The Australian MRI-Linac Program: measuring profiles and PDD in a horizontal beam

The Australian MRI-Linac consists of a fixed horizontal photon beam combined with a MRI. Commissioning required PDD and profiles measured in a horizontal set-up using a combination of water tank measurements and gafchromic film. To validate the methodology, measurements were performed comparing PDD and profiles measured with the gantry angle set to 0 and 90° on a conventional linac. Results showed agreement to within 2.0% for PDD measured using both film and the water tank at gantry 90° relative to PDD acquired using gantry 0°. Profiles acquired using a water tank at both gantry 0 and 90° showed agreement in FWHM to within 1 mm. The agreement for both PDD and profiles measured at gantry 90° relative to gantry 0° curves indicates that the methodology described can be used to acquire the necessary beam data for horizontal beam lines and in particular, commissioning the Australian MRI-linac

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

In vivo endorectal dosimetry of prostate tomotherapy using dual MOSkin detectors

Verification of dose to the anterior rectal wall in helical tomotherapy to the prostate is important due to the close proximity of the rectal wall to the treatment field. The steep dose gradient makes these measurements challenging. A phantom-based study was completed, aimed at developing a system for measurement of anterior rectal wall doses during hypofractionated prostate stereotactic body radiotherapy (SBRT) utilizing tomotherapy delivery. An array of four dual MOSkinTM dosimeters, spaced 1 cm apart, was placed on a replica Rectafix® immobilization spacer device. This Perspex probe is a more rigid alternative to rectal balloons, to improve geometric reproducibility. The doses at each point were measured in real time and compared to doses calculated by the treatment planning system (TPS). Additionally, distance-to-agreement (DTA) measurements were acquired to assist in the comparison of measured and predicted doses. All dual MOSkin detectors measured dose to within ± 5% of the TPS at the anterior rectal wall. Whilst several points were outside of experimental error, the largest deviation from the TPS predicted dose represented a DTA of only 1.3 mm, within the acceptable DTA tolerance of 3 mm. Larger deviations of up to -11.9% were observed for the posterior and side walls; however, if acceptable DTA measurements are accounted for, then an agreement of 75% was observed. Although larger differences were observed at the other rectal wall locations, the overall effect of dose at these points was not as significant, given the lower doses. Despite the very high-dose gradient region, real-time measurements of the anterior rectal wall doses were within acceptable limits of TPS-predicted doses. The differences between measured and planned data were due to difficulties in precisely locating each detector on the TPS dose grid, which presented large variations in dose between CT voxels in regions of steep dose gradients. The dual MOSkin system would, therefore, be a useful device for detecting errors in real time, such as patient shifts or incorrect setup, during tomotherapy of the prostate