Technical note: Energy dependence of the Gafchromic EBT4 film: Dose‐response curves for 70 kV, 6 MV, 6 MV FFF, 10 MV FFF, and 15 MV x‐ray beams

Abstract

Background: EBT4 was newly released for radiotherapy quality assurance to improve the signal-to-noise ratio in radiochromic film dosimetry. It is important to know its dose-response characteristics before its use in the clinic. Purpose: This study aims to investigate and compare the dose-response curves of the Gafchromic EBT4 film for megavoltage and kilovoltage x-ray beams with different dose levels, scanning spatial resolutions, and sizes of region of interest (ROI). Methods: EBT4 film (Lot#07052201) calibration strips (3.5 × 20 cm 2 ) were exposed to a 10×10 cm 2 open field at doses of 0, 63, 125, 500, 750, 1000 cGy using 6 MV photon beam. EBT4 film strips from the same lot were then exposed to each x-ray beam (6 MV, 6 MV FFF, 10 MV FFF, 15 MV, and 70 kV) at six dose values (50, 100, 300, 600, 800, 1000 cGy). A full sheet (25 × 20 cm 2 ) of EBT4 film was irradiated at each energy with 300 cGy for profile comparison with the treatment planning calculation. At two different spatial resolutions of 72 and 300 dpi, each film piece was scanned three consecutive times in the center of an Epson 10000XL flatbed scanner in 48-bit color. The scanned images were analyzed using FilmQA Pro. For each scanned image, an ROI of 2 × 2 cm 2 at the field center was selected to obtain the average pixel value with its standard deviation in the ROI. An additional ROI of 1 cm diameter circle was also used to evaluate the impact of ROI shape and size, especially for FFF beams. The dose value, average dose-response value, and associated uncertainty were determined for each energy and relative responses were analyzed. The Student's t-test was performed to evaluate the statistical significance of the dose-response values with different color channels, ROI shapes, and spatial resolutions. Results: The dose-response curves for the five x-ray energies were compared in three color channels. Weak energy dependence was found among the megavoltage beams. No significant differences (average ∼1.1%) were observed for all doses in this study among 6 MV, 6 MV FFF, 10 MV FFF, and 15 MV beams, regardless of spatial resolution and color channel. However, a statistically significant difference in dose-response was observed up to 12% between 70 kV and 6 MV beams. Conclusions: The dose-response curves for Gafchromic EBT4 films were nearly independent of the energy of the photon beams among 6 MV, 6 MV FFF, 10 MV FFF, and 15 MV. For very low-energy photons (e.g., 70 kV), a separate calibration from the same low-energy x-ray is necessary.

Characterization of scanning orientation and lateral response artifact for EBT4 Gafchromic film

“…A recent study on the new EBT4 film has reported high‐energy photon beams of nearly independent energy (6 MV, 6 MV Flattening‐Filter‐Free [FFF], 10 MV FFF, and 15 MV) were used in radiotherapy. However, for low‐energy photons (70 kV), there is an energy dependence, with a difference of up to 12% 25 . Several studies have studied characteristics of the Gafchromic EBT film series and scanning procedures, including energy dependence, 4,6,22 absorption spectra, 10 film homogeneity, 11 temperature dependence, 8,12 ambient light sensitivity, 12 post‐exposure coloration, 4,6,12 and variations among different lot numbers 13 …”

“…However, for low-energy photons (70 kV), there is an energy dependence, with a difference of up to 12%. 25 Several studies have studied characteristics of the Gafchromic EBT film series and scanning procedures, including energy dependence, 4,6,22 absorption spectra, 10 film homogeneity, 11 temperature dependence, 8,12 ambient light sensitivity, 12 post-exposure coloration, 4,6,12 and variations among different lot numbers. 13 EBT3 films exhibit two types of geometrical effects: scanning orientation 4,6,12,16,17,20 and lateral response artifact (LRA) 5,7,[15][16][17][18][19][20][21][22] effects.…”

Characterization of GafchromicTM EBT4 film with clinical kV/MV photons and MeV electrons

“…The flattening-filter-free (FFF) beams were not selected for the film characterization in our study because using the non-uniform dose profile of a FFF beam is likely to introduce large inaccuracy in the quantitative analysis if the radiation field center on the film image is not identified accurately. The characterization of EBT4 film using FFF beams can be found in the report by Chan et al 2 For electron irradiations, 6, 12, and 20 MeV beams were selected, representing low, medium, and high clinical electron energies. In the irradiation, the gantry was rotated to 0 • with a standard setup: source-to-surface distance (SSD) = 100 cm, and field size = 10 × 10 cm 2 in the isocenter plane.…”

“…There are four fitting coefficients in this function, as defined in Equation (2), where x is the gray value or net OD, y is dose, and a, b, c and d are fitting coefficients. 6 In the present study, only the net OD was used in the film calibration.…”

Dose‐rate dependence and IMRT QA suitability of EBT3 radiochromic films for pulse reduced dose‐rate radiotherapy (PRDR) dosimetry

“…Radiochromic films, such as EBT3 film or EBT4 film, are suitable candidates for PRDR dosimetry due to their energy independence, angular independence, high spatial resolution, and 2D readout 15–17 . Several investigations have previously evaluated the dose rate dependence of radiochromic films for conventional dose rates above 100 MU/min 15,18–21 .…”