Answer: Spindle cell lipoma

Histology: The lesion is a relatively well circumscribed mass composed of adipose tissue and spindle cells. The spindle cells are somewhat uniform in appearance and do not demonstrate cytologic atypia or mitotic activity. There is a moderate amount of fibrous tissue associated with the lesion. True lipoblasts are not observed. The number of spindle cells within the lesion is variable. The spindle cells are associated with a mucoid matrix and bundles of collagen.

Discussion: The major differential diagnosis on this case rests between a spindle cell lipoma and a myxoid liposarcoma. The spindle cell lipoma is a benign, often well circumscribed lesion that is characteristically seen in the shoulder and neck but can occur in other locations. It is composed predominantly of mature lipocytes. Unlike liposarcoma, lipoblasts are not identified. This lesion is often better circumscribed than a liposarcoma which is characteristically not well circumscribed. Another feature helpful in distinguishing this lesion from myxoid liposarcoma is the lack of prominent vessels. Extensive vascularization is more common in myxoid liposarcoma. The absence of lipoblasts and vascular pattern supports a benign diagnosis such as spindle cell lipoma over a myxoid liposarcoma.