CALCULATION OF PORTAL DOSE USING THE CONVOLUTION SUPERPOSITION METHOD/

The convolution/superposition method was used to predict the dose thro ughout an extended volume, which includes a phantom and a portal imagi ng device. From the calculated dose volume, the dose delivered in the portal image plane was extracted and compared to a portal dose image. This comparison aids in verifying the beam configuration or patient se tup after delivery of the radiation. The phantoms used to test the acc uracy of this method include a solid water cube, a Nuclear Associates CT phantom, and an Alderson Rando thorax phantom. The dose distributio n in the image plane was measured with film and an electronic portal i maging device in each case. The calculated portal dose images were wit hin 4% of the measured images for most voxels in the central portion o f the field for all of the extended volumes. The convolution/superposi tion method also enables the determination of the scatter and primary dose contributions using the particular dose deposition kernels for ea ch contribution. The ratio of primary dose to total dose was used to e xtract the primary dose from the detected portal image, which enhances the megavoltage portal images by removing scatter blurring. By also p redicting the primary energy fluence, we can find the ratio of compute d primary energy fluence to total dose. Multiplying this ratio by the measured dose image estimates the relative primary energy fluence at t he portal imager. The image of primary energy fluence possesses higher contrast and may be used for further quantitative image processing an d dose modeling. (C) 1996 American Association of Physicists in Medici ne.