Cell-age heterogeneity and deviations from the LQ model in the radiation survival responses of human tumour cells

Purpose: To determine whether some of the deviations from the simple linear-quadratic (LQ) theory in the radiation dose-survival responses of asynchronous cultures of human tumour cell lines are caused by the presence of cell-agespecific subpopulations which all individually follow LQ theory, but have different radiosensitivities. Materials and methods: Human tumour cells were synchronized by mitotic selection and their survival dose responses were measured at doses from 0.05Gy to 12Gy, using a high-precision survival assay. These responses were compared with a kinetic model of radiation survival in synchronized cells, which assumed that age-specific populations individually obeyed the LQ theory. The cell lines used included HT-29 and A549, which have typical dose responses, and U1, which is somewhat atypical. Results: In two of the three cell lines, A549 and HT-29, observed deviations from the LQ model were consistent with those expected from cell-age heterogeneity. In the third cell line, U1, survival r...