Chondromyxoid fibroma is an unusual, benign tumor of cartilaginous origin and represents less than 1% of all primary bone tumors. It usually involves the long bones around the knee joint or the flat bones of the pelvis or ribs. Soft tissue extension is also thought to be rare in these lesions. They are usually eccentrically located in the metaphyses of the long bones and centrally in the flat bones. The radiographic appearances are characteristically those of a single, lytic lesion with lobulated margins, septations, cortical expansion and a sclerotic rim. Histologically, they display a lobulated pattern with spindle-shaped cells lying within a myxoid matrix with areas of hyaline cartilage. The differential diagnosis includes giant cell tumor, chondroblastoma or enchondroma as well as chondrosarcoma. The rarity of these lesions may render the diagnosis difficult to make, especially when the lesion involves an unusual site such as the acromium.

A number of fibrous lesions involving bone display almost identical histologic appearances yet may represent either reactive or neoplastic conditions, resulting in a confusing nomenclature and possible diagnostic confusion. We report the case of a young man with no significant previous medical history who presented with a painful lesion in the left shoulder, which consisted almost entirely of xanthomatous material. We discuss the possible differential diagnosis of this lesion and why benign fibrous histiocytoma is the preferred terminology for this lesion. To our knowledge, this is the first reported case of a benign fibrous histiocytoma involving the acromion in an adult.

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