Benign Metastasizing Leiomyoma Presenting as a Solitary Pulmonary Metastasis.

A 52-year-old female patient was initially evaluated at another hospital for a splenic tumor. Chest computed tomography revealed a 12-mm nodule with well-defined margins and uniform internal density in the S5 segment of the right middle lobe. She was referred to our hospital for follow-up of a suspected benign lung tumor. After 9 months of outpatient observation, the nodule increased in size to 16 mm, prompting referral to our department. Thoracoscopic partial resection of the right lung was performed for both diagnostic and therapeutic purposes. Histopathological examination showed a tumor composed of densely packed spindle-shaped cells with eosinophilic cytoplasm and minimal atypia. The tumor was positive for estrogen receptor and progesterone receptor. Considering the patient' s history of uterine myoma, these findings led to a diagnosis of benign metastasizing leiomyoma (BML). BML is a rare condition in which benign uterine leiomyomas metastasize to the lungs. It should be considered in the differential diagnosis of women with a history of uterine myoma. Although BML typically presents with multiple pulmonary lesions, solitary lesions are rare. The present case, with a single lesion, posed a challenge in preoperative diagnosis. Therefore, we report this case along with a literature review.