Comparison of Cell-multiplication and Colony-formation as Criteria for Radiation Damage in Cells Grown in Vitro

Summary

Mouse L cells growing in vitro in log phase were irradiated with either ⁶⁰Co γ-rays or 220-kvp x-rays, and the inhibition of both cellular multiplication (growth) and colony-formation were studied. Growth-inhibition is greatly dependent upon the length of time after irradiation that it is measured; however, the suppression of colony-formation is relatively independent of the time of scoring. Growth-inhibition measured at 3–4 days after irradiation showed less radiation effect than did the suppression of colony-formation; whereas, growth-inhibition measured at 9–10 days after irradiation showed a greater radiation effect, even after correction for the initial mitotic lag. The greater radiation effect exhibited by growth-inhibition was present because the colonies that developed from cells surviving irradiation grew at a reduced rate. This decrease in the proliferation rate may be attributed either to an increase in the time required for the irradiated cells to complete their cell-cycle or to some of the descendants of the surviving cells failing to divide.