Altered Fractionation in Radiotherapy

Differences between late-responding (slowly proliferating) normal tissues and early-responding (rapidly proliferating) normal tissues and tumor cells and the event of tumor cell repopulation occurring during treatment have essentially led to the development of altered fractionation schemes. Altered fractionation regimens mainly refer to schedules utilising two or more (small dose) fractions per day for part of or for the entire treatment course. It must be underlined that a true standard or conventional fractionation regimen does not exist: no schedule is universally recognised as the standard of reference to be compared with. However, continental European and U.S. conventional regimens are the considered control arm with which the new experimental regimens have to be compared. For this reason they are generally recognised as the standards. The basic rationale for hyperfractionated or accelerated regimens respectively lies in the possibility (a) to deliver higher total doses reducing late-responding normal tissue damage, (b) to deliver total doses in a reduced overall treatment time to defeat tumor clonogen repopulation. Multiple fractions per day should not be delivered with interfraction intervals smaller than 6 hours. Clinical results of phase I-II and limited but convincing phase III randomised trials suggest that a therapeutic benefit can be achieved with new altered regimens.