title: Automated treatment planning for a dedicated multi-source intra-cranial radiosurgery treatment unit accounting for overlapping structures and dose homogeneity.
abstract: OBJECTIVE
  The purpose of this work is to advance the two-step approach for Gamma Knife(®) Perfexion™ (PFX) optimization to account for dose homogeneity and overlap between the planning target volume (PTV) and organs-at-risk (OARs).
  
  
  METHODS
  In the first step, a geometry-based algorithm is used to quickly select isocentre locations while explicitly accounting for PTV-OARs overlaps. In this approach, the PTV is divided into subvolumes based on the PTV-OARs overlaps and the distance of voxels to the overlaps. Only a few isocentres are selected in the overlap volume, and a higher number of isocentres are carefully selected among voxels that are immediately close to the overlap volume. In the second step, a convex optimization is solved to find the optimal combination of collimator sizes and their radiation duration for each isocentre location.
  
  
  RESULTS
  This two-step approach is tested on seven clinical cases (comprising 11 targets) for which the authors assess coverage, OARs dose, and homogeneity index and relate these parameters to the overlap fraction for each case. In terms of coverage, the mean V99 for the gross target volume (GTV) was 99.8% while the V95 for the PTV averaged at 94.6%, thus satisfying the clinical objectives of 99% for GTV and 95% for PTV, respectively. The mean relative dose to the brainstem was 87.7% of the prescription dose (with maximum 108%), while on average, 11.3% of the PTV overlapped with the brainstem. The mean beam-on time per fraction per dose was 8.6 min with calibration dose rate of 3.5 Gy/min, and the computational time averaged at 205 min. Compared with previous work involving single-fraction radiosurgery, the resulting plans were more homogeneous with average homogeneity index of 1.18 compared to 1.47.
  
  
  CONCLUSIONS
  PFX treatment plans with homogeneous dose distribution can be achieved by inverse planning using geometric isocentre selection and mathematical modeling and optimization techniques. The quality of the obtained treatment plans are clinically satisfactory while the homogeneity index is improved compared to conventional PFX plans.
authors.affiliation: Department of Mechanical and Industrial Engineering, University of Toronto, Ontario M5S 3G8, Canada. kimia@mie.utoronto.ca
authors.forename: Kimia
authors.initials: K
authors.lastname: Ghobadi
authors.forename: Hamid R
authors.initials: HR
authors.lastname: Ghaffari
authors.forename: Dionne M
authors.initials: DM
authors.lastname: Aleman
authors.forename: David A
authors.initials: DA
authors.lastname: Jaffray
authors.forename: Mark
authors.initials: M
authors.lastname: Ruschin
mesh_terms: D000465:Algorithms; D001331:Automation; D006801:Humans; D058958:Organs at Risk / Q000528:radiation effects; D011829:Radiation Dosage; D016634:Radiosurgery / Q000009:adverse effects / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012886:Skull / Q000601:surgery*