CALCULATION OF THE DOSE DISTRIBUTION IN WATER FROM GE-71 K-SHELL X-RAYS

The dose distribution in water from Ge-71 K-shell x-rays (E-ave = 9.44 keV) was calculated for various source configurations using both anal ytic and EGS4 Monte Carlo calculations. The point source kernel and th e 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 Ge-71 and Sr-90/Y shows a similar ity between their relative dose distribution in water. The dose distri bution from a disc source was calculated using the EGS4 code and compa red with the results from analytic calculation. Excellent agreement wa s observed, confirming the validity of analytic calculations. The dose rate at 0.01 cm from a Ge-71 disc source was calculated to be about 1 .3 x 10(-5) Gy MBq(-1) s(-1). Based on the results from this study, Ge -71 activity of the order of 3.7 x 10(10) Bq (-1 Ci) might be necessar y to obtain dose rates typical of Sr-90/Y ophthalmic applicators. The possibility of using Ge-71 as a source of radioactive stents was also investigated. A Ge-71 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 P-32-coate d stent. The dose rate at 0.01 cm from a Ge-71 stent has been calculat ed 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 P-32-coated stent. However, an initial source activity of the order of 3.7 x 10(7) Bq (s imilar to 1 mCi) would easily result in a typical target dose (similar to 24 Gy) needed for intravascular stent applications. In conclusion, Ge-71 sources could be used as alternatives to beta sources and, unli ke high-energy (similar to MeV) beta sources, may provide easily predi ctable dose distributions in heterogeneous media and low dose rates, w hich might be beneficial for some clinical applications.