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Comparison of Beam Data Collected with and Without Crossline Offset Correction for a 1.5 T MR-Linac

B Yang*, W Lam , M Law , Y Ho , C Ho , K Cheung , S Yu , Hong Kong Sanatorium & Hospital, Hong Kong, Hong Kong, Hong Kong Island


(Tuesday, 7/16/2019) 3:45 PM - 4:15 PM

Room: Exhibit Hall | Forum 3

Purpose: During beam data collection for MR-Linac, some literature mentioned the crossline offset caused by magnetic field could be implemented when measuring PDDs and inline profiles. This study aims to clarify the difference between two sets of beam data measured with different scanning setup. The feasibility of profile measurements for large field using microdiamond detector was also discussed.

Methods: All irradiations with field size ranging from 2×2 to 57×22 cm2 were delivered with a 7 MV flattening filter free beam generated by Elekta 1.5T MR-Linac. Beam data including crossline/inline profiles and PDDs were collected by PTW BEAMSCAN MR water phantom using microdiamond detector and Semiflex3D ionization chamber. One set of beam data was collected without any crossline offset correction, while the other was collected by shifting the detector to the offset position that was 1.5 mm in +x direction. Measured data by microdiamond and Semiflex3D were compared for large field size (57×22 cm2). Inline profiles were compared using gamma analysis with the criteria of 1%/1mm and PDDs beyond the dmax were compared using 0.5% dose difference.

Results: All inline profiles of different field size had gamma passing rates larger than 95%. More than 94% of points in PDDs were within 0.5% dose difference and those failed points were due to the measurement uncertainty. Though the microdiamond might not be considered suitable for profile measurement of large field within a magnetic field due to the angular dependence, our measurements showed the microdiamond data agreed well with those of Semiflex3D for the maximum field size, except that the latter had broader penumbras due to the volume averaging effect.

Conclusion: Both inline profiles and PDDs collected at a crossline offset position do not have significant difference from those without any correction. Microdiamond detector can be used in profile measurements for large field.


Magnetic Fields, Linear Accelerator, Commissioning


TH- External beam- photons: General (most aspects)

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