Biologic basis for altered fractionation schemes.

Conventional is commonly not universally correct, and so with dose fractionation in radiotherapy. Fractionation spares slowly responding tissues more than tissues and tumors that show an early response, suggesting that therapeutic gains may be further increased by reducing fractional doses below 1.8 to 2 Gy. The overall duration of a course of radiotherapy should not be the same for all tumors in all sites because the time of onset of regeneration after the start of radiotherapy varies from tissue to tissue and among tumors. Although growth kinetics and dose-response characteristics are known to vary, inability to identify and quantify them prospectively frustrates rational selection of patients for individualized fractionation regimens. In general, curative radiotherapy should be delivered in as short an overall time as possible using the smallest practical dose per fraction. Although 2 Gy, 5 times per week may be a reasonable "average" treatment, greater individualization should be a research goal.