Sclerotic (fibroma-like) lipoma: a distinctive lipoma variant with a predilection for the distal extremities.

This report details the clinicopathologic and immunohistochemical findings identified in 21 cases of a fibrosclerotic variant of lipoma from acral sites that is frequently misdiagnosed as a fibromatous (nonlipogenic) process. The study includes 12 males and 9 females; aged 7 to 72 (mean and median, 39). The patients presented with solitary, mostly asymptomatic, masses that ranged from 0.6 to 2.2 (median, 1.2; mean, 1.3) cm and involved fingers (n=17), hands or wrists (n=3), and toes (n=1). Microscopically, the lesions were well-circumscribed nodules that showed very low to moderately low cellularity and consisted of cytologically bland spindled and stellate-shaped cells and a minor component of randomly dispersed adipocytes embedded in a collagenous to myxocollagenous stroma. Eight tumors were "fibroma-like" owing to their paucity of spindled cells, dense fibrosclerotic stroma, and inconspicuous vasculature. One tumor showed features of spindle cell/pleomorphic lipoma, whereas another demonstrated a vague onion skin-like arrangement of collagen reminiscent of sclerotic (storiform) fibroma. In all cases, the mitotic activity was negligible. Nonlipogenic tumor cells were immunoreactive for CD99 (6 of 6 cases), CD34 (6 of 8), S-100 protein (4 of 7), and smooth muscle actin (2 of 6). Follow-up data on 8 patients (range, 1 to 20 years; median, 9.5 years) revealed no recurrence in 6, but indicated the possibility of persistent tumor in the remaining 2 individuals after simple excision. Despite histological overlap in 1 case with the sclerotic fibroma, no patient displayed definitive clinical features of Cowden syndrome. Our study indicates that fibrosclerotic lipomas demonstrate a broader histological scope than what was initially described.