Lung Colony Assays of Murine Mammary Tumor Cells IrradiatedIn VivoandIn Vitro

C3HBA tumor cells were irradiated either in vitro prior to their intravenous injection or in vivo by whole lung irradiation 24 hours after their intravenous injection into isologous tumor-free C3H∕HeJ female mice. The animals were sacrificed 28 days following treatment, and the number of lung colonies in each treatment group was expressed as the percentage of lung colonies present in untreated (control) mice. Both in vivo and in vitro survival curves had similar values of D0, but had different extrapolation numbers which were 3.4 and 1.0, respectively. Possible underlying causes for the difference in survival curves are discussed.