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[Radiobiological basis for hyperfractionated radiation therapy].

Hyperfractionated radiation therapy has been established as a radical treatment for head and neck cancer. The radiobiological hypothesis for hyperfractionation has been derived from that of fractionated radiation therapy, and it enables us to improve therapeutic gain by delivering a small fraction dose at optimal interfraction intervals, i. e., 1) reduction of normal tissue complication while maintaining tumor control probability, 2)improvement of tumor control probability while maintaining the incidence of normal tissue complications within an acceptable level. The understanding of radiobiological hypothesis or theory is important in clinical application of hyperfractionation.