ANISOTROPY OF AN (192)IRIDIUM HIGH-DOSE-RATE SOURCE MEASURED WITH A MINIATURE IONIZATION-CHAMBER

The anisotropy of a high dose rate (HDR) Ir-192 source was measured in air and in water using a miniature (0.147 cm(3)) ionization chamber. Measurements were made at a distance of 5 cm from the source center at polar angles from 10 degrees-170 degrees. The anisotropy was found to be less pronounced in water, and the anisotropy is asymmetric about t he transverse axis. The results agree with previous ionization chamber and TLD measurements to within +/-4%. Mean anisotropy factors were de termined at each angle from all existing data at 5 cm distance, and co mpared to published Monte Carlo calculations, and to the values used i n the microSelectron HDR brachytherapy planning system (BPS). The Mont e Carlo photon transport code appears to systematically underestimate the anisotropy factor by up to 4% in the forward direction and overest imate it by up to 3% in the backward direction. The mean anisotropy fa ctors also indicate that the BPS systematically underestimates the ani sotropy factor by up to 3% in the forward direction, and overestimates it by up to 15% in the backward direction. However, the 15% differenc e occurs at 180 degrees where it is not likely to be clinically signif icant. (C) 1997 American Association of Physicists in Medicine.