A fast algorithm for the dose calculation of irregularly shaped fields in stereotactic convergent beam irradiation

A fast, irregular field dose calculation algorithm is described for use in stereotactic convergent beam irradiation (CBI) with high-energy photons. The dose distribution of an irregular field is approximated by a convolution of narrow pencil beams. Decomposition of pencil beams and a 'pre-selection' algorithm is used to avoid redundant calculations. A typical dose calculation for CBI with nine couch angles and nine different irregularly shaped fields takes less than 90 s on a DEC VAXstation 3100. Fortran. The difference between the measured and the calculated dose values increases with increasing distance from the isocentre. The error in 10% isodose line is less than 3% of the isocentre dose.