Optimizing dose and fractionation for stereotactic body radiation therapy. Normal tissue and tumor control effects with large dose per fraction.

Stereotactic body radiation therapy (SBRT) is a potent noninvasive means of administering high-dose radiation to demarcated tumor deposits in extracranial locations. The treatments use image guidance and related treatment delivery technology for the purpose of escalating the radiation dose to the tumor itself with as little radiation dose to the surrounding normal tissue as possible. The local tumor control for SBRT has been higher than anything previously published for radiotherapy in treating typical carcinomas. In addition, the pattern, timing and severity of toxicity have been very different than what was seen with conventional radiotherapy. In this review, the clinical characteristics and outcomes of SBRT are presented in the context of their underlying mechanisms. While much of the material is unproven and speculative, it at least qualitatively searches for understanding as to the biological basis for the observed clinical effects. Hopefully, it will serve as a motivation for more sophisticated biological research into the effects of SBRT.