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

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Abstract
A cell separation system involving the centrifugation of mouse 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, 2 selected clonogenic cell populations were separated from a murine fibrosarcoma after exposure in situ to ionizing radiation. The clonogenicity of each population (Band 2, .hivin..rho. = 1.08 g/cm3 and Band 4, .hivin..rho. = 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 (cell sensitivity dose) of this resistant fraction was 459 rad. Comparing the survival curves of Band 4 and unseparated cells, the valudes of D0 were similar (403 and 413 rad, respectively), but the parameters describing the shoulder region of these curves differed significantly (Band 4, Dq (quasi-threshold dose) = 644 rad, n (extrapolation number) = 5.2; and unseparated cells, Dq = 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. The Band 4 population probably consisted primarily of chronically hypoxic cells since conditions of acute hypoxia had little effect on their response to radiation.

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