Two-dimensional discrete ordinates photon transport calculations for brachytherapy dosimetry applications

The DANTSYS discrete ordinates computer code system is applied to quantitat ive estimation of water kerma rate distributions in the vicinity of discret e photon sources with energies in the 20- to 800-keV range in two-dimension al cylindrical r-z geometry. Unencapsulated sources immersed in cylindrical water phantoms of 40-cm diameter and 40-cm height are modeled in either ho mogeneous phantoms or shielded by IE, Fe, and Pb filters with thicknesses o f 1 and 2 mean free paths. The obtained dose results are compared with corr esponding photon Monte Carlo simulations. A 210-group photon cross-section library for applications in this energy range is developed and applied, tog ether with a general-purpose 42-group library developed at Los Alamos Natio nal Laboratory, for DANTSYS calculations The accuracy of DANTSYS with the 4 2-group library relative to Monte Carlo exhibits large pointwise fluctuatio ns from -42 to +84%. The major cause for the observed discrepancies is dete rmined to be the inadequacy of the weighting function used for the 42-group library derivation. DANTSYS simulations with a finer 210-group library sho w excellent accuracy on and off the source transverse plane relative to Mon te Carlo kerma calculations, varying from --4.9 to 3.7%. The P-3 Legendre p olynomial expansion of the angular scattering function is shown to be suffi cient for accurate calculations. The results demonstrate that DANTSYS is ca pable of calculating photon doses in very good agreement with Monte Carlo a nd that the multigroup cross-section library and efficient techniques for m itigation of ray effects are critical for accurate discrete ordinates imple mentation.