DOSE CALCULATIONS USING CONVOLUTION AND SUPERPOSITION PRINCIPLES - THE ORIENTATION OF DOSE SPREAD KERNELS IN DIVERGENT X-RAY-BEAMS

The convolution/superposition method of dose calculation has the poten tial to become the preferred technique for radiotherapy treatment plan ning. When this approach is used for therapeutic x-ray beams, the dose spread kernels are usually aligned parallel to the central axis of th e incident beam. While this reduces the computational burden, it is mo re rigorous to tilt the kernel axis to align it with the diverging bea m rays that define the incident direction of primary photons. We have assessed the validity of the parallel kernel approximation by computin g dose distributions using parallel and tilted kernels for monoenerget ic photons of 2, 6, and 10 MeV; source-to-surface distances (SSDs) of 50, 80, and 100 cm; and for field sizes of 5 X 5, 15 X 15, and 30 X 30 cm2. Over most of the irradiated volume, the parallel kernel approxim ation yields results that differ from tilted kernel calculations by 3% or less for SSDs greater than 80 cm. Under extreme conditions of a sh ort SSD, a large field size and high incident photon energy, the paral lel kernel approximation results in discrepancies that may be clinical ly unacceptable. For 10-MeV photons, we have observed that the paralle l kernel approximation can overestimate the dose by up to 4.4% of the maximum on the central axis for a field size of 30 X 30 cm2 applied wi th a SSD of 50 cm. Very localized dose underestimations of up to 27% o f the maximum dose occurred in the penumbral region of a 30 X 30-cm2 f ield of 10-MeV photons applied with a SSD of 50 cm.