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Increasing Couch and Gantry Trajectory Resolution in Order to Approximate Dynamic Non-Coplanar VMAT

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L MacDonald

L MacDonald1*, C Kamerling2 , A Schatti2, J Robar1,3, C Thomas1,3 , (1) Dalhousie University, Halifax, NS, Canada, (2) Brainlab AG, Munich, Germany, (3) Nova Scotia Health Authority, Halifax, NS, Canada


SU-F-FS2-5 (Sunday, July 30, 2017) 2:05 PM - 3:00 PM Room: Four Seasons 2

Purpose: To quantify the dosimetric results of increasing the resolution of couch and gantry trajectory optimization in acoustic neuroma patients based on minimization of a geometric overlap-based objective function.

Methods: Nine acoustic neuroma patients were planned using a research version of Brainlab’s Cranial SRS Elements with patient-specific optimized trajectories. The trajectories used were divisions of a 360° gantry rotation into 3, 9, 18, and 36 equal sub-arcs. The couch angle at each sub-arc was optimized according to minimization of an objective function based on PTV/OAR geometric overlap, depth ratio between target and OAR, relative dosimetric constraints, collision avoidance, and factors to encourage dosimetric fall-off to the most proximal target. VMAT plans for each trajectory were generated using identical prescription dose, optimization objectives, and trajectory generation methodology. Emphasis was placed on attempting to spare the brainstem, the most proximal OAR.

Results: With increased resolution from 3 to 36 optimized sub-arc trajectories, maximum dose to the brainstem was reduced, on average, by 5.1 ± 2.3% of initial dose (dose from 3 optimized sub-arcs). The D10% of the brainstem was reduced by 21.3 ± 6.3% of initial dose. Conformity index, calculated as the inverse Paddick, was reduced from 1.35 to 1.23 on average from the 3 to the 36 optimized sub-arc plans. An increase in sparing in the highest priority OAR was seen, in general, without deterioration of target coverage.

Conclusion: Increased complexity of arc trajectories shows an increased dosimetric benefit in the ability to spare proximal normal tissues in acoustic neuroma cases with an increase in target conformity. Acoustic neuromas were selected to present a geometry in which brainstem sparing is at its most challenging. While the dosimetric benefit of dynamic non-coplanar VMAT is approximated by the 36 optimized sub-arc plans, employment of a dynamic non-coplanar trajectory would significantly increase delivery efficiency.

Funding Support, Disclosures, and Conflict of Interest: This research was funded and licensed by Brainlab AG.

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