Growth of fibroblasts as a potential confounding factor in soft agar clonogenic assays for tumour cell radiosensitivity.

Soft agar clonogenic assays are considered to be a standard method for measuring tumour cell radiosensitivity and it has been widely reported that fibroblast contamination does not occur. We report here that human fibroblasts can proliferate to form colonies in a modified form of the Courtenay-Mills soft agar clonogenic assay. It was observed that early passage skin fibroblasts could form colonies in soft agar, although the plating efficiencies were reduced compared with growth on plastic. It was demonstrated that normal lung could proliferate in agar with similar plating efficiencies to fresh tumours and that fibroblastic cells were present in these cultures. Characterisation of primary lung tumour cultures also showed that fibroblastic cells were present in these cultures. Characterisation of primary lung tumour cultures also showed that fibroblastic cells were present which lacked epithelial features and which resembled closely the cells found in cultures of normal lung. This is an important finding for workers using soft agar assays to culture human tumour cells and is of interest in understanding the processes of normal growth control of human fibroblasts.