title: Modeling the dose dependence of the vis-absorption spectrum of EBT3 GafChromic™ films.
abstract: OBJECTIVE
  The aim of this work was to model the dose dependence of the darkening of GafChromic™ EBT3 films by combining the optical properties of the polydiacetylene polymer phases, and a modified version of the single-hit model, which will take the stick-like shape of the monomer microcrystals into account. Second, a comparison is made between the quantification of the film darkening by flatbed scanning and by UV-vis absorption spectroscopy.
  
  
  METHODS
  GafChromicTMEBT3 films were irradiated with a 6 MV photon beam at dose levels between 0 and 50 Gy. The radiation-induced darkening of the films is quantified by a flatbed scanner, and by UV-vis absorption spectroscopy in the wavelength range of 220-750 nm. From the UV-vis absorption spectra, the contribution of each polymer phase to the absorbance was deduced. Next, the dose dependence of the polymer content is described by a modified single-hit model where the size distribution of polymerizable centers is approximated by way of the size distribution of the monomer microcrystals in the film.
  
  
  RESULTS
  The absorption properties of the film can be accurately quantified by UV-vis spectroscopy for dose levels between 0 and 10 Gy. Over 10 Gy, the absorption spectrum saturates due to the limited sensitivity of the spectrometer. The modified single-hit model was successful in describing the increasing polymer concentration with radiation dose, using a log-normal distribution for the length of the stick-like monomer microcrystals. The dose dependence of the polymer content, deduced from the UV-vis absorption spectrum, differs from that of the flatbed scanning method and is more sensitive to changes in dose.
  
  
  CONCLUSIONS
  The dose dependence of the polymer concentration can be modeled by taking into account the distribution of active centers using the microstructure of the active layer for dose levels between 0 and 10 Gy. The dissimilar dose dependence of the polymer concentration and the absorbance must be accounted for when modeling darkening from the kinetics of the photopolymerization reaction.
authors.affiliation: Wave Propagation and Signal Processing, KU Leuven - KULAK, Kortrijk, 8500, Belgium.
authors.forename: Maarten B
authors.initials: MB
authors.lastname: Callens
authors.affiliation: Department of Radiation Oncology, University Hospitals Leuven, Leuven, 3000, Belgium.
authors.forename: Wouter
authors.initials: W
authors.lastname: Crijns
authors.affiliation: Department of Radiation Oncology, University Hospitals Leuven, Leuven, 3000, Belgium.
authors.forename: Tom
authors.initials: T
authors.lastname: Depuydt
authors.affiliation: Department of Radiation Oncology, University Hospitals Leuven, Leuven, 3000, Belgium.
authors.forename: Karin
authors.initials: K
authors.lastname: Haustermans
authors.affiliation: Department of Electrical Engineering, ESAT/PSI, KU Leuven, Leuven, 3001, Belgium.
authors.forename: Frederik
authors.initials: F
authors.lastname: Maes
authors.affiliation: DoseVue NV, Hasselt, 3500, Belgium.
authors.forename: Emiliano
authors.initials: E
authors.lastname: D'Agostino
authors.affiliation: Department of Materials Engineering, KU Leuven, Leuven, 3001, Belgium.
authors.forename: Martine
authors.initials: M
authors.lastname: Wevers
authors.affiliation: Department of Materials Engineering, KU Leuven, Leuven, 3001, Belgium.
authors.forename: Helge
authors.initials: H
authors.lastname: Pfeiffer
authors.affiliation: Wave Propagation and Signal Processing, KU Leuven - KULAK, Kortrijk, 8500, Belgium.
authors.forename: Koen
authors.initials: K
authors.lastname: Van Den Abeele
mesh_terms: D005373:Film Dosimetry; D011829:Radiation Dosage; D013057:Spectrum Analysis
keywords: GafChromic EBT; absorption spectroscopy; film dosimetry; polymerization