Comparison of measured and calculated dose rates in water near I-125 and Ir-192 seeds.

Recent theoretical and experimental work indicates that currently accepted 125I dosimetry data may overestimate dose in water at 1 cm by 10%-24%. Among the most comprehensive measurements are those of the NCI-sponsored brachytherapy contract participants. Absolute dose rates in water calculated by the Monte Carlo method have been compared with the NCI dose measurements about 125I and 192Ir seeds embedded in solid-water phantoms. The photon transport code allows realistic geometric simulation of the complex internal seed structure, the National Institute of Standards and Technology air-kerma strength standardization geometry, and the dose measurement setup. When the appropriate measurement medium and geometry are assumed, agreement between theory and measurement is excellent, within 3% at 1 cm and averaging 3% at larger distances. However, the data do not support the water equivalence of solid water at 125I energies indicating that solid-water measurements underestimate 125I specific dose-rate constants in water by 4.3%. Because of its higher ratio of absorption to scatter, 125I dose distributions measured in solid water are less penetrating (by 35% at 10 cm) than those measured in liquid water. For model 6711, model 6702, and steel-clad 192Ir seeds, Monte Carlo calculations yielded specific dose-rate constants (assuming liquid water medium) of 0.877, 0.932, and 1.122 cGy cm2 h-1 per unit air-kerma strength, respectively. For 125I, currently accepted values are 18% and 11% larger for the two seed models.