Verification of compensator thicknesses using a fluoroscopic electronic portal imaging device

A method is presented for verification of compensator thicknesses using a fluoroscopic 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 without the compensator in the beam. The transmission is determined with the EPID by dividing two images, acquired with and without compensator inserted, which are only corrected for the nonlinear response of the fluoroscopic system. The transmission has a primary and a scatter component. The primary component is derived from the measured transmission by subtracting the predicted 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 primary components taking into account off-axis variations in beam quality. The developed method has been tested for various compensators made of a granulate of stainless steel. The compensator thicknesses could be determined with an accuracy of 0.5 mm (1 s.d.), corresponding to a change in the transmitted dose of about 1% for a 10 MV beam. The method is fast, accurate, and insensitive to long-term output and beam profile fluctuations of the linear accelerator.