Determination of Calibration Curve and Accurate Assessment of Error and Uncertainty in Radiochromic EBT2 Film Dosimetry

In radiotherapy, the delivered dose to a target tissue should be of high accuracy, where any inaccuracy in the delivered dose can affect the treatment outcomes. Radiochromic films, in this respect, have an essential role in the verification and quality assurance of the radiotheraphy treatment. Gafchromic EBT2 films are widely used in radiation dosimetry due to their advantages such as high sensitivity, good reproducibility, high spatial resolution, easy readout and portability. The first step in applying these films in dosimetry is preparing the calibration profile and determining the uncertainty and absolute error of the films calibration protocol. In this paper, first, the uniformity of a suitable flatbed scanner which is used in dosimetry readout procedure is studied. Then the calibration process of the EBT2 film is carried out using Cobalt radiotherapy. After the detailed specification of error and uncertainty of measurements, the used protocol is evaluated. Also, the analysis of the films is performed by homemade functions using the Matlab software and program along with the image processing toolbox. The percentage depth dose curve of high energy electron beam obtained by the EBT2 film is compared with the standard ionization chamber data. The results show that within the central scan region (20×20 cm2) the scanner nonuniformity is less than ±1%, and no correction method was required. By the use of the described protocol, the error for the dose measurements was evaluted to be less than 1% for a uniform field. A comparison of the percentage depth dose curves obtained from the ionization chamber and EBT2 film shows a 4% difference in values between two curves. The protocol described in this paper is recommended for accurate dosimetry using EBT2 film.