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

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, y_d, which is the microscopic analog of the dose average linear energy transfered. The quality factor used in radiation protection has been shown to be a function of the frequency average lineal energy, y_f. 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, (y_d and y_f, respectively) differ between isotopes by factors of two or even higher in comparison to megavoltage electron beams. These y_d's and y_f's for ¹⁶⁹Yb are consistently higher when compared to ⁶⁰Co or ¹³⁷Cs but are approximately equal to those for ¹²⁵I. Values of y_f, and y_d 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.