Beyond the uterus: Metastatic leiomyoma.

Uterine leiomyomas are common benign smooth muscle tumors typically confined to the uterus and hormonally responsive to estrogen and progesterone, typically regressing after menopause. Rarely, they demonstrate metastatic behavior despite benign histology, a condition known as benign metastasizing leiomyoma (BML). BML is exceedingly rare and can mimic malignancy, posing diagnostic challenges. We report a rare case of a 53-year-old woman with a history of hysterectomy for leiomyomas who presented with dyspnea and chest pain. Imaging revealed numerous bilateral pulmonary nodules, later confirmed to be smooth muscle tumors. After being lost to follow-up, she returned 2 years later with new breast and forearm masses. Imaging and biopsy confirmed metastatic smooth muscle tumors consistent with BML. Breast lesions mimicked benign cysts on ultrasound, underscoring the importance of clinical and histological correlation. The patient responded well to treatment with symptom improvement. This case highlights the rare phenomenon of BML metastasizing to multiple extrauterine sites, including the breast and soft tissues, locations infrequently reported. Imaging findings, particularly in the breast, can mimic benign lesions such as simple cysts, underscoring the need for biopsy and correlation with clinical history. BML should be considered in women with a history of uterine fibroids and indolent-appearing metastases without fludeoxyglucose uptake on positron emission tomography, computed tomography, and cystic appearance on ultrasound of breast lesions. Early recognition is crucial to avoid misclassification as malignancy and to guide effective hormonal treatment.