Anisotropy functions for Yb-169 brachytherapy seed models 5, 8 and X1267. An EGS4 Monte Carlo study

Anisotropy Functions for Yb-169 sources used in interstitial brachytherapy are investigated. A comprehensive study of several factors affecting the an gular dose distribution around four Yb-169 seed models (Amersham internatio nal) has been undertaken. Absolute dose rates around Yb-169 seed models 5, 8a, 8b and X1267 have been estimated by means of the EGS4 Monte Carlo Simul ation System. An updated cross section library (DLC-136/PHOTX). binding cor rections for Compton scattering and water molecular form factors were inclu ded in the calculations. Following the formalism developed by the Interstit ial Brachytherapy Collaborative Working Group, anisotropy functions, F(r, t heta), have been calculated and compared with other Monte Carlo results and whenever possible with experimental data. Excellent agreement is found wit h other Monte Carlo calculations. Considering the large experimental errors reported, a fairly good coincidence has been achieved between experimental and Monte Carlo data for models 8a and 8b. For model X1267 large discrepan cies with experiment are obtained. Monte Carlo calculations For all seed mo dels showed model 5 to he the least anisotropic and models 8b and X1267 to be almost identical. Statistical fluctuations can be drastically reduced co mputationally, offering an efficient alternative to measured data. Our resu lts have estimated uncertainties of 0.5%-1.0% within one standard deviation everywhere excluding the longitudinal source axis, where uncertainties are below 3% up to 5 cm, this accuracy being excellent for clinical calculatio ns.