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

Abstract

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 α and β (termed α_ac and β_ac), low dose-rate and split-dose experiments have been used to derive independent values of α and β (α_ldr and β_RR), respectively. α_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. β_RR derived from split-dose experiments represents a unique measure of recovery for each cell line. Large differences were found between individual values of β_ac and β_RR, especially in the radiosensitive cell lines. Since β_RR 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 α/β ratios. No relationship was observed between α_ac and β_ac resulting in values of α_ac/β_ac ranging from 1 to 175. In contrast a positive correlation was observed between α_ldr and β_RR in the 11 tumour cell lines, giving an α/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.