Dose Course 2018_Flipping book

Example 3: Modelling the MLC in Raystation by Raysearch RayPhysics Manual v4.7

Transmission and Tongue & Groove effect

Leaf Ends and leaf position

Leaf-end shape is modelled flat .

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• The width of the T&G area is the adjustable parameter. The transmission under this is set to the square root of the transmission through the leaves. • T&G effect only accounted for if the MLC leaf edge is exposed into the beam the MLC portal • Interleaf leakage modelling is considered part of the overall average MLC transmission

• The position of each MLC leaf pair is calculated using a method based on three user specified parameters: offset, gain, curvature. Their values are derived iteratively based on comparisons against measured data.

• MLC leaf ends are modelled straight (No explicit ray-tracing through leaf ends) • The MLC leaf tip position off-axis is corrected to account for difference in nominal position and radiation field edge • T&G modelled using an adjustable parameter (no ray tracing)

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