A microdosimetric interpretation of the radiobiological effectiveness of125I and the problem of quality factor

When considering the microdosimetric energy deposition in small biological targets the Auger electron emitter¹²⁵I behaves like a high-LET radiation. For a specific incorporation in sensitive biological volumes of 20 nm in diameter a mean lineal energy of $\bar y_F \simeq 270$ keV µm⁻¹ has been evaluated. Experimental data on transformation, mutation and chromosome aberration from DNA-bound¹²⁵I are interpreted to yield RBE-values, relative to 200 kV X rays, of 32 to 38 (transformations), 1 to 16 (mutations) and 6 to 77 (chromosome aberrations). Thus the RBE for transformation and chromosome aberration tends to be larger than the ICRP recommended high-LET quality factor (Q=20). The implications of these results for the functionQ(L) applied in radiation protection will be discussed.