Fractionation in Radiobiology: Classical Concepts and Recent Developments

Abstract

Well-focused radiation treatments such as the one utilizing stereotactic guidance for both benign and malignant tumors have gained significant popularity over the recent decades. Historically, executing the treatment course over several weeks with daily fraction of relatively low dose (termed conventional or standard fractionation) has been established as the norm for major portion of the past century. This has largely been guided by the established quantitative doctrines of clinical or classical radiation biology. However, a new trend has apparently emerged to deliver significantly fewer fractions of treatment (called hypofractionation) or even single-dose irradiation. That one can now seemingly violate the long-held tenet of fractionation radiobiology results largely from the physical advantage of ultra-precision-oriented technology, made possible by more sophisticated computerized treatment planning. Nevertheless, some new biological insights have been offered by recent investigators with renewed quantitative theories, in order to account for the observed clinical efficacy of hypofractionation. The synopsis below aims to present the shifting development in fractionation practice from the classical radiobiology viewpoints, with the emphasis on the evolution of mathematical modeling so pervasive in the clinical application of the biological principles. Readers interested in a more in-depth coverage of the background information are urged to first browse through the author’s earlier review on the subject, which followed a thread of synthesis of central ideas behind quantitative radiobiology (Lee et al. 2006). In order to maintain a self-sufficient amount of information, however, much of the previously presented exposition is summarized and at times included verbatim here for the reader’s convenience. The main addition to the antecedent discussion is the presentation of recent development of theoretical models designed to quantify more accurately the observed efficacy of hypofractionation and thus justify its clinical practice.