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 cm2 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.
Highlights
Radiotherapy Nowadays, radiochromic film is popular in medical applications to provide treatment dose verification and measure complicated 2D dose maps in external beam radiotherapy [1,2,3]
It has been reported that EBT2 film shows inhomogeneities in its response, which lead to larger uncertainties in dose determination if using single-channel radiochromic film dosimetry [10,11]
Two EBT3 film sheets were used and each sheet was cut into nine pieces of 6.8 × 8.5 cm2
Summary
Radiotherapy Nowadays, radiochromic film is popular in medical applications to provide treatment dose verification and measure complicated 2D dose maps in external beam radiotherapy [1,2,3]. The first generation GafChromic EBT film whose dosimetric properties were extensively studied [4,5,6,7,8,9], was considered to be suitable for use at the absorbed dose level for radiation therapy quality assurance. In early 2009, the EBT film was replaced by EBT2 film by arguing that this version would enhance the absorbed dose accuracy [10]. A new EBT3 film was released in late 2011 with the same chemical composition of the active layer as in EBT2 model (see Figure 1), but featuring symmetrical construction and anti-Newton ring coatings for enhanced ease-of-use and accuracy [12]
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