An important element of radiation treatment planning for cancer therapy is the selection of beam angles (out of all possible coplanar and non-coplanar angles in relation to the patient) in order to maximize the delivery of radiation to the tumor site and minimize radiation damage to nearby organs-at-risk. This category of combinatorial optimization problem is particularly difficult because direct evaluation of the quality of treatment corresponding to any proposed selection of beams requires the solution of a large-scale dose optimization problem involving many thousands of variables that represent doses delivered to volume elements (voxels) in the patient. However, if the quality of angle sets can be accurately estimated without expensive computation, a large number of angle sets can be considered, increasing the likelihood of identifying a very high quality set. Using a computationally efficient surrogate beam set evaluation procedure based on single-beam data extracted from plans employing equally-spaced beams (eplans), we have developed a global search metaheuristic process based on the Nested Partitions framework for this combinatorial optimization problem. The surrogate scoring mechanism allows us to assess thousands of beam set samples within a clinically acceptable time frame. Tests on difficult clinical cases demonstrate that the beam sets obtained via our method are superior quality.

Volumetric modulated arc therapy (VMAT) presupposes that it is beneficial to deliver radiation from all beam angles as the gantry rotates, requiring the multi-leaf collimator to maintain continuity in shape from one angle to another. In turn, radiation from undesirable beam angles could compromise the dose distribution. In this work, we challenge the notion that the radiation beam must be held on as the gantry rotates around the patient. We propose a new approach for delivering intensity-modulated arc therapy, beam-controlled arc therapy (BCAT), during which the radiation beam is controlled on or off and the dose rate is modulated while the gantry rotates around the patient. We employ linear-programming-based dose optimization to each aperture weight, resulting in some zero weight apertures. During delivery, the radiation beam is held off at control points with zero weights as the MLC shape transits to the next non-zero weight shape. This was tested on ten head and neck cases. Plan quality and delivery efficiency were compared with VMAT. Improvements of up to 17% (p-value 0.001) and 57% (p-value 0.018) in organ-at-risk sparing and target dose uniformity, respectively, were achieved. Compared to the fixed number of apertures used in single-arc and double-arc VMAT, the BCAT used 109 and 175 apertures on average, respectively. The difference in total MUs for VMAT and BCAT plans was less than 4%. Plan quality improvement was confirmed after delivery with γ analysis resulting in over 99% agreement, or 4 in 1099 points that failed.