Mb. Sharpe et Jj. Battista, DOSE CALCULATIONS USING CONVOLUTION AND SUPERPOSITION PRINCIPLES - THE ORIENTATION OF DOSE SPREAD KERNELS IN DIVERGENT X-RAY-BEAMS, Medical physics, 20(6), 1993, pp. 1685-1694
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.