Energy Dependence of the New Gafchromic EBT3 Film:Dose Response Curves for 50 KV, 6 and 15 MV X-Ray Beams

Abstract

Purpose: To study and compare the dose response curves of the new GafChromic EBT3 film for megavoltage and kilovoltage x-ray beams, with different spatial resolutions. Methods: EBT3 films (lot#A101711-02) were exposed to each x-ray beam (6 MV, 15 MV, and 50 kV) at 7 dose values (50-3200 cGy). Each film piece was scanned three consecutive times in the center of Epson 10000XL flatbed scanner in 48-bit color at two separate spatial resolutions of 75 and 300 dpi. The data were analyzed using ImageJ and, for each scanned image, a region of interest (ROI) of 2 × 2 cm 2 at the field center was selected to obtain the mean pixel value with its standard deviation in the ROI. For each energy, dose value and spatial resolution, the average net optical density (netOD) and its associated uncertainty were determined. The Student's t-test was performed to evaluate the statistical differences between the net OD/dose values of the three energy modalities, with different color channels and spatial resolutions. Results and Discussion: The dose response curves for the three energy modalities were compared in three color channels. Weak energy dependence was found. For doses above 100 cGy, no statistical differences were observed between 6 and 15 MV beams, regardless of spatial resolution and color channel. However, statistical differences were observed between 50 kV and the megavoltage beams. The degree of energy dependence (from MV to 50 kV) was found to be a function of color channel, dose level, and spatial resolution. Conclusions: The dose response curves for GafChromic EBT3 films were found to be weakly dependent on the energy of the photon beams from 6 MV to 15 MV. For very low energy photon (e.g. 50 kV), variation of more than 11% due to the energy-dependence is observed, depending on the absorbed dose, spatial resolution and color channel used.

High dose‐per‐pulse electron beam dosimetry: Usability and dose‐rate independence of EBT3 Gafchromic films

“…The local film uniformity was measured by evaluating the SD of pixel values on the ROI analyzed for dosimetry. We found that SD linearly increased with decreasing exposure and was between 0.32 and 0.85%, which is comparable to previous studies . Intra‐film uniformity was evaluated to be between 0.22 and 0.45% by comparing the MPV of small films irradiated to the same dose and coming from the same sheet.…”

“…The energy dependence of EBT3 film has been studied by various authors. Massillon et al investigated this for 50 kV, 6 MV and 15 MV photon beams. For doses higher than 1 Gy, no significant statistical differences were found between 6 and 15 MV and only small differences (11%) were detected when comparing the 50 kV beam with the megavoltage ones.…”

AAPM TG158: Measurement and calculation of doses outside the treated volume from external‐beam radiation therapy

“…This energy dependence (at out‐of‐field energies compared with a primary 6‐MV beam) is small but present. EBT film, for example, was found to under‐respond by up to 8% (for EBT), while EBT2 and 3 were found to overrespond by 5–10% . These effects depend slightly on dose, color channel, and other scan parameters …”

Impact of source position on high-dose-rate skin surface applicator dosimetry

“…The EBT3 is self-developing film in real time, almost energy independent, and near tissue-equivalent [17–20]. Film calibration, to correlate the film’s optical density with the absorbed dose, was performed with a 6MV beam from a linear accelerator in the reference conditions (field size of 10×10 cm 2 at the depth of maximum dose (d max ) of 1.4 cm) for 13 different dose levels ranging from 0 to 464 cGy.…”