Stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT)

are rapidly becoming accepted practice for the radiotherapy of certain tumors. Typically SRS and SBRT involve the delivery of one or a few large dose fractions of 8 to 30 Gy per fraction. SRS and SBRT have come about because of technological advances in image guidance and treatment delivery techniques that enable the delivery of large doses to tumors with reduced margins and high gradients outside of the target

thereby minimizing doses to surrounding normal tissues. The impressive results obtained by them have raised the following questions. Are classic radiobiological theory (5“R”) and the linear-quadratic (LQ) model appropriate for large doses per fraction? Could endothelial damage

vascular injury

tumor immunity and hypoxia play extra roles in SRS/SBRT? Here

by analyzing the studies on basic radiobiology

preclinical and clinical data

we suggest that for most tumors the standard radiobiology concepts of the 5R ′s are sufficient to explain the clinical data besides possible anti-tumor immunity in certain tumors

and the excellent results obtained from clinical studies are those from the much larger biologically effective doses (BED) that are delivered with SRS and SBRT.