Time Factor in External Radiotherapy: Radiobiological Mechanisms and Clinical Applications

This review article explores the critical role of time in external radiotherapy, focusing on the concepts of fractionation and spreading. It traces the evolution of radiobiology, highlighting key milestones that have shaped current treatment strategies. The article delves into the four fundamental principles of radiobiology—Repair, Redistribution, Reoxygenation, and Repopulation—and their implications for fractionated radiation therapy. It further discusses the clinical applications of these principles, including hyperfractionated, accelerated, and hypofractionated treatments. This review provides a comprehensive understanding of how the time factor influences the effectiveness of radiotherapy and its impact on healthy and tumor tissues.