A microdosimetric interpretation of the radiobiological effectiveness of 125I and the problem of quality factor

When considering the microdosimetric energy deposition in small biological targets the Auger electron emitter 125I behaves like a high-LET radiation. For a specific incorporation in sensitive biological volumes of 20 nm in diameter a mean lineal energy of gamma F approximately equal to 270 keV micron-1 has been evaluated. Experimental data on transformation, mutation and chromosome aberration from DNA-bound 125I 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 function Q(L) applied in radiation protection will be discussed.