Radiobiology of Stereotactic Radiosurgery and Stereotactic Body Radiation Therapy

Abstract

In recent years, increasing number of cancer patients are treated with stereotactic radiosurgery (SRS) or stereotactic body radiation therapy (SBRT), which deliver hypofractionated irradia- tion with high-dose per fraction . It is highly likely that the radiobiological principles such as 4 Rs (Reoxygenation, Repair, Redistribution, Repopu- lation) for the conventional fractionated radiother- apy with small-dose per fractions do not apply for SRS and SBRT. Reoxygenation: When tumors are exposed to high-dose per fraction, e.g. [10 Gy, significant vascular damage will occur. Conse- quently, intratumor environment becomes hypoxic and acidic, which not only will prevent reoxygen- ation of hypoxic cells but also will cause indirect cell death. Repair: delivery of SRS or SBRT lasts considerable lengths of time, which may allow repair of sub-lethal radiation damage during the irradiation exposure. Redistribution: high-dose irradiation prevents cell cycle progression and cells undergo interphase death in the cell cycle phases where they are irradiated. Repopulation: Since SRS or SBRT treatment is completed within 1-2 weeks, repopulation of tumor cells during the course of treatment may be negligible. The linear- quadratic (LQ) model , which is used to calculate isoeffect doses for different hyperfractionated irradiation schemes, may be applied for hypofrac- tionated SRS or SBRT, provided that indirect cell death due to vascular damage is negligible.