Verification of compensator thicknesses using a fluoroscopic electronic portal imaging device

A method is presented for verification of compensator thicknesses using a f luoroscopic electronic portal imaging device (EPID). The method is based on the measured transmission through the compensator, defined by the ratio of the portal dose with the compensator in the beam and the portal dose witho ut the compensator in the beam. The transmission is determined with the EPI D by dividing two images, acquired with and without compensator inserted, w hich are only corrected for the nonlinear response of the fluoroscopic syst em. The transmission has a primary and a scatter component. The primary com ponent is derived from the measured transmission by subtracting the predict ed scatter component. The primary component for each point is only related to the radiological thickness of the compensator along the ray line between the focus and that point. Compensator thicknesses are derived from the pri mary components taking into account off-axis variations in beam quality. Th e developed method has been tested for various compensators made of a granu late of stainless steel. The compensator thicknesses could be determined wi th an accuracy of 0.5 mm (1 s.d.), corresponding to a change in the transmi tted dose of about 1% for a 10 MV beam. The method is fast, accurate, and i nsensitive to long-term output and beam profile fluctuations of the linear accelerator. (C) 1999 American Association of Physicists in Medicine. [S009 4-2405(99)01908-2].