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 I-125 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 I-125 and Ir-192 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 I-125 energies indicating that solid-water measurements underestimate I-125 specific dose-rate constants in water by 4.3%. Because of its higher ratio of absorption to scatter, I-125 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 Ir-192 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 I-125, currently accepted values are 18% and 11% larger for the two seed models.