Assessing radiation and light field congruence with a video based electronic portal imaging device.

Projected light fields are used on treatment simulators and teletherapy treatment units to delineate the size and position of the radiation beam. Any discrepancy between these fields will lead to a systematic field placement error, with possibly serious implications with regard to the accuracy of the delivered dose distribution in the patient. Conventionally, film has been used for regular quality control tests of light and radiation field congruence, but this is a time consuming method and is not suitable for daily checks. A new method is described that uses a specially designed test phantom, a video-based electronic portal imaging device and a personal computer to test for radiation and light field congruence on treatment accelerators. This method consists of aligning the test phantom in the light field of a treatment linac and acquiring an electronic portal image. A computer program then automatically analyzes the image and determines the degree of congruence between 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 light and radiation fields. Two algorithms were tested for reproducibility (<0.4 mm), sensitivity to noise (<0.2 mm), 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 days demonstrated differences in the results determined by the two algorithms 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 routine start-up procedures.