A quality assurance program for an amorphous silicon electronic portal imaging device using in-house developed phantoms: a method development for dosimetry purposes

Background: Electronic portal imaging devices (EPIDs) play an important role in radia on therapy portal imaging, geometric and dosimetric verifica ons. A successful u liza on of EPIDs for imaging and dosimetric purposes requires a reliable quality control process rou ne to be carried out regularly. In this study, two in‐house phantoms were developed and analyzed for implementa on in a quality assurance program for dosimetry purposes. Materials and Methods: An amorphous silicon (a‐Si) imager (Op Vue500) was used. A low contrast resolu on phantom and an image quality phantom were constructed and implemented. Low contrast resolu on of the EPID was evaluated by coun ng the number of holes detectable in the image of phantom using human observers and a so ware. The image quality phantom was used for modula on transfer func on, contrast to noise ra o and noise level evalua ons. This phantom contains five sets of high‐contrast rectangular bar pa erns of variable spa al frequencies and six uniform regions. Results: Although the manual low contrast resolu on method was observer‐ dependent and insensi ve to ar facts, the automa c method was robust and fully objec ve but sensi ve to ar facts. The cri cal frequency values for 6 and 18 MV were 0.3558±0.006 lp/mm and 0.2707±0.006 lp/mm respec vely. The contrast‐to‐noise ra o was found to be ~ 240% higher for 6 MV compared to 18 MV. Conclusion: The developed phantoms provide a convenient process for periodic performance of an EPID. These phantoms are independent of the EPID system and provide robust tools for con nuous monitoring of image quality parameters as well as dosimetric parameters.