title: Monte Carlo-based analytical model for small and variable fields delivered by TomoTherapy.
abstract: OBJECTIVE
  Extend to very small fields the validity of a Monte Carlo (MC) based model of TomoTherapy called TomoPen for future implementation of the dynamic jaws feature for helical TomoTherapy.
  
  
  METHODS
  First, the modelling of the electron source was revisited using a new method to measure source obscuration for very small fields (<1cm). The method consisted in MC simulations simulations and measurements of the central dose in a water phantom for a 10 cm x FW field scanned to deliver a 10 x 10 cm(2) fluence. FW, the longitudinal field width, was varied from 0.4 to 5 cm. The second part of the work consisted of adapting TomoPen to account for any configuration of the jaws in a fast and efficient way by using routinely only the phase-space file of the largest field (5 cm) and interpolated analytical information of phase-space files of smaller field widths.
  
  
  RESULTS
  For the electron source fine tuning, it was shown that the best results were obtained for a 1.1mm wide spot. Our single phase-space method showed no significant differences compared to MC simulations of various field widths even though only longitudinal intensity and angular analytical functions were applied to the 5 cm phase-space.
  
  
  CONCLUSIONS
  The designed model is able to simulate all jaw openings from the 5 cm field phase-space file by applying a bi-dimensional analytical function accounting for the fluence and the angular distribution in the longitudinal direction.
authors.affiliation: Université Catholique de Louvain, Department of Radiotherapy and Oncology, Brussels, Belgium. esterpin@yahoo.fr
authors.forename: Edmond
authors.initials: E
authors.lastname: Sterpin
authors.forename: Brian T
authors.initials: BT
authors.lastname: Hundertmark
authors.forename: Thomas R
authors.initials: TR
authors.lastname: Mackie
authors.forename: Weiguo
authors.initials: W
authors.lastname: Lu
authors.forename: Gustavo H
authors.initials: GH
authors.lastname: Olivera
authors.forename: Stefaan
authors.initials: S
authors.lastname: Vynckier
mesh_terms: D000465:Algorithms; D003198:Computer Simulation; D006801:Humans; D009010:Monte Carlo Method; D009369:Neoplasms / Q000532:radiotherapy*; D011874:Radiometry / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D011881:Radiotherapy, Computer-Assisted / Q000379:methods*