Calculation of the dose distribution in water from ⁷¹Ge K-shell x-rays

The dose distribution in water from ⁷¹Ge K-shell x-rays (E(ave) = 9.44 keV) was calculated for various source configurations using both analytic and EGS4 Monte Carlo calculations. The point source kernel and the buildup factor are presented. The buildup factor for a point source in water has been found to increase up to about 1.1 as radial distance approaches 1 cm. Comparison between ⁷¹Ge and ⁹⁰Sr/Y shows a similarity between their relative dose distribution in water. The dose distribution from a disc source was calculated using the EGS4 code and compared with the results from analytic calculation. Excellent agreement was observed, confirming the validity of analytic calculations. The dose rate at 0.01 cm from a ⁷¹Ge disc source was calculated to be about 1.3 x 10^-5 Gy MBq^-1 s^-1. Based on the results from this study, ⁷¹Ge activity of the order of 3.7 x 10¹⁰ Bq (~1 Ci) might be necessary to obtain dose rates typical of ⁹⁰Sr/Y ophthalmic applicators. The possibility of using ⁷¹Ge as a source of radioactive stents was also investigated. A ⁷¹Ge stent was modelled as a cylindrical shell source and the dose rates were determined by Monte Carlo calculations. Some calculated results are compared with published values for a ³²P-coated stent. The dose rate at 0.01 cm from a ⁷¹Ge stent has been calculated to be about 6.5 x 10^-3 Gy MBq^-1 h^-1, which is much lower than the reported dose rate at the same distance from a ³²P-coated stent. However, an initial source activity of the order of 3.7 x 10⁷ Bq (~1 mCi) would easily result in a typical target dose (~24 Gy) needed for intravascular stent applications. In conclusion, ⁷¹Ge sources could be used as alternatives to beta sources and, unlike high-energy (~MeV) beta sources, may provide easily predictable dose distributions in heterogeneous media and low dose rates, which might be beneficial for some clinical applications.