Introduction: Chronic expanding hematoma is a rare persistent hematoma that can sometimes be misdiagnosed as a malignant tumor due to its clinical and radiological features. Case presentation: A 42-year-old Japanese man with a large mass in his leg, suggestive of malignancy, presented to our hospital. He had been aware of the leg swelling for the last eight years. A magnetic resonance imaging scan demonstrated a large mass with two components. One was a large, well-defined cystic mass (13×9cm) showing high intensity on T1-and T2-weighted images, and the other was a solid mass (3.5×2.5cm, adjacent to the large mass) with high intensity on T1-weighted images. Two-[18F]fluoro-2 deoxy-D glucose positron emission tomography images revealed increased uptake with a maximum standardized uptake value of 15.8 in the solid mass. As these findings were considered suggestive of hematoma associated with a malignant lesion, an open biopsy was performed. A pathological examination demonstrated a hematoma with xanthogranuloma, and no malignant cells were evident. Therefore, we resected the tumor including both components, and the histological diagnosis was chronic expanding hematoma. Clinical diagnosis based on 2-[18F]fluoro-2 deoxy-D glucose uptake is sometimes limited by the fact that 2-[18F]fluoro-2 deoxy-D glucose is taken up by not only malignant tumor cells but also macrophages and tissues with granulation or inflammation. Conclusions: Significantly increased standardized uptake value in the peripheral rim of the lesion on 2-[18F]fluoro-2 deoxy-D glucose positron emission tomography imaging, mimicking a soft tissue sarcoma, should be recognized as a potential diagnostic pitfall in cases of chronic expanding hematoma.
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