Fractionation And Late Effects

Fraction size has a major impact on the occurrence of late effects. This is because many of the cells which develop late effects have low alpha/beta values - that is, they are able to resist radiation effectively at low doses but become overwhelmed rapidly at higher doses. This can be exploited by delivering the dose in small fractions that only deliver dose that the late tissues can tolerate. This feature is uncommon in early responding tissues or tumours, which have more dependence on total dose due to a high alpha/beta ratio.

In summary, fractionation allows for a difference in cell kill to be seen between early and late responding tissues, as well as tumours. This difference is one of the primary reasons for fractionating radiotherapy - to spare late tissues from toxicity following a course of radiation.