Signet-ring cell dermatofibroma.

Dermatofibroma or cutaneous fibrous histiocytoma is a common benign skin lesion with multiple, distinct histologic variants, including cellular, aneurismal, epithelioid, atypical, lipidized "ankle-type," palisading, and cholesterotic. Although dermatofibromas are considered benign neoplasms, certain variants including cellular and aneurismal ones have shown to have a notable tendency to locally recur after excision. Indeed, although extremely rarely, metastases have been associated with the cellular and aneurysmal/atypical variants. Signet-ring cells are formed by cytoplasmic accumulations of various substances that push the nucleus toward the cellular border. The finding of signet-ring cells in a skin neoplasm always raises the suspicion of metastatic adenocarcinoma, although a number of reports have shown their occurrence in primitive cutaneous neoplasms as well. Signet-ring cell formation, however, has never been described in dermatofibroma. We present, for the first time, a new, distinctive variant of dermatofibroma, so-called signet-ring cell dermatofibroma, in a 16-year-old man with a slowly growing skin tumor on the lateral side of his right leg. Histologic examination demonstrated a striking signet-ring cell appearance of most of the cells in an otherwise fibrohistiocytic looking proliferation. Histochemical and immunohistochemical stainings confirmed the diagnosis of dermatofibroma. The phenomenon described in this case enlarges the histologic spectrum of cutaneous fibrous histiocytoma and may cause substantial differential diagnostic problems.