title: Monte Carlo calculation of depth doses for small field of CyberKnife.
abstract: OBJECTIVE
  A Monte Carlo (MC) model of CyberKnife was developed as a quality assurance tool. The percentage depth dose (%dd) was verified by using this model.
  
  
  METHODS
  An MC model was developed with Electron Gamma Shower version 4 (EGS4) in two steps: (1) a model of the CyberKnife treatment head and (2) a model of the collimator and phantom. The bremsstrahlung spectrum was calculated using the first model, and this spectrum was then used to calculate %dds with the second model. The calculated %dds for a large field (60 mm diameter) and three small fields (30, 15, and 5 mm diameter) were compared with those measured with a diamond detector.
  
  
  CONCLUSIONS
  The MC-calculated and measured %dd-curves for the 60 mm diameter field were in excellent agreement (<1.85%), thus confirming the validity of the model. Discrepancies between the calculated and measured %dd-curves increased with decreasing field size, with considerable discrepancy (11.62%) for the 5 mm diameter field due to lateral electron disequilibrium. Accurate dose can be determined with MC even in small fields.
  
  
  CONCLUSIONS
  The MC technique can provide reliable standard data for accurate dose delivery with high-technology radiotherapies using small beams.
authors.affiliation: Department of Medical Engineering, Osaka University Graduate School of Medicine, Suita, Japan.
authors.forename: Tokihiro
authors.initials: T
authors.lastname: Yamamoto
authors.forename: Teruki
authors.initials: T
authors.lastname: Teshima
authors.forename: Satoshi
authors.initials: S
authors.lastname: Miyajima
authors.forename: Masao
authors.initials: M
authors.lastname: Matsumoto
authors.forename: Hiroya
authors.initials: H
authors.lastname: Shiomi
authors.forename: Takehiro
authors.initials: T
authors.lastname: Inoue
authors.forename: Toshihiko
authors.initials: T
authors.lastname: Inoue
authors.forename: Hideo
authors.initials: H
authors.lastname: Hirayama
mesh_terms: D006801:Humans; D009010:Monte Carlo Method; D010315:Particle Accelerators; D011785:Quality Assurance, Health Care; D011829:Radiation Dosage; D016634:Radiosurgery