As expected, hematoxylin and eosin (H&E) staining (Figure 5e) and TUNEL (Figure 5f) revealed that the most severe tumor tissue destruction occurred after the tumors were exposed to 125I‐TiO2 rather than to 125I. According to immunohistochemical staining results, the expression of Ki‐67 (a cancer cell proliferation factor) was downregulated in 125I‐TiO2 group compared to that in 125I or control groups (Figure S11, Supporting Information), while the TNF‐α (a tumor necrosis factor) expressed the highest in 125I‐TiO2 group (Figure S12, Supporting Information). This evidence concerns the gene TNF and cancer.