Non-coplanar trajectories to improve organ at risk sparing in volumetric modulated arc therapy for primary brain tumors

Background and purposeTo evaluate non-coplanar volumetric modulated arc radiotherapy (VMAT) trajectories for organ at risk (OAR) sparing in primary brain tumor radiotherapy.Materials and methodsFifteen patients were planned using coplanar VMAT and compared against non-coplanar VMAT plans for three trajectory optimization techniques. A geometric heuristic technique (GH) combined beam scoring and Dijkstra's algorithm to minimize the importance-weighted sum of OAR volumes irradiated. Fluence optimization was used to perform a local search around coplanar and GH trajectories, producing fluence-based local search (FBLS) and FBLS + GH trajectories respectively.ResultsGH, FBLS, and FBLS + GH trajectories reduced doses to the contralateral globe, optic nerve, hippocampus, temporal lobe, and cochlea. However, FBLS increased dose to the ipsilateral lens, optic nerve and globe. Compared to GH, FBLS + GH increased dose to the ipsilateral temporal lobe and hippocampus, contralateral optics, and the brainstem and body. GH and FBLS + GH trajectories reduced bilateral hippocampi normal tissue complication probability (p = 0.028 and p = 0.043, respectively). All techniques reduced PTV conformity; GH and FBLS + GH trajectories reduced homogeneity but less so for FBLS + GH.ConclusionsThe geometric heuristic technique best spared OARs and reduced normal tissue complication probability, however incorporating fluence information into non-coplanar trajectory optimization maintained PTV homogeneity.