Sclerosing dermatofibrosarcoma protuberans (DFSP): an unusual variant with focus on the histopathologic differential diagnosis

A 59‐year‐old man presented with a 10‐cm × 8‐cm tumoral plaque with a superficial nodule in the interscapular region of the back (Fig. 1). The lesion had been growing for 25 years. As a cystic lesion was suspected, the superficial nodule was biopsied. The histopathologic diagnosis was low‐grade sarcoma with sclerosis. Two months after the initial biopsy, the lesion was completely excised, reaching the muscular fascia, with a 2‐cm margin and with a free graft. 1 Clinical aspect of the plaque in the lesion on the back Formalin‐fixed paraffin‐embedded samples were submitted to histologic and immunohistochemical study (4‐µm paraffin sections); frozen tissue was submitted to electron microscopy. For histopathology, sections were stained with hematoxylin and eosin. Immunohistochemistry was performed following standard avidin‐biotin immunoperoxidase procedures with primary antibodies for vimentin, CD34, smooth muscle‐specific actin, bcl‐2, S‐100, desmin, myoglobin, factor VIII, p53 (all from DAKO, Copenhagen, Denmark), HHF‐35 (Enzo Diagnostics, Farmingdale NY), cytokeratin (AE1/AE3) (Biogenex, San Ramon, CA), and factor XIIIa (Calbiochem Novabiochem Corporation, La Jolla, CA). At low magnification, the histologic study of the initial tumoral nodule revealed a poorly circumscribed mesenchymal proliferation, with fibroblastic‐like neoplastic cells arranged in a fascicular and storiform pattern, admixed with extensive areas of sclerosis. At higher magnification, tumoral cells were spindle‐shaped with hyperchromatic nuclei and scant cytoplasm. In some areas, sclerosis was so evident that a keloid‐like pattern was seen (Fig. 2a). The surgical specimen showed a fibroblastic neoplastic proliferation infiltrating the dermis and hypodermis. In the dermis, cells were arranged in a storiform pattern, whereas in the hypodermis there was a honeycomb or lace‐like pattern (Fig. 2b). There were also cellular areas alternating with sclerotic areas, with transitional zones in between, in both the dermis and hypodermis. 2 (a) Keloid‐like hyalinization area in the initial biopsy; (b) surgical specimen of the hypodermis showing honeycomb pattern The immunohistochemical study of the initial tumoral nodule and the surgical specimen showed that tumoral cells expressed vimentin, CD34 (Fig. 3), bcl‐2, HHF‐35, and smooth muscle actin. Neoplastic cells failed to show positivity with desmin, myoglobin, factor XIIIa, factor VIII, S‐100, cytokeratin (AE1/AE3), and p53. 3 CD34 positivity in tum