CALCULATING DOSE DISTRIBUTIONS AND WEDGE FACTORS FOR PHOTON TREATMENTFIELDS WITH DYNAMIC WEDGES BASED ON A CONVOLUTION SUPERPOSITION METHOD/

A convolution/superposition based method was developed to calculate do se distributions and wedge factors in photon treatment fields generate d by dynamic wedges. This algorithm used a dual source photon beam mod el that accounted for both primary photons from the target and seconda ry photons scattered from the machine head. The segmented treatment ta bles (STT) were used to calculate realistic photon fluence distributio ns in the wedged fields. The inclusion of the extra-focal photons resu lted in more accurate dose calculation in high dose gradient regions, particularly in the beam penumbra. The wedge factors calculated using the convolution method were also compared to the measured data and sho wed good agreement within 0.5%, The wedge factor varied significantly with the held width along the moving jaw direction, but not along the static jaw ol the depth direction. This variation was found to be dete rmined by the ending position of the moving jaw, or the STT of the dyn amic wedge. In conclusion, the convolution method proposed in this wor k can be used to accurately compute dose for a dynamic or an intensity modulated treatment based on the fluence modulation in the treatment field. (C) 1998 American Association of Physicists in Medicine.