PORTAL DOSE IMAGE (PDI) PREDICTION FOR DOSIMETRIC TREATMENT VERIFICATION IN RADIOTHERAPY - 1 - AN ALGORITHM FOR OPEN BEAMS

A method is presented for calculation of transmission functions for hi gh energy photon beams through patients. These functions are being use d in our clinic for prediction of portal dose images (PDIs) which are compared with PDIs measured with an electronic portal imaging device ( EPID). The calculations are based on the planning CT-scan of the patie nt and on the irradiation geometry as determined in the treatment plan ning process. For each beam quality, the required input data for the a lgorithm for transmission prediction are derived from a limited number of measured beam data. The method has been tested for a PDI-plane at 160 cm from the focus, in agreement with the fixed focus-to-detector d istance of our fluoroscopic EPIDs. For 6, 23 and 25 MV photon beams go od agreement (similar to 1%) has been found between calculated and mea sured transmissions through anthropomorphic phantoms. (C) 1998 America n Association of Physicists in Medicine. [S0094-2405(98)02006-9].