SU-F-BRD-04: Development of a Secondary MU Calculation Tool for An Integrated MR-LINAC

Purpose: To develop a fast secondary MU calculation tool for an integrated MR-Linac, an emerging technology for delivering image-guided on-line adaptive therapy. Methods: A software tool was developed to implement a modified Clarkson integration algorithm for point-dose calculation, considering the dosimtric effect of a magnetic field. A research planning system, considering the presence of a perpendicular 1.5 T magnetic field, was used to extract the beam data (e.g., depth doses and beam profiles) required for the MU calculation in homogeneous phantom of 60×60×40 cm3 for a prototype of MR-Linac integrating a 1.5 T MR scanner with 6MV photon beams. These dosimetric data may be replaced with those to be collected during the commissioning measurement of the MR-Linac. Results: The presence of the magnetic field resulted in noticeable changes including (1) shifts in beam profiles for small fields (< 2×2 cm2), and (2) changes in central-axis doses when one of field edges is < 2 cm away from the central axis. For example, the maximum change in central-axis dose was up to 8% for one of the field borders with a distance away from the central axis of <1 cm. These changes are relevant for the MU calculation particularly for IMRT and VMAT, where most of segments are small. These magnetic field affected beam profiles and depth doses were included in the software tool. The MU calculations from the software tool for selective representative plans were found to agree with those from the research planning system within 10%. Conclusion: The presence of a perpendicular magnetic field results in noticeable changes in beam profiles and depth doses for small fields. These changes are included in the secondary MU calc³³u³lation to be used as a redundancy check for an integrated MR-Linac