Use of Electronic Portal Image Device in Some Qa Tests

Abstract

Introduction: In the present study a method to investigate light and radiation field congruence utilizing an amorphous silicon electronic portal imaging device (EPID) was developed. An alternative method for routine dynamic multi-leaf collimator (DMLC) quality assurance (QA) using EPID was also described. Materials and Methods: This study employed an amorphous silicon (aSi) EPID, the associated EPI software and a diamond-shaped template. The collimator jaws of the linac (Elekta iViewGT) were aligned such that the light field fell directly on the corners of the diamond. A radiation detection algorithm within the EPI software determined the extent of the radiation field. The light and radiation field congruence was evaluated by comparing the vertices of the diamond reference structure to the detected radiation field. In addition, the digital jaw settings were recorded and later compared to the light field detected on Kodak x-omat films and EPIs. In this study an alternative method for routine dynamic multi-leaf collimator (DMLC) quality assurance (QA) using (EPID) was described. This QA was done by producing a pattern of five 0.5-cm bands 5-cm apart and performing a visual spot-check for dose peak location, dose peak values, interband distance and band gaps measured at Full Width at Half-Maximum (FWHM) of the test pattern for this QA experiment. An aSi EPID and films are used for the DMLC QA to test the practicality and efficacy of EPID vis film. Results: Light radiation field congruence tests with films and EPIs were comparable, yielding a difference of less than 1.2 mm, well within the allowed 2-mm tolerance. A disparity was observed in the magnitude of the detected light field. The X and Y dimensions of the light field measured with film differed by less than or equal to 1.6 mm from the digital collimator settings, whereas the values extracted from the EPIs differed by up to 1.8 mm. The FWHM of the multi-leaf collimator (MLC) obtained by the EPID is 6.34 mm, which is slightly higher than the 6.18 mm for the film. The lowest dose at the valley is significantly lower for the EPID than for the film. The dose peak value of the EPID is found to be 97.9 with standard deviation of ±1.59 and the dose peak value of film is found to be 96.5 with standard deviation of ±2.41, respectively. Film and EPID track the distance between peaks with a mean of 49.8 and 49.53 mm and standard deviation of ±0.3 and ±1.02 mm, respectively. EPID is able to track the location of individual dose peaks for all the leaf pairs consistently to a standard deviation of ±0.12 to ±0.47 mm, slightly better than that of ±1.00 to ±1.26 mm for the film. Conclusion: EPIs were found to be a quick and reliable alternative to film for qualitative and relative analyses. Also EPID is as good as the film in resolving the band pattern of the DMLC test field. Although the resolution of the EPID is lower than that of the film, it is high enough to faithfully reproduce the band pattern without significant distortion. Since EPID images can be acquired, analyzed an...