New Method of Quality Control Test for Light and Radiation Field Coincidence in Medical Linear Accelerators

The evaluation of X-ray and light field coincidence in linear accelerators as a quality control test is often performed subjectively, involving the manual marking of films and their visual inspection following the irradiation. Therefore, the present study aimed to develop an objective method for the performance of this test leading to the increased levels of accuracy, precision, and speed for the measurement of X-ray and light field coincidence.

The new method involved a portable, lightweight, and inexpensive device containing optically-shielded and non-shielded photodiodes to detect the location and dimensions of the light and X-ray fields. The obtained results were analyzed using purpose-written user-friendly software.

On the basis of the results, this system could be a reliable method to measure the coincidence of the two fields with the accuracy of 0.5 mm and average field size standard deviations of Elekta Presice and Siemens Primus are 22.47 mm2 and 22.36 mm2, respectively. The result was well within the tolerance recommended by the American Association of Physicists in Medicine task group report number 142.

The proposed method allows accurate and precise measurements through a largely automated process. Therefore, the measurement results benefit from the reduced level of subjectivity or human error, compared to the standard film-based technique.