Clonogenic survival of seven different cancer cell lines as determined by the conventional PE-based algorithm. For each cell line, geometric series of twelve different cell densities were seeded for each radiation dose anticipating a resulting range of ≤ 15 to an uncountable number of colonies per well (0–8 Gy; three (T47D and HCC1806) to four independent biological replicates (all other cell lines)). a Representative images of untreated plates with the number of single cells seeded in black and the number of colonies obtained in blue (upper part). The PEs derived from all countable wells are given below. p-values were calculated by one-way Kruskal–Wallis ANOVA with the factor "cell number seeded". b Heatmap of clonogenic survival results as calculated from the mean values of counted colonies at all countable cell densities according to the PE-based method. Note that the number of cells seeded varies between cell line and radiation dose. c Reduced range of results from b if only wells with 5 to 100 counted colonies are considered for calculation.