Intervals between multiple fractions per day. Differences between early and late radiation reactions.
Assuming the linear quadratic model for dose-response curves enables the proportion of repairable damage to be calculated for any size of dose per fraction. It is given by the beta (dose squared) term, and represents a larger proportion of the total damage for larger doses per fraction, but also for late-reacting than for early-reacting tissues. For example at 2 Gy per fraction, repairable damage could represent nearly half the total damage in late-reacting tissues but only one fifth in early-reacting tissues. Even if repair occurs at the same rate in both tissues, it will obviously take longer for 50% of the damage to fade to an undetectable level (3 or 5%) than for 20% to do so. This means that late reactions require longer intervals than early reactions when multiple fraction per day radiotherapy is planned, even if the half-lives of repair are not different.
Fowler
J F
JF
Gray Laboratory, Mount Vernon Hospital, Northwood, England.
D006801:Humans; D011878:Radiotherapy / Q000009:adverse effects / Q000379:methods*; D011879:Radiotherapy Dosage; D013997:Time Factors