肺転移性平滑筋腫と同時発生の乳房腫瘍：診断上の課題

Pulmonary benign metastasizing leiomyoma (PBML) is a rare condition in which histologically benign smooth muscle tumors occur in distant organs, most often the lungs. Although typically discovered incidentally, differentiating BML from metastatic malignancy can be challenging in real-world clinical settings. A 53-year-old woman with a history of hysterectomy for uterine leiomyoma presented with multiple pulmonary nodules and a cystic left breast lesion containing an irregular solid component. Fluorine-18 fluorodeoxyglucose positron emission tomography/computed tomography demonstrated no significant fluorodeoxyglucose (FDG) uptake in the pulmonary nodules but intense uptake in the cystic lesion of the left breast. Excisional biopsy of the breast revealed an intraductal papilloma, while video-assisted thoracoscopic biopsy of the lung nodules demonstrated benign smooth muscle proliferation positive for α-smooth muscle actin and estrogen receptor (ER), with a very low Ki-67 index, findings consistent with BML. This case illustrates the diagnostic challenge of distinguishing BML from metastatic malignancy in patients with concurrent lesions. Awareness of this entity and its characteristic low FDG uptake may help clinicians avoid unnecessary overtreatment and adopt an appropriate diagnostic strategy.