.2. Otto K. Volumetric modulated arc therapy: IMRT in a single gantry arc. Med Phys. 2008;35(1):310–317.3. Mijnheer B, Beddar S, Greer P, Mijnheer B, Mijnheer BJ, Mijnheer B. QA of IMRT: current practice and future trends. Radiother Oncol. 2005;76(3):231–238.4. Low DA, Moran JM, Dempsey JF, Oldham M, Son JB, Mutic S, et al. Independent dose verification in IMRT. Med Phys. 2011;38(1):135–146.5. Low DA, Klein EE, Yang D, Zhao T, Mutic S, Purdy JA. Commissioning and validation of Mobius3D for TomoTherapy. J Appl Clin Med Phys. 2019;20(4):87–96. Keywords: Mobius3D, Dosimetric Leaf Gap, Patient- specific Evaluation of a commercially available image correction algorithm for application in online adaptive radiotherapy Alex Dunlop, Michael Thomas, Pragathi Pragathi, Frances Lavender, Simeon Nill, Uwe Oelfke The Joint Department of Physics, The Institute of Cancer Research and The Royal Marsden Hospital, London, United Kingdom Purpose/Objective: In online adaptive radiotherapy (oART), new treatment plans are generated on images acquired on the treatment platform immediately prior to delivery. The images must be suitable for both contouring of targets and organs-at-risk, as well as for dose calculation. In this study we evaluate the ‘corrected CBCT’ algorithm available within the RayStation (v2024B onwards) TPS (RaySearch Laboratories, Stockholm Sweden) in terms of dose calculation accuracy for images acquired on a variety of common radiotherapy treatment platforms. Digital Poster 4234 Such validation is a key step in the move towards online adaptive workflows that utilise software independent of the treatment platform which will be crucial for oART widespread adoption. Material/Methods: The corrected CBCT algorithm was applied to images across a variety of clinical treatment indications including gynaecological (n=12), head and neck (H&N, n=6), rectum (n=6), and prostate bed (n=6), for Radixact (Accuray, Wisconsin US) ClearRT images, and Elekta (Stockholm Sweden) XVI (non-IRIS) and Varian
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