Radiobiology for IMRT

Abstract

Since the dose delivery pattern is different between intensity-modulated radiotherapy (IMRT) and conventional radiation, radiobiological assessment of the physical dose delivered by IMRT is considered necessary. In IMRT, the daily dose is usually given intermittently over a time longer than that used in conventional radiotherapy. During prolonged radiation delivery, sublethal damage repair takes place, leading to the decreased effect of radiation. This phenomenon is universarily observed in vitro. In in vivo tumors, however, this decrease in effect may be counterbalanced by rapid reoxygenation; we demonstrated it in murine tumors. Studies on reoxygenation in human tumors are warranted to better evaluate the influence of prolonged radiation delivery. Another issue related to high-dose-per-fraction intensity-modulated stereotactic radiotherapy is the mathematical model for dose evaluation and conversion. Many clinicians use the linear–quadratic (LQ) model and biologically effective dose (BED) to estimate the effects of various radiation schedules, but it has been suggested that the LQ model is not applicable to high doses per fraction. Recent experimental studies verified the inadequacy of the LQ model in converting hypofractionated doses into single doses. The LQ model overestimates the effect of high fractional doses of radiation. BED is particularly incorrect when it is used for tumor responses in vivo, since it does not take reoxygenation into account. For normal tissue responses, improved models have been proposed, but, for in vivo tumor responses, the currently available models are not satisfactory, and better ones should be proposed in future studies.