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VMAT Optimization with Dynamic Collimator Rotation

Q Lyu

Q Lyu*, D Ruan , D Nguyen , D O'Connor , K Sheng , UCLA School of Medicine, Los Angeles, CA


MO-F-FS1-8 (Monday, July 31, 2017) 4:30 PM - 6:00 PM Room: Four Seasons 1

Purpose: Existing Volumetric Modulated Arc Therapy (VMAT) optimization uses a fixed collimator angle in each arc and rotates the collimator between arcs. This method under-utilizes the collimator rotation as a control variable, resulting in the need for multiple arcs. We develop a novel integrated optimization method for VMAT, accounting for dynamic collimator angles during the arc motion.

Methods: Dynamic Collimator in VMAT (DC_VMAT) was achieved by modifying a typical dose fidelity objective with an anisotropic total variation term to regulate fluence smoothness, and a group sparsity term to control collimator rotation. A binary variable was used to form simple apertures. The optimal collimator angle for each beam angle was selected using Dijkstra's algorithm, where node costs and edge costs depend on the estimated fluence map at the current iteration. An alternating optimization strategy was implemented to solve for the fluence map, aperture shape, and collimator angle simultaneously. Feasibility of DC_VMAT using one full-arc with dynamic multi-leaf collimator (MLC) rotation was tested on a brain tumor patient. The plan was compared against a static collimator VMAT (SC_VMAT) plan using two full arcs.

Results: Compared to SC_VMAT, DC_VMAT reduced the average max and mean OAR dose by 7.07% and 4.25% of the prescription dose with the same target coverage. Collimator, gantry and MLC coordination in DC_VMAT were considered in the optimization for deliverability. There were 15 beam angles in the single-arc DC_VMAT plan requiring gantry rotation to slow down for multiple collimator angles.

Conclusion: The novel DC_VMAT approach adds a new dimension to VMAT by optimizing dynamic collimator rotation during arc delivery. By fully utilizing collimator rotation and simultaneously considering all beams, DC_VMAT affords more complex intensity modulation and more possible MLC modulation. Consequently, this single approach results in superior dosimetry than two full-arc VMAT using static collimator.

Funding Support, Disclosures, and Conflict of Interest: DOE DE-SC0017057 NIH R44CA183390 NIH R01CA188300 NIH R43CA183390 NIH U19AI067769

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