CORRECTING KERNEL TILTING AND HARDENING IN CONVOLUTION SUPERPOSITION DOSE CALCULATIONS FOR CLINICAL DIVERGENT AND POLYCHROMATIC PHOTON BEAMS/

To account for clinical divergent and polychromatic photon beams, we h ave developed kernel tilting and kernel hardening correction methods f or convolution dose calculation algorithms. The new correction methods were validated by Monte Carlo simulation. The accuracy and computatio n time of the our kernel tilting and kernel hardening correction metho ds were also compared to the existing approaches including terma diver gence correction, dose divergence correction methods, and the effectiv e mean kernel method with no kernel hardening correction. Treatment fi elds of 10x 10-40x40 cm(2) (field size at source to axis distance (SAD )) with source to source distances (SSDs) of 60, 80, and 100 cm, and p hoton energies of 6, 10, and 18 MV have been studied. Our results show ed that based on the relative dose errors at a depth of 15 cm along th e central axis, the terma divergence correction may be used for fields smaller than 10x10 cm(2) with a SSD larger than 80 cm; the dose diver gence correction with an additional kernel hardening correction can re duce dose error and may be more applicable than the terma divergence c orrection. For both these methods, the dose error increased linearly w ith the depth in the phantom; the 90% isodose lines at the depth of 15 cm were shifted by about 2%-5% of the field width due to significant underestimation of the penumbra dose. The kernel hardening effect was less prominent than the kernel tilting effect for clinical photon beam s. The dose error by using nonhardening corrected kernel is less than 2.0% at a depth of 15 cm along the central axis, yet it increased with a smaller field size and lower photon energy. The kernel hardening co rrection could be more important to compute dose in the fields with be am modifiers such as wedges when beam hardening is more significant. T he kernel tilting correction and kernel hardening correction increased computation time by about 3 times, and 0.5-1 times, respectively. Thi s can be justified by more accurate dose calculations for the majority of clinical treatments. (C) 1997 American Association of Physicists i n Medicine.