Benign metastasizing leiomyoma of the uterus: histologic and immunohistochemical characterization of primary and metastatic lesions.

Benign uterine leiomyoma metastasizing to the lung is a recognized entity that has been reported infrequently in the medical literature. There is persisting controversy regarding the pathogenesis and biology of these lesions. We report a well-studied and well-characterized case of benign leiomyoma metastasizing to the lung. The patient was a 72-year-old woman with an enlarged uterus that contained several leiomyomas with usual histology. Areas of fibrosis, hyalinization, edema, and focal infarction together with small foci with mildly increased cellularity and minimal nuclear pleomorphism were seen. Careful and repeated mitotic counts ranged from 0 to 2 mitoses per 10 high-power fields. In summary, based on histopathologic criteria, the neoplasm was determined to be a focally cellular benign leiomyoma. Four years later, the patient underwent surgical resection of a single nodule in the lung, which had been detected on routine radiographs. Histopathologic evaluation showed a low-grade leiomyosarcoma with moderate nuclear pleomorphism, necrosis, and brisk mitotic activity. Immunohistochemical studies performed on both neoplasms showed them to be of mesenchymal derivation with smooth muscle differentiation. Both neoplasms expressed estrogen receptors with moderate to strong intensity. The patient received no further treatment and, to date, shows no evidence of recurrent disease. The diagnosis of benign metastasizing leiomyoma can only be made with certainty after careful and extensive sampling of the primary tumor to exclude small foci of sarcoma and of the pulmonary tumor to rule out a primary neoplasm. Although it is biologically peculiar, benign metastasizing leiomyoma should continue to be recognized as a distinct entity because current morphologic criteria do not allow primary myometrial tumors to be reclassified as leiomyomas of uncertain malignant potential even if they have metastasized to the lung.