DOSIMETRY ON TRANSVERSE AXES OF I-125 AND IR-192 INTERSTITIAL BRACHYTHERAPY SOURCES

Dose rates along the transverse axes of 125I model 6702, 125I model 6711 and ,92Ir 0.2-mm steel sources for interstitial brachytherapy have been measured in a solid-water phantom for distances up to 10 cm using LiF thermoluminescent dosimeters (TLDs). Specific dose rate constants, the dose rates in water per unit source strength 1 cm along the perpendicular bisector of the source, are determined to be 0.90 + 0.03,0.85 + 0.03, and 1.09 + 0.03 cGy h “1 U“1for ,25I model 6702, l25I model 6711 and 192Ir 0.2-mm steel sources, respectively (1 U = unit of air kerma strength = 1/iGy m2 h_I= 1 cGy cm2h_l). In older and obsolete units of source strength (i.e., mCi apparent), these are 1.14 + 0.03, 1.08 + 0.03, and 4.59 + 0.15 cGyh_1mCi-1(apparent). Currently accepted values of specific dose rate constant for l25I sources are up to 20% higher than our measured values which are in good agreement with the results of our Monte Carlo simulations. But for 192Ir there is good agreement between our measured value of the specific dose rate constant and currently accepted values. The radial dose function for 125I model 6702 is found to be consistently larger than that for125I model 6711, with an increasing difference as the distance from the source increases. Our measured values for the radial dose function for 125I sources are in good agreement with the results of our Monte Carlo simulation as well as the measured values of Schell etal. [Int. J. Radiat. Oncol. Biol. Phys. 13, 795–799 (1987)] for model 6702 and Ling etal [Int. J. Radiat. Oncol. Biol. Phys. 9, 1747–1752 (1983)] for model 6711. However, some of the recently reported Monte Carlo values of the radial dose function for 125I sources are significantly larger than measured values; up to 18% at a distance of 5 cm. Our measured radial dose function for the 192Ir seed is in good agreement with our Monte Carlo calculated values, and with both our earlier data for the high activity 192Ir source of a remote afterloader and recommended values by Meisberger et al [Radiol. 90, 953–957 (1968)1. © 1990, American Association of Physicists in Medicine. All rights reserved.