title: Comparison of 3D and 4D Monte Carlo optimization in robotic tracking stereotactic body radiotherapy of lung cancer.
abstract: OBJECTIVE
  To investigate the adequacy of three-dimensional (3D) Monte Carlo (MC) optimization (3DMCO) and the potential of four-dimensional (4D) dose renormalization (4DMCrenorm) and optimization (4DMCO) for CyberKnife (Accuray Inc., Sunnyvale, CA) radiotherapy planning in lung cancer.
  
  
  METHODS
  For 20 lung tumors, 3DMCO and 4DMCO plans were generated with planning target volume (PTV5 mm) = gross tumor volume (GTV) plus 5 mm, assuming 3 mm for tracking errors (PTV3 mm) and 2 mm for residual organ deformations. Three fractions of 60 Gy were prescribed to ≥ 95 % of the PTV5 mm. Each 3DMCO plan was recalculated by 4D MC dose calculation (4DMCrecal) to assess the dosimetric impact of organ deformations. The 4DMCrecal plans were renormalized (4DMCrenorm) to 95 % dose coverage of the PTV5 mm for comparisons with the 4DMCO plans. A 3DMCO plan was considered adequate if the 4DMCrecal plan showed ≥ 95 % of the PTV3 mm receiving 60 Gy and doses to other organs at risk (OARs) were below the limits.
  
  
  RESULTS
  In seven lesions, 3DMCO was inadequate, providing < 95 % dose coverage to the PTV3 mm. Comparison of 4DMCrecal and 3DMCO plans showed that organ deformations resulted in lower OAR doses. Renormalizing the 4DMCrecal plans could produce OAR doses higher than the tolerances in some 4DMCrenorm plans. Dose conformity of the 4DMCrenorm plans was inferior to that of the 3DMCO and 4DMCO plans. The 4DMCO plans did not always achieve OAR dose reductions compared to 3DMCO and 4DMCrenorm plans.
  
  
  CONCLUSIONS
  This study indicates that 3DMCO with 2 mm margins for organ deformations may be inadequate for Cyberknife-based lung stereotactic body radiotherapy (SBRT). Renormalizing the 4DMCrecal plans could produce degraded dose conformity and increased OAR doses; 4DMCO can resolve this problem.
authors.affiliation: Department of Clinical Oncology, Tuen Mun Hospital, Hong Kong (S.A.R), China, ckh456@ha.org.hk.
authors.forename: Mark K H
authors.initials: MK
authors.lastname: Chan
authors.forename: Rene
authors.initials: R
authors.lastname: Werner
authors.forename: Miriam
authors.initials: M
authors.lastname: Ayadi
authors.forename: Oliver
authors.initials: O
authors.lastname: Blanck
mesh_terms: D056973:Four-Dimensional Computed Tomography / Q000379:methods*; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods*; D008175:Lung Neoplasms / Q000473:pathology / Q000601:surgery*; D009010:Monte Carlo Method; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000379:methods*; D012371:Robotics; D047368:Tumor Burden