Anisotropy functions for 169Yb brachytherapy seed models5, 8 and X1267. An EGS4 Monte Carlo study

Abstract

Anisotropy functions for 169Yb sources used in interstitialbrachytherapy are investigated. A comprehensive study of several factorsaffecting the angular dose distribution around four 169Yb seed models(Amersham International) has been undertaken. Absolute dose rates around169Yb seed models 5, 8a, 8b and X1267 have been estimated by means ofthe EGS4 Monte Carlo Simulation System. An updated cross section library(DLC-136/PHOTX), binding corrections for Compton scattering and watermolecular form factors were included in the calculations. Following theformalism developed by the Interstitial Brachytherapy Collaborative WorkingGroup, anisotropy functions, F(r,θ), have been calculated andcompared with other Monte Carlo results and whenever possible withexperimental data. Excellent agreement is found with other Monte Carlocalculations. Considering the large experimental errors reported, a fairlygood coincidence has been achieved between experimental and Monte Carlodata for models 8a and 8b. For model X1267 large discrepancies withexperiment are obtained. Monte Carlo calculations for all seed modelsshowed model 5 to be the least anisotropic and models 8b and X1267 to bealmost identical. Statistical fluctuations can be drastically reducedcomputationally, offering an efficient alternative to measured data. Ourresults have estimated uncertainties of 0.5%-1.0% within one standarddeviation everywhere excluding the longitudinal source axis, whereuncertainties are below 3% up to 5 cm, this accuracy being excellent forclinical calculations.