title: Dynamic trajectory-based couch motion for improvement of radiation therapy trajectories in cranial SRT.
abstract: OBJECTIVE
  To investigate potential improvement in external beam stereotactic radiation therapy plan quality for cranial cases using an optimized dynamic gantry and patient support couch motion trajectory, which could minimize exposure to sensitive healthy tissue.
  
  
  METHODS
  Anonymized patient anatomy and treatment plans of cranial cancer patients were used to quantify the geometric overlap between planning target volumes and organs-at-risk (OARs) based on their two-dimensional projection from source to a plane at isocenter as a function of gantry and couch angle. Published dose constraints were then used as weighting factors for the OARs to generate a map of couch-gantry coordinate space, indicating degree of overlap at each point in space. A couch-gantry collision space was generated by direct measurement on a linear accelerator and couch using an anthropomorphic solid-water phantom. A dynamic, fully customizable algorithm was written to generate a navigable ideal trajectory for the patient specific couch-gantry space. The advanced algorithm can be used to balance the implementation of absolute minimum values of overlap with the clinical practicality of large-scale couch motion and delivery time. Optimized cranial cancer treatment trajectories were compared to conventional treatment trajectories.
  
  
  RESULTS
  Comparison of optimized treatment trajectories with conventional treatment trajectories indicated an average decrease in mean dose to the OARs of 19% and an average decrease in maximum dose to the OARs of 12%. Degradation was seen for homogeneity index (6.14% ± 0.67%-5.48% ± 0.76%) and conformation number (0.82 ± 0.02-0.79 ± 0.02), but neither was statistically significant. Removal of OAR constraints from volumetric modulated arc therapy optimization reveals that reduction in dose to OARs is almost exclusively due to the optimized trajectory and not the OAR constraints.
  
  
  CONCLUSIONS
  The authors' study indicated that simultaneous couch and gantry motion during radiation therapy to minimize the geometrical overlap in the beams-eye-view of target volumes and the organs-at-risk can have an appreciable dose reduction to organs-at-risk.
authors.affiliation: Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada.
authors.forename: R Lee
authors.initials: RL
authors.lastname: MacDonald
authors.affiliation: Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada; Department of Medical Physics, Nova Scotia Cancer Centre, Queen Elizabeth II Health Sciences Centre, Halifax, Nova Scotia B3H 1V7, Canada; Department of Radiation Oncology, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada; and Department of Radiology, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada.
authors.forename: Christopher G
authors.initials: CG
authors.lastname: Thomas
mesh_terms: D000465:Algorithms; D001932:Brain Neoplasms / Q000532:radiotherapy; D004865:Equipment and Supplies, Hospital; D006801:Humans; D009038:Motion; D058958:Organs at Risk; D056888:Patient Positioning / Q000295:instrumentation; D011878:Radiotherapy / Q000295:instrumentation* / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted