Stem-cell survival and tumor control in the Lewis lung carcinoma.

The stem-cell response of the Lewis lung carcinoma to single doses of cyclophosphamide has been studied by three assay techniques: in vitro colony formation, lung colony formation, and the end-point dilution assay. These three techniques have given comparable results, and the end-point dilution results showed that 50 percent takes could be achieved with as few as 1 to 3 cells. Studies have been made of the growth of small i.m. implants and of the time following implantation at which they could be eradicated by cyclophosphamide. The results were compared with the curability that would be expected on the basis of cell survival studies. It was found that older (and therefore larger) implants were cured than might have been expected. It seems unlikely that this discrepancy was due to additional cell kill caused by an immune response. An alternative explanation, that the surviving fraction following a dose of cyclophosphamide was lower in small implants than in larger i.m. tumors, was supported by studies of cell survival in dissectable lung colonies.