Compensator thickness verification using an a-Si EPID.

Electronic portal imaging devices (EPIDs) are being increasingly employed to make therapy verification and dose measurements in the clinic. In this work, we investigate the use of an amorphous silicon (a-Si) EPID to verify the accuracy of compensator fabrication and mounting. Compensator thickness estimates on a two-dimensional grid were calculated from the primary component of transmission obtained by subtracting a modeled scatter component from the total transmission measured with the EPID. The primary component was related to the thickness via an exponential relation that includes beam hardening. Implementation of the method involved determination of: (i) a calibration curve relating EPID pixel values to energy fluence for open and attenuated fields, which was found to be linear for open fields but to have a small quadratic component for attenuated beams; (ii) EPID scatter factors to account for field size effects, which exhibited a small dependence on compensator thickness and field size; (iii) the attenuation coefficient of the steel shot compensator material, which varied slightly with off-axis distance and field size, and (iv) an analytical model to predict scatter from the compensator, which was calculated to be <4% at the standard EPID imaging distance of 140 cm. Thickness distributions were then measured for several types of attenuators including flat, test, and clinical compensators. Although uncertainties associated with compensator manufacturing were non-negligible and made assessment of thickness measurement uncertainty difficult, we estimate the latter to be approximately 0.5 mm for steel shot compensators of thickness <4 cm.