ESTRO 2025 - Abstract Book

S2758

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2025

Keywords: Hyperthermia, Robust optimization, Uncertainties

References: 1. Kroesen M, Mulder HT, Van Holthe JML, et al. Confirmation of thermal dose as a predictor of local control in cervical carcinoma patients treated with state-of-the-art radiation therapy and hyperthermia. Radiotherapy and Oncology, v 140, p. 319-328, 2019 2. S. K. Das, S. T. Clegg, and T. V. Samulski, "Computational techniques for fast hyperthermia temperature optimization," Medical Physics, vol. 26, no. 2, pp. 319-328, 1999. 3. J. A. Groen, J. Crezee, H. W. M. van Laarhoven, M. F. Bijlsma, and H. P. Kok, "Quantification of tissue property and perfusion uncertainties in hyperthermia treatment planning: Multianalysis using polynomial chaos expansion," Comput Methods Programs Biomed, vol. 240, p. 107675, 2023

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Digital Poster MRI-only workflow in Elekta Unity MR-linac and Synthetic CT Carlos Ferrer 1 , Concepción Huertas 1 , Marcos Martínez 1 , Inmaculada Navarro 2 , Beatriz Moreno 2 , Beatriz Sánchez 2 , Patricia Arroyo 2 , Daniel Camacho 2 , Moisés Sáez 1 1 Medical Physics, La Paz University Hospital, Madrid, Spain. 2 Radiation Oncology, La Paz University Hospital, Madrid, Spain Purpose/Objective: Among the different possible workflows in an Elekta Unity, the one utilised at our institution involves adquiring a simulation computed tomography (CT) image of the patient to obtain the average electron densities (EDs) of the contoured structures. Afterwards, a simulation magnetic resonance image (MRI) is acquired and registered with the previous CT. The EDs from the CT are assigned to the MRI contours and the dose distribution is calculated on the MRI. To avoid acquiring the CT image and simplify the workflow, it is possible to perform only one MRI and accomplish the dose calculation and contouring prior to the first fraction using a synthetic CT (sCT). This work studies EDs and dose distribution differences between CT/MRI and sCT to validate the only MR workflow. Material/Methods: The MR-only workflow was studied in a prostate patient to be treated with an Elekta Unity MR-Linac. MRI images for simulation, contouring and treatment planning were adquired on a Philips Ingenia Ambition 1.5T MR, which has Philips MRCAT (Magnetic Resonance for Calculating Attenuation) software that produces a sCT with 5 tissue types (air, fat, water, bone marrow and cortical bone) via multiple MRI sequences. sCT was adquired immediately after the simulation MRI to avoid significant changes in the contoured structures. The treatment plan was calculated on the MR to deliver 36.25 Gy in 5 fractions, with the EDs overridden by the CT values. The dose was then recalculated in the sCT with no changes to the original plan and no optimization. Finally, the dose was again recalculated performing a full optimization, to test if it is possible to optimize directly in the sCT and achieve similar results. Results: Comparison between dose distributions produced a gamma pass index of 95.9% for 3%/3mm acceptance criteria and 95.4% for 3%/2mm. Mean differences in DVH were lower than 10%. PTV coverage and doses to OAR were comparable. Difference in Eds were below 1% except for femoral heads and pelvic bone, which presented a 6% and 4.4% lower ED with sCT. Similar results were obtained with a full optimisation on the sCT and only 1 MU difference was obtained between the MR original plan and the sCT optimized plan.