Radiation Response of Cell Populations Irradiated in Situ and Separated from a Fibrosarcoma

A cell separation system involving the centrifugation of cells in linear density gradients of Renografin provides a useful method for the in vivo study of the various clonogenic subpopulations comprising a solid tumor. Following this procedure, two selected clonogenic cell populations were separated from a murine fibrosarcoma after exposure in situ to ionizing radiation. The clonogenicity of each population (Band 2, <tex-math>$\overline{\rho}=1.08\ {\rm g}/{\rm cm}^{3}$</tex-math> and Band 4, <tex-math>$\overline{\rho}=1.14\ {\rm g}/{\rm cm}^{3}$</tex-math>) was determined using a lung colony assay. Survival curves were constructed and compared with data from an unseparated population. The Band 2 population, when irradiated in air-breathing animals, had a relatively large fraction of sensitive cells and a more resistant component whose response was apparent at doses greater than 1000 rad. The D₀ of this resistant fraction was 459 rad. Comparing the survival curves of Band 4 and unseparated cells, the values of D₀ were similar (403 and 413 rad, respectively), but the parameters describing the shoulder region of these curves differed significantly (Band 4, <tex-math>$D_{q}=644$</tex-math> rad, n = 5.2; and unseparated cells, <tex-math>$D_{q}=344$</tex-math> rad, n = 2.3). Under conditions of acute hypoxia, Band 2 cells became significantly more radioresistant. No change in radiation sensitivity was apparent, however, for Band 4 cells. It was concluded that the Band 4 population probably consisted primarily of chronically hypoxic cells since conditions of acute hypoxia had little, if any, effect on their response to radiation.