Technical Note: Experimental characterization of the dose deposition in parallel MRI-linacs at various magnetic field strengths
Purpose:Dose deposition measurements for parallel MRI-linacs have previously only shown com-parisons between 0 T and a single available magnetic field. The Australian MRI-Linac consists of amagnet coupled with a dual energy linear accelerator and a 120 leaf Multi-Leaf Collimator with theradiation beam parallel to the magnetic field. Two different magnets, with field strengths of 1 and1.5 T, were used during prototyping. This work aims to characterize the impact of the magnetic fieldat 1 and 1.5 T on dose deposition, possible by comparing dosimetry measured at both magnetic fieldstrengths to measurements without the magnetic field.Methods:Dose deposition measurements focused on a comparison of beam quality (TPR20/10),PDD, profiles at various depths, surface doses, and field size output factors. Measurements wereacquired at 0, 1, and 1.5 T. Beam quality was measured using an ion chamber in solid water atisocenter with appropriate TPR20/10buildup. PDDs and profiles were acquired via EBT3 film placedin solid water either parallel or perpendicular to the radiation beam. Films at surface were used to determine surface dose. Output factors were measured in solid water using an ion chamber at isocen-ter with 10 cm solid water buildup.Results:Beam quality was within0.5% of the 0 T value for the 1 and 1.5 T magnetic fieldstrengths. PDDs and profiles showed agreement for the three magnetic field strengths at depthsbeyond 20 mm. Deposited dose increased at shallower depths due to electron focusing. Output fac-tors showed agreement within 1%.Conclusion:Dose deposition at depth for a parallel MRI-linac was not significantly impacted byeither a 1 or 1.5 T magnetic field. PDDs and profiles at shallow depths and surface dose measure-ments showed significant differences between 0, 1, and 1.5 T due to electron focusing.