Optimum absorbed dose versus energy response of Gafchromic EBT2 and EBT3 films exposed to 20–160 kV x-rays and ⁶⁰ Co gamma

Abstract

High-accuracy in the absorbed-dose and small energy-dependence are important features of a dosimeter to be considered ideal for medical applications. This work investigated the relation between the optimum absorbed-dose in terms of combined standard-uncertainty and energyresponse of EBT2 and EBT3 films exposed to five x-ray beams and 60 Co. A systematic analysis was performed to control the fitting procedure in order to find the parameter-values that best describe the experimental data. Six EBT2 and three EBT3 film pieces per absorbed-dose were exposed to 0.05-100 Gy and 0.1-15 Gy, respectively. HP Scanjet-7650 and Epson Expression-11000XL document scanners were used to read the EBT2 and EBT3 films, respectively both in transmission mode, 48-bit at 300 dpi spatial-resolution. Images were analysed using ImageJ-software. The results indicate that, independent of the photon-energy beam, taking or not into account the uncertainty of the experimental data during the fitting process has a notable impact on the fit parameters for describing the measurements as well as on the uncertainty in the absorbed dose that can be measured with both films. Furthermore, the degree of energy dependence of both films depends on the absorbed-dose and the minimum absorbed-dose that can be measured with them is an intrinsic characteristic of each film model and depends on the photon energy as well as the colour channel. Lower the photon-energy, smaller the combined standard uncertainty, possibly associated to higher ionisation-density pattern of secondary-electrons released locally within the film active layer by low photon-energy comparing to high-energy. Thus, based on the results of this work, we suggest the inclusion of the uncertainty on the experimental data in the fitting procedure of the film calibration curve and an optimum absorbed-dose of 10 Gy for EBT3 in green-channel and 3 Gy for EBT2 in redchannel when use in modern radiotherapy techniques.

Absorbed dose distribution in liquid water for a CyberKnife VSI using radiochromic EBT3 film

“…This was done to avoid scattering radiation from the acrylic material to the beam centre. Based on the results recently reported, the absorbed dose delivered to the film in the Cyberknife was around 10 Gy at dmax and analysed in the green channel in order to decrease the combined standard uncertainty [9], which was confirmed from the uncertainty analysis made in this work. All the results were compared with that measured with an IBA stereotactic field diode (SFD).…”

“…The films were cut into pieces of 1.2 cm x 1.2 cm for the calibration and of 7 cm x 25.4 cm for the measurements in the Cyberknife and scanned 24 hours pre-and post-irradiation according to our protocol reported elsewhere [9]. The reading process was made using an EPSON 11000-XL document flatbed scanner at 300 dpi spatial-resolution and 48-bit colour depth.…”

“…The reading process was made using an EPSON 11000-XL document flatbed scanner at 300 dpi spatial-resolution and 48-bit colour depth. The analysis of the response as well as the uncertainty evaluation in the absorbed dose was done following the same process described and reported in ref [9]. The calibration was performed for absorbed dose from 0.25 Gy up to 60 Gy in liquid water under the reference conditions of a 6 MV X-ray beam produced by a linac Varian iX previously calibrated using an A12 reference class ionization chamber calibrated at the National Institute of Standards and Technology (NIST) [10], i.e.…”

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

“…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. The same authors also found a slight energy dependence of EBT3 when exposing them to low energy X‐rays (between 20 and 160 kV) compared to Co‐60 irradiations . Sorriaux et al determined calibration curves for EBT3 films for five different beams, namely 6 and 18 MV photon, 60 and 100 MeV proton, and 6 MeV electron beams.…”

Evaluation of dose rate and photon energy dependence of gafchromic EBT3 film irradiating with 6 MV and Co-60 photon beams

“…Gafchromic™ films have lots of advantages and they are broadly utilized in radiotherapy; for example, verifying treatment planning systems, evaluating two-dimensional ambient absorbed dose, and investigating dose distribution in small radiotherapy fields. [4] The EBT and EBT2 films are typical types of Gafchromic™ films which are used in radiation therapy quality assurance (QA) and have several important advantages including better spatial resolution, energy and dose rate independency, effective atomic number and a density close to the soft tissue or water, lower sensitivity to room light (resulting in easy handling), and low postirradiation growth. [567] In addition, these films are self-developing; therefore, they do not need the conventional chemical processing[6] and also, they can be shaped to fit in any phantom.…”