A DUAL SOURCE PHOTON-BEAM MODEL USED IN CONVOLUTION SUPERPOSITION DOSE CALCULATIONS FOR CLINICAL MEGAVOLTAGE X-RAY-BEAMS/

A realistic model of photon beams generated by clinical linear acceler ators has been incorporated in a convolution/superposition method to c ompute dose distributions in photon treatment fields. In this beam mod el, a primary photon source represents photons directly from the targe t, and an extra-focal photon source represents scattered photons from the primary collimator and the flattening filter. Monte Carlo simulati on was used to study clinical linear accelerators producing photon bea ms. From the output of the Monte Carlo simulation, the fluence and spe ctral distributions of each photon component, as well as the geometric al characteristics of each photon source with respect to its distance to the isocenter and its source distribution, were analyzed. These qua ntities were used to reproduce realistic photon distributions in treat ment fields, and thus to compute dose distributions using the convolut ion method. Our results showed that compared to the primary photon flu ence, the extra-focal photon fluence from the primary collimator and t he flattening filter was 11%-16% at the isocenter, among which 70% was contributed by the flattening filter. The variation of extra-focal ph otons in different treatment fields was predicted accurately by accoun ting for the finite size of the extra-focal source. Compared to measur ements, dose distributions in photon treatment fields, including those of asymmetric jaw settings and at different SSDs were calculated accu rately, particularly in the penumbral region, by using the convolution method with the new dual source photon beam model. (C) 1997 American Association of Physicists in Medicine.