Malignant pilomatricoma. An immunohistochemical study with antihair keratin antibody.

A case is reported of malignant pilomatricoma confirmed by immunohistochemistry using anti-human hair keratin (anti-HHK) antibody prepared by the authors. The tumor occurred in the soft tissue of the inguinal region of an 88-year-old woman, with later invasion of the epidermis. No other possible primary lesion was found at autopsy. Histologically, the tumor was squamous cell carcinoma with nests of tumor cells and shadow cell-like necrotic cells showing central keratinization and focal calcification. Immunohistochemically, the hair keratin was positive in this tumor and in benign pilomatricomas exclusively. All other skin lesions and various squamous cell carcinomas examined were negative for this antigen. The staining patterns of commercial antiepidermal keratin and antiinvolucrin antibodies were significantly different from that of anti-HHK in normal skin and in these lesions. To the authors' knowledge, this is the first case of malignant pilomatricoma tested with anti-HHK staining. Malignant pilomatricoma is generally a low-grade malignant tumor, but it can metastasize and be fatal as it was in this case.