title: Effect of beam blocking on linac head scatter factors.
abstract: OBJECTIVE
  This study aimed to investigate the influence of beam blocking on head scatter within the context of a two-component x-ray source model.
  
  
  METHODS
  Head scatter factors were measured for open rectangular fields defined by X and Y jaws and for fields blocked by a multileaf collimator (MLC) for a 6-MV photon beam from a Varian 2300CD accelerator. A simple formula based on consideration of linac head geometry and an x-ray source model was derived to determine when the head scatter factor for a shaped field depends only on jaw settings and when it is influenced by the blocking (cerrobend blocks or MLC) itself.
  
  
  RESULTS
  Experimental data demonstrate that the assumption that head scatter is unaffected by the introduction of beam blocking is not always acceptable, particularly for fields blocked by an MLC. The ratios of open to blocked field dimensions were compared with simple functions characterizing the accelerator head geometry to predict changes in the behavior of head scatter factors observed experimentally.
  
  
  CONCLUSIONS
  A simple formula can be used to determine when head scatter factors are influenced by beam blocking (cerrobend blocks or MLC).
authors.affiliation: Department of Medical Physics, Cross Cancer Institute, Edmonton, Alberta, Canada.
authors.forename: M K
authors.initials: MK
authors.lastname: Yu
authors.forename: R S
authors.initials: RS
authors.lastname: Sloboda
mesh_terms: D004867:Equipment Design; D008962:Models, Theoretical; D010315:Particle Accelerators / Q000295:instrumentation*; D011874:Radiometry; D012542:Scattering, Radiation