Retreatment Tolerance of Normal Tissues

The experimental data for reirradiation of various normal tissues are reviewed. Retreatment tolerance is discussed in relation to the interval between treatments and the initial treatment dose delivered. To facilitate a comparison between tissues with different radiosensitivities, both initial and retreatment doses are described in terms of percent extrapolated tolerance dose, using linear-quadratic fraction concepts. The data indicate that rapidly dividing tissues, such as skin and intestine, recover rapidly and can be reirradiated to almost full tolerance within 1 to 3 months. Some slowly dividing tissues, eg, lung and spinal cord, are capable of a more limited long-term recovery and can be reirradiated with partial tolerance doses. For these tissues, reirradiation tolerance increases with time between treatments and is inversely related to the initial dose. In other tissues, eg, bladder, the reirradiation tolerance is independent of retreatment interval suggesting permanent residual injury. In the kidney, reirradiation tolerance actually decreases with time, indicative of further progression of subclinical damage from the first treatment.