ABSTRACT
The radiation exposure of normal tissues must be considered over a wide range of overall treatment durations (Fig. 11.1). First, on a short scale of minutes to hours, incomplete recovery of sublethal damage may reduce the radiation tolerance of a tissue. Second, over a range of days to weeks (i.e. during a course of fractionated radiotherapy with varying overall treatment times) radiation-induced tissue regeneration (‘repopulation’) may modulate radiation tolerance. This radiation-induced regeneration response is seen in early-responding tissues. Third, over a range of months to years, long-term restoration can occur in some tissues, which renders them more resistant to re-irradiation. Also, long-term progression of the damage can occur in other tissues, which causes decreased re-irradiation tolerance.
