Anisotropy functions for 125I and 103Pd sources.

The anisotropy function was defined by AAPM Task Group 43 in a recent report, and reference data were provided. However, some of the data appeared to have large experimental errors. In this work, anisotropy functions are presented for three low-energy photon-emitting sources commonly used for brachytherapy: 125I source models 6702 and 6711, and the model 200 103Pd source. The data were generated via a two-step process: the sources' intrinsic radiation emission pattern was determined from in-air measurements, and these data were used as input to Monte Carlo calculations of the fluence distribution in water. The new anisotropy function values for iodine compare well with earlier data from diode measurements, and seem more physically realistic than some data provided by TG43. Previous palladium data measured with TLDs are more variable, but on the average are similar to the data presented here. The new data have estimated uncertainties of 3%-6%; this level of accuracy should be completely adequate for clinical calculations.