title: Prediction of stopping-power ratios in flattening-filter free beams.
abstract: OBJECTIVE
  In recent years, there has been an increasing interest in flattening-filter free (FFF) beams. However, since the removal of the flattening filter will affect both the mean and the variance of the energy spectrum, current beam-quality specifiers may not be adequate for reference dosimetry in such beams. The purpose of this work was to investigate an alternative, more general beam-quality specifier.
  
  
  METHODS
  The beam-quality specifier used in this work was a combination of the kerma-weighted mean and the coefficient of variation of the linear attenuation coefficient in water. These parameters can in theory be determined from narrow-beam transmission measurements using a miniphantom "in-air," which is a measurement condition well suited also to small and nonstandard fields. The relation between the Spencer-Attix stopping-power ratios and this novel beam-quality specifier was described by a simple polynomial. For reference, the authors used Monte Carlo calculated spectra and stopping-power data for nine different beams, with and without flattening filter.
  
  
  RESULTS
  The polynomial coefficients were obtained by least-squares optimization. For all beams included in this investigation, the average of the differences between the predicted and the Monte Carlo calculated stopping-power ratios was 0.02 +/- 0.17% (1 SD) (including TomoTherapy and CyberKnife example beams).
  
  
  CONCLUSIONS
  An alternative dual-parameter beam-quality specifier was investigated. The evaluation suggests that it can be used successfully to predict stopping-power ratios in FFF as well as conventional beams, regardless of filtration.
authors.affiliation: Department of Medical Radiation Physics, Lund University, Box 117, SE-221 00 Lund, Sweden. crister.ceberg@med.lu.se
authors.forename: Crister
authors.initials: C
authors.lastname: Ceberg
authors.forename: Stefan
authors.initials: S
authors.lastname: Johnsson
authors.forename: Mårten
authors.initials: M
authors.lastname: Lind
authors.forename: Tommy
authors.initials: T
authors.lastname: Knöös
mesh_terms: D003198:Computer Simulation; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005374:Filtration / Q000295:instrumentation; D015233:Models, Statistical; D020266:Radiotherapy, Conformal / Q000295:instrumentation*; D015203:Reproducibility of Results; D012542:Scattering, Radiation; D012680:Sensitivity and Specificity