Sci-Fri PM: Planning-02: MRI-based radiation treatment planning for an MRI-linac system

At Cross Cancer Institute, we are investigating a novel MRI-linac system consisting of a bi-planar 0.2 T permanent magnet coupled with a 6 MV Linac. The system can freely revolve axially around the patient to deliver dose from any desired angle. For such a system, the radiation treatment planning procedure is expected to rely on the MR images only, i.e. MRI Simulation. Replacing the current CT/CT+MRI-based RTP procedure with MRI Simulation will eliminate the need for the planning CT scanning sessions (no additional x-ray exposure) and consequently the image fusion between MRI and planning CT. In this work, we propose a comprehensive MRI-based RTP procedure for an MRI-Linac system. Specifically, the method consists of a) data acquisition, b) analysis and correction of image artifacts caused by the scanner-related and patient-induced distortions, c) segmentation of organ structures relevant to dosimetric calculations (e.g. soft tissue, bone, air), d) conversion of MR images into CT-like images by assigning bulk electron density values to organ contours defined at step c), e) dose calculations in external magnetic field, and f) plan evaluation. Monte Carlo simulations were performed to determine the linac-MRI scanner's magnetic field induced effects on the dose deposited patterns using patient data. Specifically, we investigated the dosimetric differences between the corresponding MRI-based RT plans simulated at zero and 0.2 T. We found that the maximum percent differences for brain studies were within 4%. Most of these differences occurred at the inferior field edge and superficially at beam exits.