S1865
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Digital Poster 2020
cancer.The reduction in dose to the normal tissue achieved with the arc/static-port technique may be due to a shorter cumulative path length, resulting from the STAMP at 0° with collimator angle 90°, effectively blocking central OIs, but also increasing the weight on tangential beam angles for the lymph node volumes.
Improving calculation time on the MR-Linac: a correlation between applied electron density and calculation time Sandra Fisher 1,2 , Leah McDermott 1 , Ricky O'Brien 3 , Sweet Ping Ng 1,2 1 Department of Radiation Oncology, Austin Health, Melbourne, Australia. 2 Department of Surgery, University of Melbourne, Melbourne, Australia. 3 Medical Radiations, School of Health and Biomedical Science, RMIT University, Melbourne, Australia Purpose/Objective: Monaco’s Unity MR-Linac calculation algorithm requires users to assign relative electron density (RED) either through scanner specific Hounsfield Unit (HU) to RED conversion tables or assigning REDs to a region of interest defined by a given structure (range 0.01-15). Long optimization and calculation times were being observed during online calculations adding to patient time on bed. This study investigates the effect of capping the RED assignment on optimisation and calculation times. Material/Methods: A retrospective analysis was performed on 11 brain and head & neck patients scanned on a Siemens Somatom Definition AS CT with high density material in/near the treatment region and extended HU and iMAR (iterative metal artefact reduction) settings turned on. Extended and capped RED curves with maximum REDs of 9.0 and 2.36 respectively, were used to recalculate the approved reference plan to determine the impact of capping the maximum density in the plan. Re-optimising plans using batch optimisation (with no user interaction) was also tested. Computational times were compared between plans with capped and non-capped EDs. Tests using the capped ED-curve and user assigned high-density overrides were also performed to further understand how maximum density ranges impact calculation times Results: Eleven patients enrolled in the MOMENTUM registry were included in this study – 6 brain and 5 head & neck. Capped maximum EDs reduced the median calculation/optimisation times for head & neck cases from 7.4mins to 1.3mins (2mm grid) and 27.1mins to 15.7mins (3mm grid). For brain cases, median calculation/optimisation times were reduced from 3.5mins to 0.5mins (2mm grid) and 11.4mins to 4.0mins (3mm grid). Time saved was found to be directly proportional to the ratio of the capped/non- capped maximum ED of the calculation dose grid. Overall, the 2mm grid calculation time saving ratios ranged from 3.3-7.2 corresponding to an absolute time saving of 1.7 - 6.9mins and 3mm grid
Conclusion: The arc/static-port technique can achieve dose reduction to the normal tissue volume for oropharynx cancer patients, not just a redistribution of the dose, compared to the reference 2-arc VMAT technique. References: 1. Johnson et al. Secondary cancer risk in six anatomical sites when using PAT, IMPT, and VMAT/IMRT radiotherapy. Radiother Oncol. 2024;199:110421. Keywords: Hybrid IMRT, integral dose, head and neck cancer
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