The effect of recovery from potentially lethal damage on the determination of reoxygenation in a murine tumour.

The pattern of reoxygenation in the murine anaplastic MT tumour was investigated using the established method of determining the hypoxic fraction, at intervals after a priming X-ray dose, from test doses given either to unclamped or clamped-off tumours. Little reoxygenation was apparent whilst the tumour was increasing in size for 12--72 hours after a single dose of 20.3 Gy, but extensive reoxygenation was evident whilst the tumour was shrinking at nine days after a dose of 50 Gy. However, the degree of reoxygenation may have been underestimated, especially after the smaller priming dose. This is because only the chronically hypoxic cells in this tumour have the ability to recover from potentially lethal damage (PLD) and so are more radioresistant than cells rendered acutely hypoxic by clamping. Because of this, even when tumours are clamped off during irradiation, the resulting survival curve is biphasic and the apparent effect of the clamp becomes a function of the X-ray dose used. The larger the dose, the smaller the observed effect of the clamp, so the greater the apparent hypoxic fraction and hence the smaller the apparent degree of reoxygenation.