Inherent radiosensitivity and split-dose recovery in plateau-phase cultures of 10 human tumour cell lines.

The radiation response of 10 human tumour cell lines, eight of them established in our Institute, was analysed. Single dose acute survival curves were constructed and fitted with the linear-quadratic (LQ) model. The mean inactivation dose (D) was also calculated, together with D(o) and n. In order to measure both split-dose recovery and delayed plating recovery of plateau-phase cells, confluent cultures were subjected to two doses of 2 Gy 24 h apart, and plated 24 h later, simulating clinical fractionation. Survival of these cells (S2 x 2) was compared to that following 4 Gy given to cells plated at low density and an overall recovery factor (RF) was derived, including both types of recovery. S2 x 2 and RF were compared to the intrinsic radiosensitivity parameters. The three melanoma cell lines differed in radiation response, and the three squamous cell carcinoma cell lines were rather radioresistant. The neuroblastoma cell line was highly sensitive, yet it expressed the highest beta and the highest RF. Using a non-parametric correlation test, S2 x 2 was found to be related to D, whereas RF was not related to the radiosensitivity parameters. However, the two cell lines with the lowest D and the lowest S2 expressed the highest RF. These results suggest that radiosensitive cell lines may have a considerable capacity to recover if confluent cultures are exposed to fractionated irradiation. The overall recovery factor (RF) used here is proposed as a useful measure of cellular recovery.