Initial slope of radiation survival curves is characteristic of the origin of primary and established cultures of human tumor cells and fibroblasts.

The published survival curves of 110 human tumor cell lines and 147 nontransformed human fibroblast strains have been reanalyzed using three different statistical methods: the single hit multitarget model, the linear-quadratic model, and the mean inactivation dose. The 110 tumor cell lines were classified in two ways: (a) into three categories defined by clinical radiocurability criteria, and (b) into seven categories based on histopathology. The 147 fibroblast strains were divided into eight genetic groups. Differences in the radiosensitivities of both the tumor cell and fibroblast groups could be demonstrated only by parameters that describe the slopes of the initial part of the survival curves. The capacity of the survival level to identify significant differences between groups was dose dependent over the range 1 to 6 Gy. This relationship showed a bell-shaped curve with a maximum at 1.5 Gy for the tumor cell lines and 3 Gy for the fibroblasts. Values for intrinsic radiosensitivity for a number of groups of tumors have also been obtained by primary culture of tumor cells. These values are strictly comparable to those obtained by clonogenic methods. This confirms that intrinsic radiosensitivity is a determinant of the response of tumor cells to radiotherapy and suggests that tissue culture methods may be used as a predictive assay.