ESTRO 2025 - Abstract Book

S3247

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2025

2180

Digital Poster MR- Linac Successfully Guided the Treatment of a Free-Moving Mesenteric Mass Xin Wang 1 , Tze Yee Lim 1 , He Wang 1 , Bouthaina Dabaja 2 , Jinzhong Yang 1 1 Radiation Physics, MD Anderson Cancer Center, Houston, USA. 2 Radiation Oncology, MD Anderson Cancer Center, Houston, USA Purpose/Objective: To report on the usefulness of MR-Linac in administering daily adaptive radiation to a highly mobile tumor with unpredictable inter-fraction motion. Material/Methods: A patient presenting with stage I follicular lymphoma was dispositioned for a definitive and curative radiation dose of 2400 cGy in 12 fractions. However, on two consecutive diagnostic images spaced less than one-week apart, the mesenteric mass exhibited substantial movement. We performed a simulation on the MR-Linac to determine the best MR sequence for tumor visualization and evaluated the capability of Elekta Unity’s adapt-to-shape (ATS) planning given the significant tumor shifts. For each fraction, the physician manually delineated the PTV with a generous 2-3 cm margin to minimize contouring time and ensure target coverage. ATS planning was performed to generate an optimum plan with PTV coverage ≥95%. Plan quality was quantified using the conformity index (CI), heterogeneity index (HI), and near maximum dose (D 0.1cc ) to the spinal canal and kidneys. To characterize tumor mobility, each fraction’s GTV was delineated offline. The displacement of the fractional GTV relative to the dry-run GTV was calculated. The complexity of the GTV’s shape was calculated as the ratio of the volume of an equivalent sphere (with the same surface area as the GTV) to the GTV volume.

Results: