  Purpose: To improve the delivery efficiency of VMAT by extending the recently
published VMAT treatment planning algorithm vmerge to automatically generate
optimal partial-arc plans.
  Methods and materials: A high-quality initial plan is created by solving a
convex multicriteria optimization problem using 180 equi-spaced beams. This
initial plan is used to form a set of dose constraints, and a set of
partial-arc plans is created by searching the space of all possible partial-arc
plans that satisfy these constraints. For each partial-arc, an iterative
fluence map merging and sequencing algorithm (vmerge) is used to improve the
delivery efficiency. Merging continues as long as the dose quality is
maintained above a user-defined threshold. The final plan is selected as the
partial arc with the lowest treatment time. The complete algorithm is called
pmerge.
  Results: Partial-arc plans are created using pmerge for a lung, liver and
prostate case, with final treatment times of 127, 245 and 147 seconds.
Treatment times using full arcs with vmerge are 211, 357 and 178 seconds. Dose
quality is maintained across the initial, vmerge, and pmerge plans to within 5%
of the mean doses to the critical organs-at-risk and with target coverage above
98%. Additionally, we find that the angular distribution of fluence in the
initial plans is predictive of the start and end angles of the optimal
partial-arc.
  Conclusions: The pmerge algorithm is an extension to vmerge that
automatically finds the partial-arc plan that minimizes the treatment time.
VMAT delivery efficiency can be improved by employing partial-arcs without
compromising dose quality. Partial arcs are most applicable to cases with
non-centralized targets, where the time savings is greatest.
