A Fast-growing Benign Metastasizing Leiomyoma Mimicking Malignancy.

We herein report a rare case of fast-growing benign metastasizing leiomyoma. A 52-year-old woman was admitted to our hospital with abnormal chest shadows. Chest computed tomography showed well-circumscribed cystic tumors. Because malignancy could not be completely distinguished in fast-growing tumors, video-assisted thoracic surgery was performed. The pathological findings revealed many cysts and the proliferation of smooth muscle cells. According to the Stanford criteria, the tumor was diagnosed as benign metastasizing leiomyoma. One possible reason for the fast growth of the tumor was enlargement of the cysts. Malignant diseases characterized by cystic tumors are rare but occasionally reported. Therefore, differentiation by a pathological examination is essential.