SU‐E‐T‐860: Developing a Fast‐Monotonic Descent (FMD) Algorithm for RapidArc Radiotherapy
Purpose: To develop a fast‐monotonic descent (FMD) algorithm for optimization of VMAT treatment planning, and investigate the impacts of arc numbers and beam interval on the plan dose quality. Methods: Fast‐monotonic descent (FMD) algorithm, proved to be one of the fastest convergent methods for optimizing conventional IMRT plans, was used to enhance the aperture optimization in VMAT. The principle consideration in this new algorithm is that the intensities of the to‐be‐delivered apertures will be re‐optimized to take the contribution of the delivered apertures into account. Leaf segmentation algorithm based on power of 2 was implemented to generate deliverable apertures based on power of 2 and a simplified primary beam only pencil beam model was employed. Several typical clinical sites (prostate, spine, lung, and brain) were tested and the effects of various parameter settings on plan dose quality were investigated. For each case, plans using both conventional IMRT and VMAT techniques were created, and their dosimetric metrics were assessed. Results: The mean value of beam intensity per beam varies drastically as beam interval falling into the range [6° ∼ 20°]. It is less obvious as beam interval outside this range. With limited aperture number per beam, the plan dose quality is affected significantly by the beam interval. It is hard to achieve a decent plan with comparable dose to the conventional IMRT one using only one arc and with beam interval less than 3°. Conclusions: To achieve a decent plan dose comparable to the conventional IMRT plan, a VMAT plan needs at least 2 arcs and the minimum beam interval 6°. Highly modulated intensity maps may require more arcs. © 2011, American Association of Physicists in Medicine. All rights reserved.
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