Non-linearity of colony formation by human tumour cells from biopsy samples

The relationship between colony numbers and concentration of cells plated is an important parameter of clonogenic assay systems. The cloning efficiency for an ideal sample should be independent of cell concentration, thus giving a straight line through the origin when colony numbers are plotted against cell concentration. A simple statistical method has been developed to test if this is the case for individual tumour samples. Colony data from 51 freshly obtained tumour samples, which had sufficient cells to plate 3 or more dilutions and gave at least 20 colonies per plate at one or more of the dilutions, were tested. The results indicated that colony formation was linear for 27 (53%) of the samples. The remaining 24 samples could be classified into 2 groups: type I, in which cloning efficiencies increased with increasing cell concentration and type II, which had reduced cloning efficiencies at high cell concentrations. Fifteen (29%) of the samples had type I non-linearity and 9 (18%) exhibited non-linearity of type II. These findings indicate that the relationship between colonies and cells plated should be examined for each biopsy sample particularly in each experiment where the effects of cytotoxic drugs are tested.