Is the mean inactivation dose a good measure of cell radiosensitivity?

The relationship between the mean inactivation dose, D̄, and cell radiosensitivity, particularly in the low-dose region, was investigated for linear-quadratic (LQ) and two-component (TC) cell survival curves. Although D̄ is approximately equal to D₄₀ for most survival curves, the results showed that there is no consistent relationship, in theory between D̄ and the extent of cell killing in the low-dose region. Curves with equal D̄ values could potentially represent cell populations with markedly different levels of survival after 200 rad, and conversely, curves with equal surviving fractions at 200 rad could have D̄ values that differ by a factor of two or more. The variability in replicate estimates of D̄ was also investigated and compared with that of SF200, the surviving fraction at 200 rad. The results of a simulation experiment suggest that estimates of D̄ are somewhat more variable than those of SF200 and are more dependent on the choice of cell survival model and the range of available data used for the estimation. It is concluded that SF200 is a more reliable and numerically stable parameter of cell radiosensitivity than D̄.