BACKGROUND
Chronic expanding hematoma (CEH) is a benign lesion characterized by slow enlargement over a prolonged period, and it can closely mimic soft tissue tumors. Differentiating CEH from malignant tumors is particularly challenging in patients with a long clinical course.
CASE REPORT
A 66-year-old man presented with a mass in the distal left upper arm that had been present for >10 years. He was referred to the Orthopaedic Department for suspected malignancy. Initial contrast-enhanced magnetic resonance imaging revealed a cystic lesion, and positron emission tomography/computed tomography revealed fluorodeoxyglucose (FDG) uptake in the septal components of the lesion and in the ipsilateral axillary lymph nodes. Incisional biopsy identified only hematoma components, and the lesion was diagnosed as consistent with CEH. Although conservative treatment, including embolization, drainage, and sclerotherapy, was applied, the mass rapidly enlarged 10 months later. Follow-up imaging revealed a markedly enlarged multilocular cystic lesion containing newly developed solid components with markedly increased FDG uptake. Left shoulder disarticulation with axillary lymph node dissection was subsequently performed. Histopathological examination of the resected tissue sample revealed an undifferentiated malignant tumor that could not be distinguished as a sarcoma or a carcinoma.
CONCLUSIONS
This case shows that lesions initially considered consistent with CEH can later reveal an underlying malignant tumor or possible malignant transformation. Careful interpretation of histopathology results, awareness of potential sampling error, and meticulous radiological evaluation with close follow-up are essential.

Keywords: Hematoma; Magnetic Resonance Imaging; Soft Tissue Neoplasms; Positron-Emission Tomography; Sarcoma; Carcinoma