Answer: Spindle cell lipoma

Histology: The lesion consists of predominantly mature adipose tissue admixed with small hyalinized, collagenous bundles. Areas of the lesion show a myxoid component composed of short bland spindle cells with scattered mast cells.

Discussion: Although the collagenous bundles seen in this case are similar to those seen in neurofibroma, recognizing that the lesion is a circumscribed mass and that the adipose tissue is integral to this lesion allows one to categorize this lesion as an adipose tissue neoplasm. Myxoid liposarcomas uncommonly have a prominent mature adipose tissue component. Furthermore, rather than bundles of collagen intersecting the lesion, one would expect a more prominent myxoid matrix with a background fine, delicate vascular network. Fibrolipomas are lipomas with collagen. The presence of the myxoid component with short bland spindle cells admixed with mast cells would not be seen within a lipoma which had undergone fibrous change. Spindle cell lipomas may show varying amounts of the spindle cell component. In this case, the spindle cell component is rather subtle. The location of this lesion in the upper back of a middle-aged man is classic for this entity. The lesion is entirely benign and treated by local excision.