*2.4. Colony Formation*

For the colony formation assay, MCF-7 cells (1000 cells/well) were grown on 6-well plates and maintained in a humidified chamber comprising 95% air and 5% CO<sup>2</sup> at 37 ◦C for 14 days. Cells were then fixed with 3.7% paraformaldehyde at room temperature for 10 min and stained using crystal violet solution (0.2% crystal violet and 1X PBS) at room temperature for 30 min. Stained cells were washed with 1× PBS and air-dried at room temperature. The numbers of colonies were quantified using the Image J program (versionv1.53e). The survival fraction was calculated as the number of colonies counted/the number of cells inoculated × plating efficiency at 0 Gy. Colonies consisting of 50 or more cells were counted as clonogenic survivors.
