DOSIMETRIC CHARACTERISTICS, AIR-KERMA STRENGTH CALIBRATION AND VERIFICATION OF MONTE-CARLO SIMULATION FOR A NEW YB-169 BRACHYTHERAPY SOURCE

Purpose: Ytterbium-169 (Yb-169) is a promising new isotope for brachyt herapy with a half life of 32 days and an average photon energy of 93 KeV. It has an Ir-192-equivalent dose distribution in water but a much smaller half-value layer in lead (0.2 mm), affording improved radiati on protection and customized shielding of dose-limiting anatomic struc tures. The goals of this study are to: (a) experimentally validate Mon te Carlo photon transport dose-rate calculations for this energy range , (b) to develop a secondary air-kerma strength standard for Yb-169, a nd (c) to present essential treatment planning data including the tran sverse-axis dose-rate distribution and dose correction factors for a n umber of local shielding materials. Methods and Materials: Several int erstitial Yb-169 sources (type 6) and an experimental high dose-rate s ource were made available for this study. Monte-Carlo photon-transport (MCPT) simulations, based upon validated geometric models of source s tructure, were used to calculate dose rates in water. To verify MCPT p redictions, the transverse-axis dose distribution in homogeneous water medium was measured using a silicon-diode detector. For use in design ing shielded applicators, heterogeneity correction factors (HCF) arisi ng from small cylindrical heterogeneities of lead, aluminum, titanium, steel and air were measured in a water medium. Finally, to provide a sound experimental basis for comparing experimental and theoretical do se-rate distributions, the air-kerma strength of the sources was measu red using a calibrated ion chamber. To eliminate the influence of meas urement artifacts on the comparison of theory and measurement, simulat ed detector readings were compared directly to measured diode readings . The final data are presented in the format endorsed by the Interstit ial Collaborative Working Group. Results: The in-air calibration revea led that the air-kerma strength per unit activity (mCi), as quoted by the vendor, varied from 1.30 to 1.57 cGy . cm2/mCi . h depending on se ed design. The maximum difference between measured and MCPT-simulated absolute diode readings on the transverse axis was less than 2%, indic ating that MCPT accurately predicts dose rate in medium for brachyther apy sources in this energy range. Comparison of measured and simulated HCFs for each of the 16 different cylindrical heterogeneities demonst rated 1-3% agreement. The HCFs vary by as much as 200% with respect to distance and by as much as 48% as a function of disk diameter, showin g that HCF is strongly dependent on heterogeneity location and lateral dimensions as well as thickness. The dose-rate constant for water med ium was found to be 1.225 cGy/h per kerma unit air-strength and 1.962 cGy/h per unit mCi as measured by the vendor. Conclusion: Monte Carlo simulation is an accurate and powerful tool for dosimetric characteriz ation of brachytherapy sources in this energy range. Thin lead foils p roduce shielding factors comparable to standard shielded applicators f or Cs-137. Meaningful theoretical absolute dose calculations in brachy therapy require accurately implemented air-kerma strength standards.