Bizarre soft tissue tumour on the gluteus muscle: a case report of spontaneous chronic expanding haematoma.

Chronic expanding haematoma (CEH) is rare and refers to a gradually increasing haematoma that is not absorbed after surgery and trauma. This report highlights unusual mass occurring on the gluteus muscle, and the aim is to indicate the diagnostic method. It is necessary to consider the mechanism of the occurrence and to know the characters of CEH. The patient was a 51-year-old man who had noticed a soft mass on his right hip. The mass had gradually increased to 10 cm in size over the year. CT images revealed a haematoma. However, MRI showed a rare biphasic fluid-fluid layer inside the mass and indicated a different pattern compared with that of a normal haematoma. Because the mass was affecting the patient's social life, and the diagnosis was difficult to confirm, surgical treatment was elected. Intraoperatively, the mass contained a large amount of a brown mud-like substance and showed the bizarre appearance inside. The mass was diagnosed as CEH based on both the clinical findings and the histopathological diagnosis. The patient had no traumatic event and no previous surgery. In the absence of the clinical history and the unique imaging findings, it was difficult to diagnose the mass as CEH. It is important to clarify a patient's underlying disease, history, and lifestyle and to consider any correlation between the mass location and the patient's condition carefully. Considering the character of the mass and the lack of a preoperative definitive diagnosis, we recommend performing complete surgical resection.