Dosimetry on transverse axes of ¹²⁵l and ¹⁹²lr 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.