INVESTIGATION OF THE CONVOLUTION METHOD FOR POLYENERGETIC SPECTRA

The distribution of absolute dose per unit fluence from polyenergetic photon beams impinging upon a water phantom was calculated using two c onvolution approaches that properly account for beam hardening effects . Dose deposition kernels calculated previously using the EGS4 Monte C arlo code are convolved with the primary terma to give the dose for mo noenergetic photon beams of energies ranging from 100 kev to 50 MeV. A polyenergetic dose distribution is composed of separately calculated monoenergetic components, which are appropriately weighted with the fl uence spectrum to yield the polyenergetic dose distribution. Alternati vely, a single convolution for the polyenergetic beam is considered, w here a composite polyenergetic kernel is convolved with the respective polyenergetic terma. The effects of the polyenergetic kernel variance due to beam hardening as well as the effect of tilting the kernels fo r a diverging beam geometry were also examined. The depth dose data pr oduced using the two proposed methods were compared with measured data and Monte Carlo simulations and showed good agreement.