Comparison between in vitro radiosensitivity and in vivo radioresponse in murine tumor cell lines. II: In vivo radioresponse following fractionated treatment and in vitro/in vivo correlations.

Survival in the low-dose region of in vitro radiation survival curves for human tumor cell lines may be correlated with the expected clinical radiocurability of tumors of similar histopathological type. The present investigation examined this hypothesis using a series of transplantable murine solid tumors. The in vivo radioresponse of the tumors following fractionated dose (10 fractions of 2 Gy given at 4 hr intervals) and single dose (20 Gy) radiation treatment was measured by growth delay and tumor cell survival assays. The measurements of tumor cell survival were initiated either immediately or 8 hr after the end of radiation treatment. There was no evidence for repair of potentially lethal damage in vivo in any of the tumors. The measurements of in vivo response were compared to parameters of in vitro radiation survival curves for the same tumor cell lines, which were measured concurrently. The tumor cell survival following 10 fractions of 2 Gy correlated best with the measured in vitro survival at 2 Gy, although good correlations were also observed with two parameters calculated from the in vitro data; the value of alpha from fitting the linear-quadratic (LQ) model and the mean inactivation dose (MID). Correlations were also observed between specific growth delay measured in vivo following 10 fractions of 2 Gy and these in vitro parameters. Despite the correlations observed, the measured survival values following 10 fractions of 2 Gy in vivo did not agree quantitatively with the theoretical values predicted from the in vitro survival data assuming equal effect for each fraction. This discrepancy indicates that other factors also contribute to the overall response of a tumor to fractionated irradiation. Nevertheless, these findings support the idea that intrinsic radiosensitivity plays a significant role in determining the overall response of a tumor to fractionated irradiation and provides strong support for the concept of testing the in vitro radiation sensitivity of biopsy specimens as a predictive assay of clinical radiocurability.