The tumor radiobiology of SRS and SBRT: are more than the 5 Rs involved?

Review of the radiobiology behind SRS and SBRT. It lists the five Rs — repair, repopulation, redistribution, reoxygenation and intrinsic radiosensitivity — notes that the first four were described by Withers and the fifth added by Steel, and explains that redistribution and reoxygenation increase cell kill over a fractionated course while repair and repopulation increase cell survival. It argues that hypoxia is likely to matter more for SRS/SBRT than for conventional fractionation, and concludes that for most tumors the classic 5 Rs and the linear-quadratic model are sufficient, with the excellent clinical results explained by the much larger BEDs delivered (with the likely exception of enhanced antitumor immunity for some tumors). Read in PubMed Central (author manuscript, PMC3893711).

A clear explanation of the 5 Rs and how they pull in opposite directions — then a look at whether SRS and SBRT need anything more to explain their results.