A test tool for the visual verification of light and radiation fields using film or an electronic portal imaging device

We describe the design and evaluation of a simple test tool which can be us ed in conjunction with either film or an electronic portal imaging device ( EPID) to verify light and radiation fields and their congruence. The precis ion of the technique is better than 0.5 mm under all conditions tested. Whe n used with film the accuracy or offset of the technique (the difference be tween test tool observations and a scanned conventional film) is better tha n 0.5 mm but, with an EPID as the image receptor, the accuracy dropped to, in one trial, 0.86 mm. The offset may be due to a systematic observer bias in determining the 50% O.D, level on the image, compounded, in the case of EPID measurements, by image acquisition and display parameters. Thus, when used with an EPID, calibration of the system will be required if absolute f ield dimensions are required. When used with film, the test tool method des cribed here is of sufficient accuracy and precision to confirm the complian ce of light and radiation field parameters with currently accepted quality control protocols. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)00902-5].