Fig. 2

The relevance of hypoxia for conventional fractionated RT and SBRT. In conventional low dose fractionated radiotherapy, the hurdle of tumor hypoxia is overcome by iterative reoxygenation of radiation-resistant hypoxic tumor cells after irradiation and cell killing of oxygenated radiation-sensitive tumor cells (a). In SBRT (b), single high doses of ionizing radiation will effectively kill tumor cells and damage the tumor vasculature, leading to secondary tumor cell death (c) or will induce hypoxia-related resistance mechanisms (d). Radiotherapy-induced toxicities in co-irradiated normal tissues often limit the required dose escalation for both low dose fractionated RT and SBRT to achieve tumor control