Microdosimetric single event spectra of Ytterbium-169 compared with commonly used brachytherapy sources and...

Ytterbium-169 has been developed as a possible replacement for Iridium-192 and Iodine-125. The Theory of Dual Radiation Action predicts that the initial slope of the cell survival curve and therefore the relative biological effect at low dose rate is proportional to dose average lineal energy, yd, which is the microscopic analog of the dose average linear energy transferred. The quality factor used in radiation protection has been shown to be a function of the frequency average lineal energy, yf. Single event microdosimetric spectra for 60Co, 137Cs, 192Ir, 125I and 169Yb were measured in air and at several depths in phantom with a Rossi proportional counter. These spectra show marked differences between sources. The microscopic analogs of the track average and dose average LET, (yd and yf, respectively) differ between isotopes by factors of two or even higher in comparison to megavoltage electron beams. These yd's and yf's for 169Yb are consistently higher when compared to 60Co or 137Cs but are approximately equal to those for 125I. Values of yf and yd for 192Ir are intermediate between 60Co and 169Yb. The Theory of Dual Radiation Action predicts a low dose rate RBE (assuming a 1 micron effective site diameter) compared to 60Co (in air) of: 1.00 for 137Cs, 1.29 for 192Ir, 1.60 for 169Yb and 1.77 for 125I.