Purpose of this work is to investigate the effects of varying the active layer composition of external beam therapy (EBT) GafChromicTM films on the energy dependence of the film, as well as try to develop a new prototype with more uniform energy response at low photon energies (⩽100 keV).
First, the overall energy response (S AD, W(Q)) of different commercial EBT type film models that represent the three different generations produced to date, i.e., EBT, EBT2, and EBT3, was investigated. Pieces of each film model were irradiated to a fixed dose of 2 Gy to water for a wide range of beam qualities and the corresponding S AD, W(Q) was measured using a flatbed document scanner. Furthermore, the DOSRZnrc Monte Carlo code was used to determine the absorbed dose to water energy dependence of the film, f(Q). Moreover, the intrinsic energy dependence, k bq (Q), for each film model was evaluated using the corresponding S AD, W(Q) and f(Q). In the second part of this study, the authors investigated the effects of changing the chemical composition of the active layer on S AD, W(Q). Finally, based on these results, the film manufacturer fabricated several film prototypes and the authors evaluated their S AD, W(Q).
The commercial EBT film model shows an under response at all energies below 100 keV reaching 39% ± 4% at about 20 keV. The commercial EBT2 and EBT3 film models show an under response of about 27% ± 4% at 20 keV and an over response of about 16% ± 4% at 40 keV.S AD, W(Q) of the three commercial film models at low energies show strong correlation with the corresponding f − 1(Q) curves. The commercial EBT3 model with 4% Cl in the active layer shows under response of 22% ± 4% at 20 keV and 6% ± 4% at about 40 keV. However, increasing the mass percent of chlorine makes the film more hygroscopic which may affect the stability of the film's readout. The EBT3 film prototype with 7.5% Si shows a significant improvement in the energy response at very low energies compared to the commercial EBT3 films with 4% Cl. It shows under response of 15% ± 5% at about 20 keV to 2% ± 5% at about 40 keV. However, according to the manufacturer, the addition of 7.5% Si as SiO2 adversely affected the viscosity of the active fluid and therefore affected the potential use in commercial machine coating. The latest commercial EBT3 film model with 7% Al as Al 2O3 shows an overall improvement in S AD, W(Q) compared to previous commercial EBT3 films. It shows under response at all energies <100 keV, varying from 20% ± 4% at 20 keV to 6% ± 4% at 40 keV.
The energy response of films in the energy range <100 keV can be improved by adjusting the active layer chemical composition. Removing bromine eliminated the over response at about 40 keV. The under response at energies ≤30 keV is improved by adding 7% Al to the active layer in the latest commercial EBT3 film models.
The authors would like to thank Mr. Michael D. C. Evans (M.Sc., FCCPM) for careful reading of the paper and his suggestions. This work was supported by the Natural Sciences and Engineering Research Council of Canada Contract No. 386009. S.D. is a Research Scientist supported by the Fonds de Recherche en Santé du Québec (FRSQ).
II. MATERIALS AND METHODS
II.A. Radiochromic films
II.B. Reference dose delivery
II.B.1. Reference dosimetry and film irradiations at orthovoltage beam qualities
II.B.2. Reference dosimetry and film irradiation at 60Co and 6 MV
II.B.3. Reference dosimetry and film irradiation at 192Ir
II.C. Film net optical density evaluation
II.D. Investigating the film extrinsic energy dependence
II.E. Uncertainties associated with net optical density and absorbed dose to water determination
III. RESULTS AND DISCUSSION
III.A. Experimental evaluation of the relative energy response and absorbed dose to water energy dependence of commercial EBT, EBT2, and EBT3 film models
III.B. Effect of film active layer composition on the film absorbed dose to water energy dependence and energy response
III.C. Reducing the energy dependence of the film response in the energy range <100 keV by adjusting the composition of the film active layer
III.C.1. Effect of Cl and Br on the absorbed dose to water energy dependence, and relative energy response of the film
III.C.2. Effect of adding Si or Al in addition to Cl and K on the energy dependence of the film
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