title: Non-coplanar trajectories to improve organ at risk sparing in volumetric modulated arc therapy for primary brain tumors.
abstract: To evaluate non-coplanar volumetric modulated arc radiotherapy (VMAT) trajectories for organ at risk (OAR) sparing in primary brain tumor radiotherapy.
  
  
  
  Fifteen 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.
  
  
  
  GH, 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.
  
  
  
  The geometric heuristic technique best spared OARs and reduced normal tissue complication probability, however incorporating fluence information into non-coplanar trajectory optimization maintained PTV homogeneity.
authors.affiliation: Joint Department of Physics at The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, United Kingdom. Electronic address: greg.smyth@icr.ac.uk.
authors.forename: Gregory
authors.initials: G
authors.lastname: Smyth
authors.affiliation: Centre for Vision, Speech and Signal Processing, University of Surrey, Guildford, United Kingdom.
authors.forename: Philip M
authors.initials: PM
authors.lastname: Evans
authors.affiliation: Joint Department of Physics at The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, United Kingdom.
authors.forename: Jeffrey C
authors.initials: JC
authors.lastname: Bamber
authors.affiliation: The Royal Marsden NHS Foundation Trust, London, United Kingdom.
authors.forename: Henry C
authors.initials: HC
authors.lastname: Mandeville
authors.affiliation: The Royal Marsden NHS Foundation Trust, London, United Kingdom.
authors.forename: Liam C
authors.initials: LC
authors.lastname: Welsh
authors.affiliation: The Royal Marsden NHS Foundation Trust, London, United Kingdom.
authors.forename: Frank H
authors.initials: FH
authors.lastname: Saran
authors.affiliation: Joint Department of Physics at The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, United Kingdom.
authors.forename: James L
authors.initials: JL
authors.lastname: Bedford
mesh_terms: D000465:Algorithms; D001932:Brain Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D006801:Humans; D008954:Models, Biological; D058958:Organs at Risk / Q000000981:diagnostic imaging / Q000528:radiation effects*; D011336:Probability; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*
keywords: Hippocampal sparing; Non-coplanar; Optimization; Trajectory; Treatment planning; VMAT