C Ding*, A Godley, K Westover, R Timmerman, J Park, The University of Texas Southwestern Medical Ctr, Indianapolis, IN

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  C Ding

PO-GePV-M-174 (Sunday, 7/25/2021)   [Eastern Time (GMT-4)]

Purpose: To compare dose distribution of stereotactic body radiation therapy (SBRT) treatment for lung cancer with or without the presence of magnetic field on 3D and IMRT plans using MR-Linac.

Methods: A comparison for a stage T3N0 lung cancer patient, which was originally treated on a conventional linac using SBRT with a prescription dose of 50Gy in 5 fractions covering at least 95% of PTV, is presented in this study. Retrospectively, 3D and IMRT based SBRT plans with the same PTV dose prescription were generated using the Monaco planning system for Unity MR-Linac (Elekta, Stockholm, Sweden) without magnetic field correction. Those plans were re-calculated using magnetic field corrected beam data to represent the presence of magnetic field during treatment. Dose distributions with and without magnetic field in exactly same beams were compared on 3D and IMRT plans. Thereafter, the IMRT plan was re-optimized using magnetic field corrected beam data with the same target and normal tissue dose constrains. Dose distribution of IMRT plans optimized with and without magnetic field were also compared.

Results: The dose on lung side of lung-target interface was reduced with the presence of magnetic field for both 3D and IMRT plans. Therefore, the target coverage was compromised by the presence of magnetic field if the planning system ignored the effects of the field. After re-optimization, the IMRT plan can compensate for the reduced coverage of the target while keep sparing of the normal tissue.

Conclusion: The presence of magnetic field changes the dose distribution of lung treatment and the lung target interface. Re-optimization is required for IMRT plans with magnetic field corrected beam data.

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