Fibrohistiocytic lipoma: twelve cases of a previously undescribed benign fatty tumor.

A lipoma with a spindled proliferation within it, resembling known (myo)fibroblastic lesions such as fibrous histiocytoma or dermatofibrosarcoma protuberans, (ie, fibrohistiocytic lipoma), has not been previously reported. This tumor varies from other classic lipoma variants, including spindle cell lipoma, myolipoma, angiolipoma, and fibrolipoma. We examine the clinicopathologic findings of this new lipoma variant. The Soft Tissue Pathology Registry of the Armed Forces Institute of Pathology was searched for patients with "lipoma with fibrohistiocytic proliferation." Lesions that were better classified as other entities were excluded. Patient slides and clinical history, including associated lesions, family history, duration of symptoms, history of trauma, natural progression, and treatments, were reviewed. Immunohistochemistry was performed on cases with available material (n = 6). Twelve patients with fibrohistiocytic lipoma were included. All tumors revealed a well-distributed quilt-like proliferation or solid focus of slightly plump to relatively bland spindled cells with collagenous stroma in short fascicular and storiform growth patterns. These spindled cells resembled those seen in either fibrous histiocytoma or dermatofibrosarcoma protuberans. However, the spindled proliferation was all within a well-circumscribed lipoma. The lesions lack the dermal involvement or plump pleomorphism of fibrous histiocytoma and the dermal involvement or infiltrative growth pattern of dermatofibrosarcoma protuberans. The fatty component demonstrated heterogeneously sized adipocytes, as those seen in other lipomas. Inflammation and hemosiderin were minimal. Mast cells were not identified. The tumors were typically found in the subcutis of the trunk of men (10 of 12; one each on the wrist and leg; mean age, 31 years). The average size of the lesions was 3.0 cm, and they were present for a mean duration of 10 months prior to surgical excision. One patient had two concurrent lesions; all others had solitary tumors. Another patient had a intracranial dermoid cyst removed during childhood. Four patients had a personal or family history of hypercholesterolemia, hypertension, or myocardial infarction. There was no history of antecedent trauma. Cases studied were positive for vimentin, calponin (5 of 5), CD34 (3 of 5), and occasionally KP-1 or lysozyme in the spindled component, and all cases studied were negative for the actins, caldesmon, S-100 protein, desmin, cytokeratins, and epithelial membrane antigen. Although the actins were negative in our laboratory, the more sensitive calponin positivity suggests myofibroblastic phenotype of the spindled component of this lesion. CD34-positive fibroblasts were present in three of five cases. Of eight patients with follow-up, there were no recurrences; all patients were alive and free of disease over a mean of 10 years (range, 2 months to 31 years). We have identified a lipoma variant, fibrohistiocytic lipoma, that has not been previously described. In our experience the morphology and calponin positivity suggest myofibroblastic phenotype for the spindled cells, within a lipoma. This entity can be distinguished from fibrous histiocytoma, fibromatosis, dermatofibrosarcoma protuberans, spindle cell lipoma and other lipoma, and liposarcoma variants.