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Method for Optimizing MLC Beam Model Parameters in RayStation for VMAT Deliveries

J Hansen*, M Belanger, A Shepard, S Frigo, University of Wisconsin-Madison, Madison, WI


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: In RayStation beam modeling, leaf tip width (LTW) is used to account for x-ray transmission through the rounded end of an MLC. Additionally, tongue-and-groove width (TGW) accounts for transmission along exposed leaf sides in an MLC-defined aperture. We varied LTW and TGW values to find optimal agreement between RayStation calculations and ionization chamber measurements for representative VMAT plans.

Methods: Analysis was performed for 6, 10, and 15 MV flattened beams on a TrueBeam in RayStation 7. LTW and TGW values were varied between 0.00-1.00 cm and 0.00-0.25 cm, respectively, while all other model parameters were fixed. Dose to a cylindrical Solid Water® phantom was calculated for seven VMAT test plans, four geometrically-based per TG-119 recommendations (small and large cylinders, c-shape, and off-axis target) and three anatomically-based (unilateral neck, chest wall, and lung). For each plan, average dose calculated to an ionization chamber ROI was compared to measurements using calibrated A1SL chambers at six positions within the high dose region. Corrections were made for accelerator output measured same-day using a water tank.

Results: Comparing calculated and measured values at each chamber position, disagreement up to 9.2% and 19.4% was observed for variations in LTW and TGW, respectively. Percent difference averaged over all plans was found to trend linearly with increasing LTW and decreasing TGW leading to larger calculated doses. Optimized LTW and TGW parameter values for each beam energy ranged from 0.33-0.36 cm and 0.00-0.08 cm, respectively. These findings should not necessarily be considered typical as they are specific to the beam model, machine, and test plans used.

Conclusion: A suite of VMAT test plans was utilized to optimize RayStation LTW and TGW values. This approach can be followed to assess various parameters in a treatment planning system when creating a beam model for dynamic delivery techniques.

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Treatment Planning, Modeling, MLC


TH- External Beam- Photons: treatment planning/virtual clinical studies

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