title: Incorporating geometric ray tracing to generate initial conditions for intensity modulated arc therapy optimization.
abstract: OBJECTIVE
  Intensity modulated arc therapy (IMAT) is a rotational variant of Intensity modulated radiation therapy (IMRT) that is achieved by allowing the multileaf collimator (MLC) positions to vary as the gantry rotates around the patient. This work describes a method to generate an IMAT plan through the use of a fast ray tracing technique based on dosimetric and geometric information for setting initial MLC leaf positions prior to final IMAT optimization.
  
  
  METHODS
  Three steps were used to generate an IMAT plan. The first step was to generate arcs based on anatomical contours. The second step was to generate ray importance factor (RIF) maps by ray tracing the dose distribution inside the planning target volume (PTV) to modify the MLC leaf positions of the anatomical arcs to reduce the maximum dose inside the PTV. The RIF maps were also segmented to create a new set of arcs to improve the dose to low dose voxels within the PTV. In the third step, the MLC leaf positions from all arcs were put through a leaf position optimization (LPO) algorithm and brought into a fast Monte Carlo dose calculation engine for a final dose calculation. The method was applied to two phantom cases, a clinical prostate case and the Radiological Physics Center (RPC)'s head and neck phantom. The authors assessed the plan improvements achieved by each step and compared plans with and without using RIF. They also compared the IMAT plan with an IMRT plan for the RPC phantom.
  
  
  RESULTS
  All plans that incorporated RIF and LPO had lower objective function values than those that incorporated LPO only. The objective function value was reduced by about 15% after the generation of RIF arcs and 52% after generation of RIF arcs and leaf position optimization. The IMAT plan for the RPC phantom had similar dose coverage for PTV1 and PTV2 (the same dose volume histogram curves), however, slightly lower dose to the normal tissues compared to a six-field IMRT plan.
  
  
  CONCLUSIONS
  The use of a ray importance factor can generate initial IMAT arcs efficiently for further MLC leaf position optimization to obtain more favorable IMAT plan.
authors.affiliation: Department of Medical Biophysics, University of Western Ontario, London, Ontario N6A 5C1, Canada. michael.oliver@lhsc.on.ca
authors.forename: Mike
authors.initials: M
authors.lastname: Oliver
authors.forename: Adam
authors.initials: A
authors.lastname: Gladwish
authors.forename: Jeff
authors.initials: J
authors.lastname: Craig
authors.forename: Jeff
authors.initials: J
authors.lastname: Chen
authors.forename: Eugene
authors.initials: E
authors.lastname: Wong
mesh_terms: D000465:Algorithms; D004307:Dose-Response Relationship, Radiation; D004867:Equipment Design; D006258:Head and Neck Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D006801:Humans; D008297:Male; D015233:Models, Statistical; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011471:Prostatic Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D011874:Radiometry; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000379:methods*; D015203:Reproducibility of Results