ASSESSING RADIATION AND LIGHT-FIELD CONGRUENCE WITH A VIDEO-BASED ELECTRONIC PORTAL IMAGING DEVICE

Projected light fields are used on treatment simulators and teletherap y treatment units to delineate the size and position of the radiation beam. Any discrepancy between these fields will lead to a systematic f ield placement error, with possibly serious implications with regard t o the accuracy of the delivered dose distribution in the patient. Conv entionally, film has been used for regular quality control tests of li ght and radiation field congruence, but this is a time consuming metho d and is not suitable for daily checks. A new method is described that uses a specially designed test phantom, a video-based electronic port al imaging device and a personal computer to test for radiation and li ght field congruence on treatment accelerators. This method consists o f aligning the test phantom in the light field of a treatment linac an d acquiring an electronic portal image. A computer program then automa tically analyzes the image and determines the degree of congruence bet ween the two fields. The final result of the test is a go, warning, or no go decision depending on the extent of misalignment between the li ght and radiation fields. Two algorithms were tested for reproducibili ty (<0.4 mm), sensitivity to noise (<0.2 mmr, and positional accuracy (<0.4 mm) and are shown to give results comparable to the conventional film method. Daily testing of field congruence over a period of 84 da ys demonstrated differences in the results determined by the two algor ithms of less than 0.1+/-0.2 mm (standard deviation) at 6 MV and 0.22/-0.13 mm at 23 MV. Routine testing is possible as the effort and time required are minimal, and the test can be performed during daily rout ine start-up procedures. (C) 1996 American Association of Physicists i n Medicine.