Changes in the repair ability of continuously irradiated lymphosarcoma LS/BL cells.

The radiosensitivity and repair ability of LS/BL lymphosarcoma cells were studied by means of a serial dilution method using host survival-time as the end point and by means of liver colony assay. The survival curves of LS/BL cells originally showed an exponential dose-response relationship without any shoulder. During the years of cultivation in vivo and progressive adaptation to ascites growth the survival curves of LS/BL cells became less steep and the D0 values increased from 1.0 to 1.5 Gy. The continuous irradiation in vivo of LS/BL cells at the exposure rate of 4.7 cGy h-1 did not lead to any remarkable change of the D0 values, but from the 150th week of irradiation the initial shoulder appeared on the survival curve and its width enlarged with the time of irradiation. The extrapolation number (n) equalled 1.0 at the onset of continuous irradiation and increased to 8.4 after 350 weeks of exposure. The results of experiments in which the split-dose technique was used proved the reappearance of the initial shoulder.