Benign metastasizing leiomyoma (BML) is a rare disease that occurs in patients with benign leiomyomatous lesions, predominantly in women with a previous history of uterine leiomyoma [1]. Surgical resection is usually performed for histological diagnostic and/or curative purposes, while endocrine therapy [2] and a wait-and-see strategy [3] are also common approaches to this disease since the clinical course is typically indolent [4]. Clinically, it is important to distinguish between BML and metastatic leiomyosarcoma (LMS) because the therapeutic strategies and prognoses for BML and LMS are quite different; however, the understanding of typical imaging findings of BML is incomplete.

A 44-year-old woman who had undergone a simple hysterectomy for uterine cellular leiomyoma 30 months earlier was found to have asymptomatic multiple pulmonary nodules, according to a chest X-ray. Chest computed tomography (CT) scans showed bilateral multiple pulmonary nodules (Figures 1(a)-(d)). Chest magnetic resonance imaging (MRI) revealed T1-low intensity/T2-high intensity nodules (Figures 2(a) and (b)), and contrastenhanced MRI demonstrated that the nodules were well enhanced (Figures 2(c) and (d)), which suggested that the nodules were blood flow-rich lesions. Interestingly, the nodules did not take up ¹⁸[F]-fluorodeoxyglucose (FDG) during an ¹⁸F-FDG positron emission tomography (PET)-CT scan (Figures 3(a),(b)). The largest 2 nodules were removed for histological diagnosis during videoassisted thoracic surgery, and both nodules were confirmed as pulmonary BML because non-atypical proliferating α-SMA-positive leiomyoma cells were observed without hemorrhage or necrosis (Figures 4(a)-(c)).

Because of its rarity, typical imaging findings for pulmonary BML have not been established, with the exception of CT findings, which include multiple well-defined rounded nodules. Thus, we newly report that the lesions

in the present case appeared as T1-low/T2-high intensity nodules on MRI and as blood flow-rich tumors on contrast-enhanced MRI. A few studies reported ¹⁸F-FDG PET-CT findings in pulmonary BML, and all appeared as avid-mild accumulating nodules [5-8]. In accordance with previous reports, the lesions in the present case did not take up ¹⁸F-FDG, suggesting that BML is a blood flow-rich tumor with low metabolic activity.

The main clinical interest is to distinguish BML from metastatic LMS. Ogawa and his collaborator reported that pulmonary BML with malignant transformation showed high ¹⁸F-FDG uptake with a maximum SUV of 18.8 [9], suggesting that ¹⁸F-FDG PET-CT might be a useful tool to distinguish BML from LMS. On the other hand, it was reported that uterine leiomyoma showed ^99mTc uptake [10] and that pulmonary BML showed high ^99mTc uptake but low ¹⁸F-FDG uptake [8], suggesting that ¹⁸F-FDG PET-CT combined with ^99mTc scintigraphy might be a useful diagnostic option to distinguish BML from other diseases.

In conclusion, we experienced a case of BML. The lesions were well enhanced on contrast-enhanced MRI, while no metabolic activity was indicated on ¹⁸F-FDG PET-CT, suggesting that BML is a blood flow-rich tumor with low metabolic activity.