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, p = 1.08 g/cm³ and Band 4, p = 1.14 g/cm³) 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, D(q) = 644 rad, n = 5.2; and unseparated cells, D(q) = 344 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.