Recovering the central axis from the crossline dose profiles in Elekta Unity MR-Linac.

This study quantifies the lateral shifts of the crossline dose profiles from the central axis (CAX) under the influence of the 1.5 T magnetic field of an Elekta Unity MR-linac using two parameters: the midpoint between the 50% value of the dose profile (Mid-D50%) and the midpoint between the inflection points (Mid-IP). The dependence of the shifts of Mid-D50% and Mid-IP on depth, source-to-surface distance, and field size was studied through simulations in Elekta's Monaco treatment planning system. Coefficients for the linear fit of the shifts are presented to enable the determination of the position of the CAX from either Mid-D50% or Mid-IP. Our results show that not only is the deviation of Mid-IP from the CAX smaller than that of Mid-D50%, but it is also less dependent on depth and field size. The ability to recover the CAX from the asymmetric dose profiles enables the measurement of the radiation isocenter size of the MR-linac using a star-shot test. The radii of the isocenters determined using the CAX recovered from either Mid-D50% or Mid-IP are both consistent with the expected result of the machine, demonstrating the accuracy of the recovered CAX and offering an alternative approach for conducting the star-shot test on the Unity MR-linac.