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Secondary MU Check Using RadCalc for Elekta Unity MR-Linac

J Yang1*, S Vedam1 , C Brink2 , M Nielsen2 , M Ji3 , L Bist3 , J Ohrt1 , J Wang1 , P Balter1 , (1) MD Anderson Cancer Center, Houston, TX, (2) Odense University Hospital, Odense, Denmark, (3) Elekta Inc, Maryland Heights, MO


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

Room: Exhibit Hall | Forum 5

Purpose: Current commercial products for secondary MU check do not explicitly support treatment plans with 1.5T magnetic field. Specific modelling and validation is necessary in order to use them for this purpose. Here we analyzed a RadCalc model for clinical use for the Elekta Unity 1.5T MR-Linac.

Methods: The beam data for the RadCalc model needed special processing before they can be used. (1) Due to scanning space limitation within the bore of the Unity measurements had to be supplemented with calculations in Monaco treatment planning system using a virtual water phantom to create full percent depth dose data. (2) The largest field size of Unity is 57x22 cm². However, RadCalc accepts only square field sizes. For the rectangular fields, only cross-plane profiles were imported and were faked as square fields. This RadCalc model was tested using 5 plans generated from Monaco TPS (4 phantom plans and 1 patient plan). Cross-comparison was performed with a RadCalc model developed by Odense University Hospital for their Unity MR-Linac system.

Results: To reduce the effect of the magnetic field, we used a radical sampling distance of 5 mm, which is larger than the standard model, within the Clarkson integration in the RadCalc model. Of all tests the point dose accuracy when comparing with TPS calculation was within 5%. For individual beams, the largest deviation was 11.8%. Due to the assumption of cylindrically symmetric beams in RadCalc points close to the side of the MLC had the largest deviation. Differences in model predictions for the two institutions were within 5% for all beams.

Conclusion: RadCalc could be used as an independent secondary MU check for Unity MR-Linac system to identify gross errors, though its accuracy may not be perfectly consistent with the TPS calculation, since effects related to the magnetic field is not included.

Funding Support, Disclosures, and Conflict of Interest: MD Anderson is a founding member of the Elekta Unity MRL consortium.


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