RADIATION-INDUCED NEOPLASTIC TRANSFORMATION OF STATIONARY-PHASE C3H 10T1/2 CELLS IN RESPONSE TO DIFFERENT DOSE-RATES AND TO POSTIRRADIATIONTREATMENT WITH TUMOR PROMOTER/

The induction of neoplastic transformation by exposure to high (HDR, 0 .66 Gy/min) or very low (LDR, 4.8 x 10(-4) Gy/min) dose rates of Cs-13 7 gamma-rays was studied in C3W/10T1/2 mouse embryo fibroblasts. Cells in stationary phase were exposed in the dose range 1-6 Gy in combinat ion with a post-irradiation treatment with the tumor promoter, 12-O-te tradecanoylphorbol-13-acetate (TPA). The post-irradiation treatment wi th TPA during 6 weeks of transformation assay did not induce any notab le increase in the slope of the dose-response curves for transformatio n frequency, compared to the conditions without TPA treatment. The lac k of an enhancing TPA effect at both dose rates applied in this study may be related to the fact that the cells were irradiated in the stati onary growth phase. Thus, the results differ from those generally obta ined when exponentially growing cells are exposed to gamma-rays and af terwards treated with TPA in the transformation assay. Earlier studies of exponentially growing C3H/10T1/2 cells exposed to different dose r ates show a significantly higher transformation frequency for high dos e rate. This study, using stationary phase cells, also shows that the slopes of dose response curves for transformed foci were somewhat high er (about 1.5-fold) for HDR exposure compared with LDR exposure. Howev er, the difference was not statistically significant.