Atypical Fibrous Histiocytoma of the Skin

Atypical fibrous histiocytoma is an uncommon, poorly documented variant of cutaneous fibrous histiocytoma. We studied 59 cases of atypical fibrous histiocytoma to better characterize the clinicopathologic spectrum. There were 33 males and 26 females (median age 38 years; range 5-79 years) with solitary lesions arising on lower (25 cases) and upper (17 cases) extremities, trunk (6 cases), head and neck (4 cases), and vulva (1 case); anatomic location was not stated in six cases. Lesions measured 0.4-8 cm in diameter (median 1.5 cm) and clinically were nodules (40 cases), polypoid tumors (18 cases), or a slightly elevated plaque (1 case). Histologically, the lesions were primarily dermal with superficial involvement of the subcutis in one third of the cases. Salient features included a proliferation of pleomorphic, plump, spindle, and/or polyhedral cells with mainly large, hyperchromatic, irregular, or bizarre nuclei, set in a background of classic features of fibrous histiocytoma, including spindle cell areas showing a storiform pattern and entrapped thickened, hyaline collagen bundles, especially at the periphery. Multinucleated giant cells, often with bizarre nuclei and foamy, sometimes hemosiderin-rich, cytoplasm were also variably present. The degree of pleomorphism varied from only focal and minimal (14 cases) or moderate (24 cases) to marked (21 cases). Mitotic activity was observed in 55 lesions, and the number of mitotic figures ranged from 1 to 15 per 10 high power fields. Atypical mitoses were noted in 20 lesions. Furthermore, some cases of atypical fibrous histiocytoma displayed other worrisome features less often observed in ordinary FH, including unusually large size (diameter >2 cm, 8 cases), involvement of the superficial subcutis (19 cases), and geographic necrosis (7 cases). Immunohistochemical studies performed in 42 cases showed only focal smooth muscle actin (10 cases) and CD34 (4 cases) positivity, whereas CD68, S-100 protein, desmin, pan-keratin, and epithelial membrane antigen were negative. Clinical follow-up data available in 21 patients (mean duration of follow-up 50.6 months, median 43 months) revealed local recurrences in three patients (one repeated); two patients developed distant metastases, one of whom died after 96 months. These two cases were not histologically distinct from the group as a whole. We conclude that atypical fibrous histiocytoma has a broader clinicopathologic spectrum than previously realized. Lesions with floridly atypical features represent potential pitfalls for overinterpretation as pleomorphic sarcoma, which would appear to be inappropriate in most cases. Provided that atypical fibrous histiocytoma is treated by complete excision, a benign outcome is to be expected in most cases. However, similar to the cellular and aneurysmal variants of fibrous histiocytoma, atypical fibrous histiocytoma shows a higher tendency to recur locally than ordinary fibrous histiocytoma and may rarely metastasize.