Benign metastasizing leiomyoma of the lung with malignant transformation mimicking mediastinal tumor.

We report a rare case of benign metastasizing leiomyomas (BMLs) showing malignant transformation in one of them. A 65-year-old woman had a fever. Computed tomography (CT) revealed a mass on the left side of the anterior mediastinum and multiple nodules in bilateral lungs. The masses proved to be leiomyomas and a leiomyosarcoma. The latter showed heterogeneous contrast enhancement on CT and high heterogeneous uptake of (18)F-fluorodeoxyglucose (FDG) on positron emission tomography (PET)-CT. This case suggested the potential usefulness of PET-CT for differentiating these entities.