The initial part of the survival curve: does it predict the outcome of fractionated radiotherapy?

The publication of a number of single-cell survival curves in vitro has stimulated radiobiologists and radiotherapists to analyze the survival characteristics of these curves for their ability to be predictive of the radioresponses of the tumors from which they were derived. Parameters of interest have been the steepness of the initial slope, the single-dose survival at 2 Gy, the mean inactivation dose, and the extrapolation number along with the D0 dose. An assessment of these correlations shows considerable overlap between the values of particular survival parameters even when tumors thought to be the most responsive are compared to those thought to be the least responsive. The importance of the full repair of sublethal damage in the analysis is noted, and a number of factors which may limit effective correlations between cell survival parameters and tumor response are discussed.