Answer: Spindle cell lipoma

Histology: The lesion forms a relatively discrete nodule within the subcutaneous fat. The nodule is comprised of myxoid areas admixed with mature fat. Dispersed within the myxoid areas are hyperchromatic spindle cells with elongated tapered nuclei separated by bands of ropey collagen. The spindle cells and the more mature adipocytes lack atypia, and mitotic figures are not identified.

Discussion: Spindle cell lipoma is a distinctive type of lipoma composed of varying proportions of mature adipocytes and spindle cells admixed with rope-like collagen and myxoid stroma. They have a strong predilection for men, and almost always arise from the subcutaneous tissues of the posterior neck and upper back. When the histologic picture is dominated by spindle cell, pathologists sometimes fail to recognize the lipomatous nature of this neoplasm and may mistake it for some other mesenchymal tumor such as nuchal fibroma. Nuchal fibromas are more heavily collagenized and often entrap nerves. In contrast to liposarcomas, spindle cell lipomas are relatively circumscribed lesions that lack the infiltration of liposarcomas. The bland appearance of the spindle cells, the absence of lipoblasts, and the superficial location also support the diagnosis of spindle cell lipoma over liposarcoma.