Monte Carlo calculated absorbed‐dose energy dependence of EBT and EBT2 film

Abstract

Purpose:The absorbed-dose energy dependence of GAFCHROMIC EBT and EBT2 film irradiated in photon beams is studied to understand the shape of the curves and the physics behind them. Methods: The absorbed-dose energy dependence is calculated using the EGSnrc-based EGS_cham-ber and DOSRZnrc codes by calculating the ratio of dose to water to dose to active film layers at photon energies ranging from 3 keV to 18 MeV. These data are compared to the mass energy absorption coefficient ratios and the restricted stopping power ratios of water to active film materials as well as to previous experimental results. Results: In the photon energy range of 100 keV to 18 MeV the absorbed-dose energy dependence is found to be energy independent within Ϯ0.6%. However, below 100 keV, the absorbed-dose energy dependence of EBT varies by approximately 10% due to changes in mass energy absorption coefficient ratios of water to film materials, as well as an increase in the number of electrons being created and scattered in the central surface layer of the film. Results are found to disagree with previous experimental studies suggesting the possibility of an intrinsic energy dependence at lower photon energies. For EBT2 film the absorbed-dose energy dependence at low photon energies varies by 50% or 10% depending on the manufacturing lot due to changes in the ratio of mass energy absorption coefficients of the active emulsion layers to water. Conclusions: Caution is recommended when using GAFCHROMIC EBT/EBT2 films at photon energies below 100 keV. It is recommended that the effective atomic number of future films be produced as close to that of water and that thicker active layers are advantageous.

Experimental assessment of out‐of‐field dose components in high energy electron beams used in external beam radiotherapy

“…However, energy dependence and dose dependence of EBT films become more significant when both beam energy and dose quantity are lower 21 , 22 . This can explain the increasing discrepancies between the film and the two other dosimeters with off‐axis distances, as well as with depth.…”

Radiochromic film based dosimetry of image‐guidance procedures on different radiotherapy modalities

“…For MV beams, EBT, EBT2, and EBT3 GAFCHROMIC film models were used extensively to measure therapeutic doses over wide dose range and the same dosimetry protocols apply for imaging. The dosimetric properties of these films were tested widely in the literature in terms of absorption spectra, (24) postirradiation waiting time, 24 , 25 scanning mode and exposure to light, 25 , 26 high temperature behavior, (25) performance in water, (27) depth dose measurements, (28) and energy dependence 29 , 30 …”

Effective energy measurement using radiochromic film: application of a mobile scanner

“…The calibration curve takes various forms depending on the type of radiochromic films [7,8]. The calibration curve of GAFCHROMIC EBT2 (GAF-EBT2: International Specialty Products, Wayne, NJ) shows minimal curvature in the recommended dose range by manufacture [7].…”

“…Therefore, measurement of the second half-value layer was impossible due to the energy-dependent influence in the highenergy range. On the other hand, GAF-EBT2 has been shown to be capable of measuring the second HVL, because it shows only slight energy dependency errors in comparison with GAF-R and other radiochromic films [8,[11][12][13][14][15]. For these reasons, in HVL measurements of devices operating in the high-energy range (X-ray CT, radiotherapy machines, and so on), it is suggested that GAF-EBT2 offers higher measurement precision than GAF-R.…”