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Yu Jin,1 Yalin Kang,1 Xiaohong Peng,1 Li Yang,2 Qianxia Li,1 Qi Mei,1 Xinyi Chen,1 Guangyuan Hu,1 Yang Tang,1 Xianglin Yuan1 1Department of Oncology, Tongji Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, Hubei Province, People’s Republic of China; 2Department of Hematology, Tongji Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, Hubei Province, People’s Republic of ChinaCorrespondence: Yang Tang; Xianglin YuanDepartment of Oncology, Tongji Hospital, Tongji Medical College, Huazhong University of Science and Technology, 1095 Jiefang Avenue, Wuhan, Hubei Province, ChinaTel +86-27-83663342Email tangyangtjmu@126.com; yuanxianglin@hust.edu.cnBackground and Purpose: Brain metastasis is among the leading causes of death in patients with non-small-cell lung cancer (NSCLC). Through yet unknown mechanisms, prophylactic cranial irradiation (PCI) can significantly decrease the incidence of brain metastases. Given that PCI probably exerts indirect anti-tumoral effects by turning cerebral “soil” unfavorable for the colonization of metastatic tumor “seeds”. This study aims to reveal how PCI regulates the brain microenvironment conducing to a reduction in brain metastases.Materials and Methods: Key markers of M1/M2 microglia types and mesenchymal-to-epithelial transition (MET) were analyzed by qRT-PCR and Western Blot in vitro. The target miR-9 was obtained by miRNA array analysis and confirmed by qRT-PCR in microglia. We used miRTarBase and TargetScan to analyze the target genes of miR-9 and confirmed by luciferase activity assay. Anti-metastatic effects of irradiation on the brain were evaluated by intravital imaging using a brain metastatic A549-F3 cell line in a nude mouse model.Results: Irradiation induced M1 microglia activation, which inhibited the MET process of A549 cell lines. Furthermore, levels of miR-9 secreted by irradiated M1 mic