S2794
RTT - RTT contouring, target definition, and treatment planning
ESTRO 2026
glands (Fig2). Conclusion:
Magnetic resonance imaging (MRI)-based radiotherapy planning has sparked a lot of interest from the community over the year as it represents a way for hypoxia-informed treatment planning. However, MRI- based radiotherapy dose calculations are difficult because MRI lacks physical information about interactions between particles and matter. This work aims to present a physics-informed, MRI-based method for dose calculations in Volumetric Modulated Arc Therapy (VMAT) and intensity-modulated proton therapy (IMPT) for head-and-neck cancer relying on available MRI sequences applicable to clinical acquisitions. Material/Methods: 3T MRI and computed tomography (CT) images were acquired on patients treated with radiotherapy providing informed consent (R201-004–314). Among the MR images, ultrashort echo time (UTE) and q- Dixon sequences were acquired.Using the UTE images to estimate the proton density [3] and a tissues mask derived from the q-Dixon maps, the electron density estimation maps were computed using a formula derived from the literature [1][2] expressing the electron density as a function of the proton density and tissues mass density. The electron density values were then converted into Hounsfield units using a clinically -used CT calibration curve to obtain a synthetic CT (sCT). VMAT clinical treatment plans with prescription going up to an RBE weighted dose of 70 Gy from ten patients were recomputed on patient MRI scans without re-optimization using MONACO. Comparison with the dose distributions computed on the CT was done with gamma pass rate computation and dose differences calculation in tumoral volumes and organs-at-risks.IMPT dose distributions were also computed using Raystation and compared following the same methodology.
Those preliminary results are a first insight that a physics-informed, MR-based synthetic CT can be an efficient and accurate replacement to the traditional CT for both proton and photon therapy. Improvements are still necessary to insure proper segmentation of the mandible for proton therapy planning. Expansion to a higher number of patients is underway.
References: [1] – Seco et Evans. Assessing the effect of electron density in photon dose calculations Medical Physics 2006;33(2):540-552[2] – Demol et al. Monte Carlo calculation based on hydrogen composition of the tissue for MV photon radiotherapy Journal of Applied Clinical Medical Physics 2015; 16(5):117-130[3] – Sayaque et al. Magnetic resonance imaging with ultra- short echo time sequence for head and neck radiotherapy planning Physica Medica 2025; 133. Keywords: Head-and-Neck, MRI-based Radiotherapy clinical trial quality assurance in locally advanced rectal cancer: assessing target volume conformity & delineation through peer review James Iddenden 1 , Debra Howard 2 , Mark Saunders 3 , Simon Gollins 4 , Rebecca Muirhead 5,6 , Rushil Patel 2 , Ane Appelt 7,8 , Mark Teo 9 , Claire Arthur 3 1 National RTQA, Radiotherapy Trials Quality Assurance (RTTQA) Group, Liverpool, United Kingdom. 2 National RTQA, Radiotherapy Trials Quality Assurance (RTTQA) Digital Poster Highlight 3760
Results: Analysis of VMAT dose maps resulted in a gamma passing rate of 99.27% at 3%/3mm and absolute RBE weighted dose differences that where below 1 Gy in most organs at risk and tumoral volumesFor IMPT, the preliminary results between the sCT and the CT show good dose map similarity with a mean gamma pass rate at 3%/3mm of 92.40%. Absolute dose differences in organs-at-risk are below 4 Gy organs are in most organs except in the mandible and submandibular
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