New electrical device for field coincidence quality control test in linear accelerators

Testing of the coincidence of a linear accelerators X-ray field and the light field as a quality control test is often done in a subjective method, involving the manual marking of pieces of film and the visual inspection of the film after irradiation. The purpose of this study was to develop an objective method for performing this test, while also increasing the accuracy, precision and the speed of the test. The new method uses a portable, lightweight an inexpensive device containing optically-shielded and nonoptically-shielded photo diodes to detect the location and dimensions of the light and x-ray fields. The measurement results are analyzed and drawn using purpose-written user friendly software. This system is able to reliably measure the coincidence of the two fields with 0.5 mm accuracy and a standard deviation of 0.02 mm, which is well within the tolerance recommended by AAPM Task Group 42. This proposed method allows accurate and precise measurements to be obtained, through a largely automated process, resulting in measurements that have substantially reduced sensitivity to subjectivity    or human error, compared with the standard film-based technique.