The Phenomenon of Reoxygenation and Its Implications for Fractionated Radiotherapy

The radiobiological oxygen effect is critical in radiotherapy, as all experimental tumors that have been tested contain large proportions (approximately 1–50%) of severely hypoxic cells, and many human tumors are partially hypoxic on histopathological grounds. In designing fractionated dose regimens, it is essential to recognize the special importance of the reoxygenation of preferentially surviving hypoxic cells. Most animal tumors have been shown to reoxygenate rapidly enough that the hypoxic fraction is restored to the preirradiated level within approximately 24 hours. Different tumors may reoxygenate at different rates. It is presently impossible to identify the mechanism(s) responsible for reoxygenation.