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RayStation MLC Parameter Value Determination Using Raytracing

S Frigo*, D Adam, University of Wisconsin-Madison, Madison, WI

Presentations

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

Room: AAPM ePoster Library

Purpose: To determine MLC model parameter values in RayStation that are optimal for both static and dynamically delivered beams. The software only provides the ability to fit dose profile data from scans of static beams in water, and these data are not sufficiently sensitive to guide parameter value determination. We outline an alternative method performed outside the treatment planning system.

Methods: The transmission function of a Varian TrueBeam Millenium MLC was modeled by raytracing, considering the leaf geometry and measured full leaf transmission. Transmission curves were generated for various leaf positions, and then analyzed for their total area above baseline as well as for the 50% transmission location. The MLC Calibration was assigned to the latter, while the MLC Leaf Tip Width (LTW) was calculated after equating above-baseline curve area to the two-rectangle RayStation transmission function area.

Results: We arrived at physically underpinned values for the MLC Calibration of around 0.20 to 0.22 cm over the leaf travel range. This is at or just above the level of 3D scanning tank position resolution. The resulting LTW values range from 0.170 to 0.524 cm over the leaf travel range, similar to the physical rounded tip depth along the leaf axis. We then used a value of 0.21 cm for the MLC Calibration and averaged 0.283 cm value for the LTW in RayStation. Dose calculated for a set of VMAT test plans showed agreement with measurement better than 2%, while a profile-optimized model could be up to 5% off.

Conclusions: Fitting MLC profile data is not sufficient in order to generate optimal MLC parameter values in RayStation. Sensible parameter values for the approximate RayStation MLC Transmission function can be determined from the more accurate transmission generated by raytracing and mapping the curve properties to the appropriate RayStation parameters.

Download ePoster [PDF]

Keywords

Not Applicable / None Entered.

Taxonomy

TH- External Beam- Photons: General (most aspects)

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