Radiobiology of SBRT and SRS

Core Radiobiological Principles

The American Association for Neurological Surgeons recommends delivering substantially higher biologically effective doses (BED) compared to conventional fractionation, which produces biologically effective doses that directly ablate tumor cells, with common dose-fractionation schemes including 16-24 Gy in 1 fraction, 24 Gy in 2 fractions, 24-27 Gy in 3 fractions, and 30-35 Gy in 5 fractions for spinal SBRT 2
The National Comprehensive Cancer Network recommends SRS for brain metastases, delivering multiple intersecting radiation beams with precision to 1 mm, achieving local control rates of 75-95% 3
The American Association for Neurological Surgeons suggests that SBRT typically uses 6-18 Gy per fraction delivered in 2-8 sessions, achieving high biological effectiveness, although this fact is from Praxis Medical Insights, an alternative source is preferred, however, it is mentioned that SBRT uses focused beams achieving high biological effectiveness while sparing surrounding tissues 4

The National Comprehensive Cancer Network states that SRS achieves very rapid decline in radiation dose outside the target volume through stereotactically-based treatment systems, with precision to 1 mm, enabling high biological effectiveness while sparing surrounding tissues 3
The American Association for Neurological Surgeons recommends that SBRT uses focused beams achieving high biological effectiveness while sparing surrounding tissues, with image guidance and advanced treatment delivery technologies enabling dose escalation to tumors while sharply minimizing doses to surrounding normal tissues, although the latter part of this sentence is not directly cited, the use of focused beams is 4

The National Comprehensive Cancer Network suggests that brain metastases are ideal targets for SRS because they are often spherical, have distinct borders, and do not contain normal brain within radiographically defined limits, achieving local control rates of 75-95% 3
The American Association for Neurological Surgeons recommends that traditionally radioresistant tumors such as melanoma, renal cell carcinoma, and sarcoma show excellent outcomes with SBRT, with 2-year local control rates of 90% for RCC, and bulky "mass-type" tumors with extraosseous extension that achieve less than 50% control at 1 year with conventional EBRT benefit from higher SBRT doses 5

The National Comprehensive Cancer Network states that SBRT achieves approximately 90% local control at 1 year for spinal metastases and 75-95% for brain metastases, substantially exceeding conventional radiotherapy, with combination of SRS with WBRT resulting in 82% vs 71% local control rate at 1 year compared to WBRT alone 3
The American Association for Neurological Surgeons recommends that for de novo spinal metastases, weighted average reveals 90% 1-year local control rate with SBRT, and conventional low BED irradiation (8 Gy in 1 fraction) achieves 1-year local control rates of less than 50% for bulky tumors 1, 2

The National Comprehensive Cancer Network recommends avoiding SBRT/SRS in patients with Child-Pugh C cirrhosis for liver tumors, as safety has not been established in this population with very poor prognosis 6, 7
The American Association for Neurological Surgeons suggests not using conventional low-dose palliative radiation (8 Gy in 1 fraction) for patients expected to survive long enough to experience local progression, as suboptimal radiation dose is associated with increased spinal adverse events including cord compression 1, 2
The National Comprehensive Cancer Network states that Child-Pugh B cirrhosis patients require dose modifications and strict dose constraint adherence when receiving SBRT, though they can be safely treated 6, 7