Gafchromic EBT3 film dosimetry in electron beams - energy dependence and improved film read-out

Abstract

For megavoltage photon radiation, the fundamental dosimetry characteristics of Gafchromic EBT3 film were determined in 60Co gamma ray beam with addition of experimental and Monte Carlo (MC)-simulated energy dependence of the film for 6 MV photon beam and 6 MeV, 9 MeV, 12 MeV, and 16 MeV electron beams in water phantom. For the film read-out, two phase correction of scanner sensitivity was applied: a matrix correction for scanning area and dose-dependent correction by iterative procedure. With these corrections, the uniformity of response can be improved to be within ± 50 pixel values (PVs). To improve the read-out accuracy, a procedure with flipped film orientations was established. With the method, scanner uniformity can be improved further and dust particles, scratches and/or dirt on scan-ner glass can be detected and eliminated. Responses from red and green channels were averaged for read-out, which decreased the effect of noise present in values from separate channels. Since the signal level with the blue channel is considerably lower than with other channels, the signal variation due to different perturbation effects increases the noise level so that the blue channel is not recommended to be used for dose determination. However, the blue channel can be used for the detection of emulsion thickness variations for film quality evaluations with unexposed films. With electron beams ranging from 6 MeV to 16 MeV and at reference measurement conditions in water, the energy dependence of the EBT3 film is uniform within 0.5%, with uncertainties close to 1.6% (k = 2). Including 6 MV photon beam and the electron beams mentioned, the energy dependence is within 1.1%. No notable differences were found between the experimental and MC-simulated responses, indicating negligible change in intrinsic energy dependence of the EBT3 film for 6 MV photon beam and 6 MeV-16 MeV electron beams. Based on the dosimetric characteristics of the EBT3 film, the read-out procedure established, the nearly uniform energy dependence found and the estimated uncertainties, the EBT3 film was concluded to be a suitable 2D dosimeter for measuring electron or mixed photon/electron dose distributions in water phantom. Uncertainties of 3.7% (k = 2) for absolute and 2.3% (k = 2) for relative dose were estimated.

Figure 1

The verification of transversal correction of the film scanner. Measured relative dose profiles in ${\hspace{0.17em}}^{60}\text{Co}$ gamma ray beam for $10\hspace{0.17em}\text{cm}\times 10\hspace{0.17em}\text{cm}$ field at depth of 5 cm in a phantom. $\text{Blue}=\text{IC measurement},\text{Red}=\text{EBT}3\hspace{0.17em}\text{measurement with the matrix correction}$, and $\text{Green}=\text{EBT}3$ measurement with the matrix correction and dose‐dependent correction.

Figure 2

The relative change of OD in the middle of a film sheet in a rotation point as a function of the angle of rotation. The maximum difference is with 90° and 270°. In zero angle position the longer edge of the film sheet is parallel to scanning direction. OD value is a mean OD in a circular area of 30 mm in diameter. $\text{Blue}=\text{blue channel measurement},\text{Green}=\text{green channel measurement}$, and $\text{Red}=\text{red channel measurement}$.

Figure 3

Procedure for scanning the EBT3 film image in four different orientations. Only 180° of rotation is made.

Figure 4

The net OD values for all color channels as a function of the absorbed dose to water in ${\hspace{0.17em}}^{60}\text{Co}$ gamma ray beam (dose rate 1.4 Gy/min) with dose levels up to 8 Gy. A linear fit has been applied between data points. Film lot: 01171401 (exp. Jan. 2016). $\text{Red}=\text{red channel measurement},\text{Green}=\text{green channel measurement}$, and $\text{Blue}=\text{blue channel measurement}$.

Figure 5

Relative OD as a function of time from exposure to read‐out. Irradiation with ${\hspace{0.17em}}^{60}\text{Co}$ gamma ray beam. Time in days and OD normalized to OD value at four days. $\text{Blue}=\text{background}$ (unexposed EBT3 film) and $\text{Red}=\text{exposed}$ to 2 Gy absorbed dose.

Figure 6

The response of the EBT3 film for absorbed dose to water as a function of electron beam nominal energy and normalized to the response for 6 MV photon beam. $\text{Blue}=\text{EBT}3$ measurement and $\text{Red}=\text{MC simulation}$. The uncertainty of measured absolute dose for 6 MV photon beam is 1.3% $(\mathrm{k}=1)$ and the main contribution of uncertainty comes from ${\mathrm{k}}_{\mathrm{Q}}$ factor of the IC. The contribution to the uncertainty from the film measurement is 0.5% $(\mathrm{k}=1)$. The statistical uncertainty of MC simulations was within 0.3%.