The lipoma-like hibernoma: A case report of a rare entity.

Hibernoma is a rare tumor developing from fat cells. It is a slowly evolving, benign tumor that is rarely pain-inducing. The most frequently encountered histological form is the typical hibernoma. The main differential diagnosis is liposarcoma. Here we present a case of a lipoma-like tumor of the arm: a rare variant of hibernoma. A 45-year-old man presents with a swelling of the left arm evolving for one year. Physical examination revealed a mobile, firm, and well-defined mass of the lateral left upper arm measuring 5 cm in length with no cutaneous lesions overlying. MRI and ultrasound confirmed the presence of a highly vascularized mass suggestive of a liposarcoma. A biopsy of the mass was performed concluding to a lipoma with no evident signs of malignancy. The patient underwent a surgical resection of the mass. Histopathological examination showed a well-differentiated adipose proliferation arranged in diffuse patterns of mature adipocytes. Large hibernoma-like foci were also noted. The diagnosis of a lipoma-like hibernoma was confirmed. Hibernoma represents is an uncommon benign tumor. It usually occurs in areas where the brown fat persists, including the thighs, shoulders, back, and neck in decreasing frequency. Commonly, this tumor occurs between the second and third decades of life. Clinically, it presents as a slow growing, painless mass. It may occasionally be painful due to compression of the surrounding structures. MRI shows T1w and T2w hyperintensity, with contrast enhancement after gadolinium injection. On histopathological examination, the structure is distinguished by an association of mature cells, round cells with central nuclei and eosinophilic cytoplasm, and multivacuolated cells. Surgical excision is the optimal treatment. The differential diagnosis concerns lipomas and well-differentiated liposarcomas. Lipoma-like hibernoma is an uncommon benign tumor which might imitate a liposarcoma clinically and radiologically. Histopathological examination is necessary to establish the diagnosis.