Biological Factors Influencing Optimum Fractionation in Radiation Therapy

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Abstract
Optimum fractionation in radiotherapy occurs when tumor control is improved without enhancement of complications. The main influence on choice of overall time, total dose and fraction size is biological: the proliferation status of tumors. For rapidly proliferating tumors, shorter schedules than 6 to 8 weeks are necessary. Optimum overall time is similar to Tk, the time after beginning cytotoxic treatment when rapid proliferation in tumors starts: 21 to 35 days in head and neck tumors. These, and non-small cell lung tumors, have a clonogenic cell doubling time during radiotherapy of about 3 days. New developments in designing optimum schedules for such tumors are presented: carefully regulated hypofractionation (CRH). For slowly proliferating tumors, especially prostate adenocarcinoma, intracellular repair is large, so larger doses per fraction will be necessary. New evidence is presented showing that their f g ratio may indeed be lower than 3 Gy. For an entirely different reason from that above, hypofractionation should be tested.

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