SU-E-T-627: Optimal Partial Arcs in VMAT Planning

To describe a method for producing minimal delivery time partial arc VMAT plans.

We begin with the assumption that dose quality is the primary treatment planning goal. Therefore the first step in the partial arc computation is a 180 beam equi-spaced IMRT multi-criteria optimized treatment plan, which serves as an ideal plan, along with a set of user- specified allowable deviations from this plan. This defines a set of target coverage and healthy organ sparing constraints. We then seek a partial arc plan which recovers this ideal plan but is minimal in delivery time. The search for the optimal partial arc which fulfills the hard constraints is done by wrapping a VMAT fluence map optimization/merging/simplification algorithm called VMERGE. The search is performed over all possible partial arcs, with start and end locations discretized to 20 degree increments, and respecting that the gantry cannot pass underneath the couch. This results in 169 partial arcs. For the ones that yield feasible plans, the complete VMERGE algorithm is run, which minimizes the delivery time for that arc. The minimal delivery time plan that fulfills the dosimetric requirements is returned.

We apply the method to a lung and liver case. The time savings are as follows: (full arc time, optimal partial arc time): lung (185 s, 94 s), liver (263 s, 165 s). The optimal arc for the lung lesion, a left anterior target, is 140 degrees centered at 50 degrees. The optimal arc for the liver lesion is 160 degrees centered at -90 degrees.

By wrapping a fast VMAT optimization/sequencing routine by an exhaustive search over 169 possible partial arcs, we are able to determine the fastest delivery partial arc. The use of partial arcs can significantly shorten delivery time in VMAT delivery. The project described was supported by Award Number R01CA103904 from the National Cancer Institute. The content is solely the responsibility of the authors and does not necessarily represent the ocial views of the National Cancer Institute or the National Institutes of Health.