Definition – Target Theory
An early model suggesting that a cell must have one or more “targets” (usually DNA sites). If all essential targets are inactivated by radiation, the cell dies.

• Single-target model: one hit on the critical target = death (too simplistic).

• Multitarget model: multiple hits required before death occurs; explains the “shoulder” on survival curves.

Survival curve features

• D0 (mean lethal dose): the dose that reduces survival to 37%.

• n (extrapolation number): reflects width of the curve’s shoulder.