Abstract: Objective To investigate the clinicopathological features, immunophenotype, pathogenesis, differentiation and management of pulmonary benign metastasizing leiomyoma(PBML). Methods The medical records and specimen data of 7 cases of PBML treated in the Department of Pathology, The First Affiliated Hospital of Soochow University during Aug. 2015 and Jan. 2022 were collected. All patients were female, aged 35 to 62 years(median 53 years), and all had a history of uterine leiomyoma surgery. The diagnosis was confirmed with chest CT, histopathology and immunohistochemistry. The imaging, pathological features and immunophenotypes were analyzed, and relevant literature was collected. Results CT images showed single or multiple quasi-circular hyperdense lesions of varying sizes. PBML was morphologically analogous to uterine leiomyoma. Immunohistochemically, all 7 cases were diffusely and strongly positive for Desmin and SMA. All 7 cases were ER positive, 4 cases were PR positive; 1 case had Ki-67 proliferation index of 10%, the other 6 had 1%-5%. All cases were negative for S-100, CD117, CD34 and HMB45. The diagnosis of this disease required a combination of history, imaging, pathology, and immunohistochemistry, and needed to be differentiated from leiomyosarcoma, fibroleiomyomatous hamartoma, inflammatory myofibroblastic tumor, and solitary fibrous tumor. The prognosis of PBML was generally favorable. Conclusion If CT images show single or multiple quasi-circular hyperdense lesions of varying sizes and the patient has a history of uterine fibroids or surgery, the possibility of PBML should be considered. Surgical resection is preferred for treatment.

Key words: Benign metastasizing leiomyoma, Rare pulmonary disease, Pathological features, Immunohistochemistry, Wedge resection of lung