The Intrinsic /β Ratio for Human Tumour Cells: Is It a Constant?

The radiation response of 15 mammalian cell lines comprising 11 human tumour, two human fibroblast and two murine lymphoma cell lines, has been analysed using the linear-quadratic equation. As well as using conventional analysis of acute dose-survival curves to derive values for and β (termed ac and βac), low dose-rate and split-dose experiments have been used to derive independent values of and β (ldr and βRR), respectively. ldr provides a measure of irrecoverable damage, the magnitude of which agreed well with the initial slope of the acute survival curve for most cell lines. βRR derived from split-dose experiments represents a unique measure of recovery for each cell line. Large differences were found between individual values of βac and βRR, especially in the radiosensitive cell lines. Since βRR is a functional measure of recovery we suggest that this is the more relevant parameter in studies of dose sparing. The most striking result of this analysis was found in considering the /β ratios. No relationship was observed between ac and βac resulting in values of ac/βac ranging from 1 to 175. In contrast a positive correlation was observed between ldr and βRR in the 11 tumour cell lines, giving an /β ratio of 9·4 ± 1·8 Gy. This observation of the relative constancy of the ratio for human tumour cells leads to an hypothesis about the role of initial damage as a determinant of radiosensitivity