Fetal deciduous placental tissues were obtained from a healthy mother after informed consent. The pMSCs were isolated and expanded according to a previously reported protocol,9 (link) and these were cultured in low-glucose Dulbecco’s Modified Eagle’s Medium (DMEM; Thermo Fisher Scientific, Waltham, MA, USA) containing 10% fetal bovine serum (Thermo Fisher Scientific), penicillin (100 U/mL), and streptomycin (100 μg/mL). Purified pMSCs at passages 8–10 and grown to 85%–90% confluency (Figure S1A and B) were subjected to 80, 160, 240, and 320 Gy irradiation using RS-2000 biological irradiator (RadSource), then stored frozen at −80°C for the entire series of experiments. Lewis lung carcinoma (LL2) cell line was obtained from American Type Culture Collection (Manassas, VA, USA) and cultured in DMEM/F12 integrative medium (Thermo Fisher Scientific) for spheroid formation with stem cell-like transition. The spheroid-bearing LL2 cells at logarithmic phase were harvested and irradiated for the experiments (Figure S2A–E).