The potential benefit from a perfect radiosensitizer and its dependence on reoxygenation

The potential benefit of a perfect radiosensitizer has been assessed by computing the sensitization ratios that would be observed in a mixed population of oxic and hypoxic cells if different reoxygenation rates existed. The sensitizer has been assumed to be as effective as oxygen, completely non-toxic and freely diffusible to all hypoxic cells within the tumour. The calculations have been made for several different clinical fractionation regimes, namely 30, 20, 9 or 6 fractions, all with the same ret dose (NSD = 1700 rets). These calculations have allowed us to deduce how large the observed sensitization would be for differing rates of reoxygenation and for the different fractionation schemes. The size of the extrapolation number is seen to be an important parameter in these calculations. They have allowed us to indicate how much reoxygenation would be needed to abolish the benefit from (and hence the need for) a perfect radiosensitizer.