Recovery from sub-letahl damage by hypoxic tumour cells in vivo

A method has been developed whereby the survival of tumour cells irradiated in vivo can be measured in terms of their colony forming ability in vitro. Using this technique, survival curves have been constructed for tumours irradiated in various conditions of oxygenation. When tumours were exposed to two equal doses of radiation separated in time, cells were able to recover from their sub-lethal damage. The doses chosen were such that the ratio of surviving cells that were hypoxic at the time of the first dose of radiation to those that were well-oxygenated was at least 20 to 1. A consideration of the shapes of survival curves showed that not only were these hypoxic cells capable of recovery from sub-lethal damage, but they remained hypoxic for the time during which this process took place. The survival curve for cells from tumours exposed to a first dose of 1,000 rads and graded second doses five hours later confirmed this deduction.