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Prediction of stopping‐power ratios in flattening‐filter free beams
Author(s) -
Ceberg Crister,
Johnsson Stefan,
Lind Mårten,
Knöös Tommy
Publication year - 2010
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3314074
Subject(s) - monte carlo method , beam (structure) , stopping power , kerma , laser beam quality , dosimetry , flattening , computational physics , mathematics , materials science , statistics , optics , physics , nuclear medicine , medicine , laser , detector , laser beams , composite material
Purpose: 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.