Gafchromic EBT2 film dosimetry in reflection mode with a novel plan‐based calibration method

Abstract

Purpose: A dosimetric system formed by Gafchromic EBT2 radiochromic film and Epson Expression 10000XL flatbed scanner was commissioned for dosimetry. In this paper, several open questions concerning the commissioning of radiochromic films for dosimetry were addressed: a) is it possible to employ this dosimetric system in reflection mode; b) if so, can the methods used in transmission mode also be used in reflection mode; c) is it possible to obtain accurate absolute dose measurements with Gafchromic EBT2 films; d) which calibration method should be followed; e) which calibration models should be used; f) does three-color channel dosimetry offer a significant improvement over single channel dosimetry. The purpose of this paper is to help clarify these questions.Methods: In this study, films were scanned in reflection mode, the effect of surrounding film was evaluated and the feasibility of EBT2 film dosimetry in reflection mode was studied. EBT2's response homogeneity has been reported to lead to excessive dose uncertainties. To overcome this problem, a new plan-based calibration method was implemented. Plan-based calibration can use every pixel and each of the three color channels of the scanned film to obtain the parameters of the calibration model. A model selection analysis was conducted to select lateral correction and sensitometric curve models. The commonly used calibration with fragments was compared with red-channel plan-based calibration and with three-channel plan-based calibration.Results: No effect of surrounding film was found in this study. The film response inhomogeneity in EBT2 films was found to be important not only due to differences in the fog, but also due to differences in sensitivity. The best results for lateral corrections were obtained using absolute corrections independent of the dose. With respect to the sensitometric curves, an empirical polynomial fit of order 4 was found to obtain results equivalent to a gamma-distributed single hit model based on physical assumptions. Three-channel dosimetry was found to be substantially superior to red-channel dosimetry.Conclusions: Reflection mode with Gafchromic EBT2 radiochromic film was found to be a viable alternative to transmission mode. The same methods that are used in transmission mode can be followed in reflection mode. A novel plan-based method was developed for calibration and multichannel dosimetry. This novel method offers 2 increased robustness against film response inhomogeneities and reduces considerably the time required for calibration. a) nmendez@onko-i.si 3

Amplitude gating for a coached breathing approach in respiratory gated 10 MV flattening filter‐free VMAT delivery

“…While one study (23) has suggested a dependence of the dose in an ROI to the pixel intensity of any surrounding film, Mendez et al (21) has not corroborated the finding and believed it may be a scanner‐specific effect. We have also not observed any significant difference in the accuracy of the two modes as long as the appropriate procedures were followed.…”

“…While the use of reflective mode for the film scanning under Bouchard's protocol is not in common use, reflective mode has been examined in previous papers 20 , 21 to produce comparable results to transmission mode (22) . While one study (23) has suggested a dependence of the dose in an ROI to the pixel intensity of any surrounding film, Mendez et al (21) has not corroborated the finding and believed it may be a scanner‐specific effect.…”

Dosimetric comparison between XR-RV3 and EBT2 radiochromic film in megavoltage photon beams

“…Some discrepancies occur in the inferior part of the 250 cGy isodose line, but the dose gradient here are not steep. EBT2 ilm has been proven to be accurate at measuring dose distributions (12) . Since XR-RV3 results compares well with EBT2 ilm in this study, it implies satisfactory accuracy as well.…”

On multichannel film dosimetry with channel-independent perturbations

“…Our previous results 18 found better fit when lateral corrections are absolute corrections independent of dose, and sensitometric curves are polynomial fits of order four. Hence, lateral corrections were calculated as…”

“…They also compared different single, dual and triple channel methods, and found better agreement between planned and calculated dose distributions using the average dose of all three channels in comparison to using the channel-independent perturbations method. In an earlier article 18 , our group suggested calculating the film dose as the weighted mean dose of all three channels. For each channel, the inverse of the mean square error obtained during the film calibration was used as weight.…”