Measurement of dose rate from exposure-calibrated 125I seeds

Dose rate in water 1 cm transverse to an 125I seed calibrated for air kerma strength is not well established; 125I dosimetry calculations are, however, based on this constant. The specific dose constant was obtained from a series of dose rate measurements using thermoluminescent dosimetry (TLD) in a rigid geometry, full scatter acrylic phantom for individual model 6711 seeds. With a statistical precision of approximately +/- .5%, the dose rate to an infinitesimal mass of water located in acrylic at a perpendicular distance of 1 cm from the seed was found to be 0.977 cGy/h per microGy-m2/h of air kerma strength. Dose rate in a water phantom was calculated using a model that takes into account differences in both attenuation and scatter between water and acrylic. The specific dose constant in water was determined to be 0.932 (1.184 cGy-cm2/mCi-h, for the conventional exposure rate constant of 1.45 R cm2/mCi-h). This value is 7.5% less than dose rate in water from an unattenuated point source, and 9.7% less than the value commonly used for dosimetry calculations. The results suggest that most clinical 125I dosimetry estimates to date should be reconsidered for a possible reduction by about 10%. Relative scatter attenuation factors at 3 and 5 mm are also presented.