title: Radiobiology of hypofractionated stereotactic radiotherapy: what are the optimal fractionation schedules?
abstract: In hypofractionated stereotactic radiotherapy (SRT), high doses per fraction are usually used and the dose delivery pattern is different from that of conventional radiation. The daily dose is usually given intermittently over a longer time compared with conventional radiotherapy. During prolonged radiation delivery, sublethal damage repair takes place, leading to the decreased effect of radiation. In in vivo tumors, however, this decrease in effect may be counterbalanced by rapid reoxygenation. Another issue related to hypofractionated SRT is the mathematical model for dose evaluation and conversion. The linear-quadratic (LQ) model and biologically effective dose (BED) have been suggested to be incorrect when used for hypofractionation. The LQ model overestimates the effect of high fractional doses of radiation. BED is particularly incorrect when used for tumor responses in vivo, since it does not take reoxygenation into account. Correction of the errors, estimated at 5-20%, associated with the use of BED is necessary when it is used for SRT. High fractional doses have been reported to exhibit effects against tumor vasculature and enhance host immunity, leading to increased antitumor effects. This may be an interesting topic that should be further investigated. Radioresistance of hypoxic tumor cells is more problematic in hypofractionated SRT, so trials of hypoxia-targeted agents are encouraged in the future. In this review, the radiobiological characteristics of hypofractionated SRT are summarized, and based on the considerations, we would like to recommend 60 Gy in eight fractions delivered three times a week for lung tumors larger than 2 cm in diameter.
authors.affiliation: Department of Radiology, Nagoya City University Graduate School of Medical Sciences, Nagoya 467-8601, Japan yshiba@med.nagoya-cu.ac.jp.
authors.forename: Yuta
authors.initials: Y
authors.lastname: Shibamoto
authors.affiliation: Department of Radiology, Nagoya City University Graduate School of Medical Sciences, Nagoya 467-8601, Japan.
authors.forename: Akifumi
authors.initials: A
authors.lastname: Miyakawa
authors.affiliation: Department of Radiology, Okazaki City Hospital, Okazaki 444-8553, Japan.
authors.forename: Shinya
authors.initials: S
authors.lastname: Otsuka
authors.affiliation: Nagoya Proton Therapy Center, Nagoya City West Medical Center, Nagoya 462-8505, Japan.
authors.forename: Hiromitsu
authors.initials: H
authors.lastname: Iwata
mesh_terms: D004307:Dose-Response Relationship, Radiation; D006801:Humans; D000860:Hypoxia / Q000473:pathology; D000069473:Radiation Dose Hypofractionation; D011853:Radiobiology; D016634:Radiosurgery; D012062:Relative Biological Effectiveness
keywords: LQ model; SBRT; SLDR; reoxygenation, fractionation; stereotactic radiotherapy