Anisotropy of an 192iridium high dose rate source measured with a miniature ionization chamber.

The anisotropy of a high dose rate (HDR) 192Ir source was measured in air and in water using a miniature (0.147 cm3) 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 the transverse axis. The results agree with previous ionization chamber and TLD measurements to within +/- 4%. Mean anisotropy factors were determined at each angle from all existing data at 5 cm distance, and compared to published Monte Carlo calculations, and to the values used in the microSelectron HDR brachytherapy planning system (BPS). The Monte Carlo photon transport code appears to systematically underestimate the anisotropy factor by up to 4% in the forward direction and overestimate it by up to 3% in the backward direction. The mean anisotropy factors also indicate that the BPS systematically underestimates the anisotropy factor by up to 3% in the forward direction, and overestimates it by up to 15% in the backward direction. However, the 15% difference occurs at 180 degrees where it is not likely to be clinically significant.