A 58-year-old woman was referred to our institution for investigation of pulmonary nodules identified on external chest radiograph performed at the time of a road traffic accident.
Repeated chest radiograph demonstrated unchanged multiple, bilateral pulmonary nodules, the largest in the right upper zone. A contrast-enhanced CT of the thorax, abdomen and pelvis was performed, which demonstrated multiple, bilateral, well-circumscribed, rounded pulmonary nodules, measuring up to 1.5 cm. A well-circumscribed, rounded, enhancing 2.3 cm mass was also noted in the posterior uterine myometrium. The patient underwent PET/CT, which demonstrated bilateral pulmonary nodules, at least one of which showed increased radiotracer uptake and an FDG-avid nodule in the posterior fundal myometrium.
The combination of CT TAP and PET/CT findings was suspicious for a primary endometrial tumour with haematogenous metastases to lung. A gynaecology opinion was recommended and the patient underwent hysteroscopy and curettage, which revealed multiple benign post-menopausal polyps only, with no evidence of hyperplasia, dysplasia or malignancy. Following multidisciplinary discussion, transbronchial biopsy of one of the pulmonary nodules was attempted but was non-diagnostic. Percutaneous CT-guided lung biopsy was subsequently performed, the histology of which revealed a bland smooth muscle tumour, suggestive of metastasising leiomyoma. The patient subsequently underwent total hysterectomy and bilateral salpingo-oophorectomy. The surgical specimen demonstrated multiple benign leiomyomata. No further intervention was required. Follow-up imaging demonstrates slow interval growth of the pulmonary lesions. The patient remains asymptomatic, 5 years later.
Benign metastasising leiomyoma (BML) is a rare condition, with only approximately 214 published research articles relating to the subject.  It was first described by Steiner in 1939.  It refers to extra-uterine metastases of benign smooth muscle tumours. As in our case, the lungs are the most frequent site of metastases.  BML is most common in asymptomatic, perimenopausal women , like our patient. They usually have a history of uterine fibroids. Imaging typically demonstrates well-circumscribed, non-calcified, non-enhancing pulmonary nodule(s), ranging in size from a few millimetres to a few centimetres. Endobronchial and pleural sparing are typical. Rarely, miliary/cavitating nodules, cysts and interstitial disease have been described.  The accumulation of 18-FDG in pulmonary BML varies significantly, with tumours more likely to be non-avid than FDG-avid.  Histopathology confirms the presence of smooth muscle cells with low mitotic activity, limited vascularisation, no anaplasia/necrosis and positive immunohistochemical staining for smooth muscle markers, i.e. actin, desmin and hormonal receptors, i.e. oestrogen/progesterone.  Although hormone-dependent, BML has a favourable long-term prognosis. Targeted hormonal therapies and/or surgery can be used in disease management, with disease control or regression in 79% of patients treated with hormonal manipulation. 
BML, although rare, should be considered in the differential diagnosis for women presenting with multiple pulmonary nodules, especially those with a history of uterine fibroids.
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