Growth and clonogenic assays compared for irradiated MCF-7 and Colo-205 cell lines.

Clonogenic assays have been the golden standard for the assessment of cytotoxic injury from irradiation or drugs. Since such assays are time consuming, growth assays, often with automatic quantifying equipment, are frequently used. Since these procedures do not immediately reflect loss of clonogenic capacity, it was considered important to validate the two procedures using gamma-irradiation (0, 2 and 4 Gy) of two human cell lines (MCF-7 and Colo-205). The cells were growing exponentially in 96-well plates and crystal violet staining resulted in optical densities proportional to cell number. The homogeneity of optical densities within the plates was optimal if the wells to be measured were surrounded by liquid-containing ones. The slopes of the exponential growth curves were decreased upon irradiation. An "apparent cell survival", the mean of the three lowest ratios between irradiated and control cells, was defined. It was compared with the SF2 and SF4 as found in parallel Courtenay-Mills assays. In this work we found a modest underestimation of cell survival using the growth assay, ranging from 0 to 17 per cent in absolute terms.