Hemosiderotic fibrohistiocytic lipomatous lesion: ten cases of a previously undescribed fatty lesion of the foot/ankle.

We address the clinicopathologic features of a previously undescribed heavily-pigmented spindle cell proliferation within a circumscribed benign lipomatous lesion that occurs mainly in the ankle region of older females. Patients with "lipoma with fibrohistiocytic proliferation" were retrieved from our files. Slides and clinical information were reviewed, and immunohistochemistry was performed (n = 5). Ten patients with hemosiderotic fibrohistiocytic lipomatous lesions were identified. All cases demonstrated a well-circumscribed fatty lesion with random focal proliferations of plump, slightly pleomorphic spindled cells, scattered inflammatory cells, and abundant iron pigment. The spindled cells had vesicular nuclei with indistinct nucleoli; occasional hyperchromatism was observed. No nuclear cytoplasmic inclusions were identified. The spindled component had a reactive appearance. In most cases, the fatty component, with homogeneously sized adipocytes, predominated. The lesions occurred in the foot/ankle region (8/10, one each cheek and hand) of primarily females (8/10) with a mean age of 50.6 years (range 42-63 years), size of 7.7 cm (range 2.5-17 cm), and prior duration of 3.1 years. Seven of eight patients had a history of prior trauma. The spindled component was positive for vimentin, calponin, CD34, and occasionally KP-1 or lysozyme and negative for caldesmon, S100, and desmin. Follow-up on eight patients revealed four with recurrences or residual disease over three years, requiring re-excision. No cases metastasized or caused patient death (mean 12 years, range 1-23 years). We describe a predominantly fatty lesion that is hemosiderin rich with a "fibrohistiocytic" proliferation, composed of histiocytes, myofibroblasts, and C34-positive fibroblasts, which occurs predominantly in the ankle region of middle-aged females. We believe that this is a reactive process due to antecedent trauma, the inflammatory cells, hemosiderin, mixed spindled cells, and homogeneous non-neoplastic appearance of the fat. HFLL can be distinguished from previously described lesions. Correct identification of hemosiderotic fibrohistiocytic lipomatous lesion is important, as it may locally recur.