[Metastasizing Leiomyoma of the Lung Detected on Chest X-ray after Surgery for Breast Cancer；Report of a Case].

A 60-year-old postmenopausal woman presented with a lung nodule, which was detected on a chest X-ray 2 years after surgery for breast cancer. Pulmonary metastasis from the breast cancer was suspected and surgical resection was performed. On histopathological examination, the lung nodule showed a smooth muscle cell tumor, and immunohistochemical staining was positive for estrogen and progesterone receptors. As a mass in the uterine was detected by computed tomography images before surgical resection, benign metastasizing leiomyoma of the lung was suspected. Subsequently, the patient underwent hysterectomy and bilateral salpingo-oophorectomy. The pathological findings revealed that the mass in the uterine was atypical leiomyoma, which was consistent with the primary lesion of the lung metastasis. Atypical leiomyoma is classified as a benign tumor, but in this case, careful follow-up is required because of its clinical course and histological features.