Yinglai Huang,1,2,* Lin Pan,3,* Khalil Helou,2 Qisheng Xia,3 Toshima Z Parris,2 Hongyan Li,3 Bo Xu,3 Hon Li3 1Division of Breast and Endocrine Surgery, Department of Surgery, Borås Hospital, Borås, 2Department of Oncology, Institute of Clinical Sciences, Sahlgrenska Cancer Center, Sahlgrenska Academy at University of Gothenburg, Gothenburg, Sweden; 3Department of Biochemistry and Molecular Biology, China-Japan Friendship Hospital, Beijing, People’s Republic of China *These authors contributed equally to this work Background/purpose: The aim of this study was to test the hypothesis that mechanical ventilation (MV) during cancer surgery induces lung stroma/tissue milieu changes, creating a favorable microenvironment for postoperative lung metastatic tumor establishment.Materials and methods: In Protocol A, female BALB/c mice were divided into an MV group and a control (no MV) group, both of which were anesthetized and subjected to intravenous injection of green fluorescent protein (GFP)-labeled mouse mammary carcinoma cell line (4T1) cells. After 24 h, the lung tissue was removed and the number of GFP-labeled 4T1 cells was calculated. In Protocol B, the clinically relevant mouse model of spontaneous breast cancer lung metastasis was used with surgical resection of the primary tumor to investigate the MV event that dictates postoperative lung metastasis. Female BALB/c mice were inoculated in the mammary fat pad with 4T1 cells. After 14-d growth, mice were anesthetized and divided into an MV group and a control (no MV) group during surgical procedures (mastectomy). Metastatic tumor burden was assessed two weeks after mastectomy by both macroscopic metastatic nodule count, hematoxylin–eosin histology, immunohistochemistry for the macrophage marker (CD68), and epithelial cell adhesion molecule (EpCAM).Results: MV was associated with a significant increase in the number of circulating breast tumor cells (GFP-labeled 4T1 cells) remaining in the microvasculature of the lung (P