Abstract： Objective:To investigate the potential role of radiotherapy-induced polyploid tumor cells in triple negative breast cancer(TNBC)recurrence.Methods:The cultured human TNBC cell line MDA-MB-231 was divided into three groups.Two groups of MDA-MB-231 cells were given radiation dose of 7 Gy and 14 Gy respectively under 6MV-X-ray mode.After further incubation for 3 days, the collected cells were divided into the 7 Gy group and 14 Gy group, and the cells without radiotherapy were selected as the control group.Cell morphology was observed under microscope, DNA ploidy was identified by flow cytometry, cell proliferation was detected by 3-(4, 5)-dimethylthiahiazo(-z-y1)-3, 5-diphenytetrazoliumromide(MTT)assay, cell apoptosis was detected by Annexin-V/propidium iodide(PI)double staining flow cytometry, cell migration and invasion were detected by Transwell assay.The expression levels of C-myc, Bcl-xl, Mcl-1, Survivin, β-catenin and Vimentin was detected by Western blot.Results:The cell volume of 7 Gy group and 14 Gy group was significantly increased, and the proportion of polyploid cell subsets(DNA content>4N)was significantly higher than that of the control group.With the increase of radiotherapy dose, the proportion of polyploid cell subsets in 14 Gy group was significantly higher than that in 7 Gy group, with statistically significant differences( P<0.05).Compared with the control group, the proliferation of polyploid MDA-MB-231 cells in 14 Gy group was inhibited( P<0.05).Compared with the control group, the total apoptosis and late apoptosis of polyploid MDA-MB-231 cells in both groups were significantly reduced( P< 0.05).The migration and invasion abilities of polyploid MDA-MB-231 cells in both groups were significantly reduced, and the migration and invasion abilities of polyploid MDA-MB-231 cells in 14 Gy group were weaker than those in 7Gy group, the differences were statistically significant( P<0.05).The expressions of Bcl-xl, Mcl-1 and Survivin were up-regulated and the expressions of C-myc, β-catenin and Vimentin were down-regulated in radiotherapy induced polyploid MDA-MB-231 cells, the differences were statistically significant( P<0.05). Conclusions:The proliferation, migration and invasion of polyploid MDA-MB-231 cells induced by induced radioactivity are reduced, but the apoptosis rate of polyploid tumor cells is decreased and the expression of anti-apoptosis related proteins is up-regulated.These characteristics suggest that polyploid tumor cells may be a potential risk factor for recurrence of triple negative breast cancer after radiotherapy.