Dosimetric investigation and portal dose image prediction using an amorphous silicon electronic portal imaging device

A two step algorithm to predict portal dose images in arbitrary detector sy stems has been developed recently. The current work provides a validation o f this algorithm on a clinically available, amorphous silicon flat panel im ager. The high-atomic number, indirect amorphous silicon detector incorpora tes a gadolinium oxysulfide phosphor scintillating screen to convert deposi ted radiation energy to optical photons which form the portal image. A wate r equivalent solid slab phantom and an anthropomorphic phantom were examine d at beam energies of 6 and 18 MV and over a range of air gaps (similar to 20-50 cm). In the many examples presented here, portal dose images in the p hosphor were predicted to within 5% in low-dose gradient regions, and to wi thin 5 mm (isodose line shift) in high-dose gradient regions. Other basic d osimetric characteristics of the amorphous silicon detector were investigat ed, such as linearity with dose rate (+/-0.5%), repeatability (+/-2%), and response with variations in gantry rotation and source to detector distance . The latter investigation revealed a significant contribution to the image from optical photon spread in the phosphor layer of the detector. This phe nomenon is generally known as "glare," and has been characterized and model ed here as a radially symmetric blurring kernel. This kernel is applied to the calculated dose images as a convolution, and is successfully demonstrat ed to account for the optical photon spread. This work demonstrates the fle xibility and accuracy of the two step algorithm for a high-atomic number de tector. The algorithm may be applied to improve performance of dosimetric t reatment verification applications, such as direct image comparison, backpr ojected patient dose calculation, and scatter correction in megavoltage com puted tomography. The algorithm allows for dosimetric applications of the n ew, flat panel portal imager technology in the indirect configuration, taki ng advantage of a greater than tenfold increase in detector sensitivity ove r a direct configuration. (C) 2001 American Association of Physicists in Me dicine.