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Optimization and Evaluation of N-Level Collimation for Intensity Modulation

A Morrow*, H Chung , Baylor Scott & White Health, Temple, TX


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: Intensity modulation of photon beams for radiotherapy can be achieved through the use of multi-leaf collimators (MLC). Multiple levels of leaves can improve the quality of beam delivery and increase the efficiency of intensity modulated radiotherapy (IMRT). In this work, we demonstrate that the thickness of each leaf level can be optimized to further increase the quality of IMRT treatment. By varying the thickness of each leaf level, more fluence options are available per beamlet for a given leaf pattern. The optimization objective was to minimize the difference between any two adjacent transmission levels.

Methods: Leaf patterns using varying numbers of leaf levels and control points were created for linear and quadratic shaped ideal fluences. This was repeated for both equal and optimized leaf thicknesses. The accuracy of delivery was compared between these two sets, calculated by the average absolute difference between the ideal and delivered fluences (<|Δ|>).

Results: For single control point patterns delivering a quadratic shaped fluence with two leaf levels, the <|Δ|> for optimized leaf levels (leaf levels are not the same thickness) and equal leaf levels were 13.4% and 9.1%, respectively. For five leaf levels, this improves the <|Δ|> to 8.0% (optimized) and 4.6% (equal). For fields with five or ten control points, there is little difference between the optimized and equal leaves.

Conclusion: Using optimized leaf thicknesses is important in the context of VMAT where there is only a single control point per gantry angle. The difference between equal and optimized leaf thicknesses is mitigated by using more control points and disappears when ten control points are used, making optimized leaf thicknesses less useful in the context of IMRT when the gantry does not rotate.




TH- External beam- photons: Development (new technology and techniques)

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