Atypical mixed tumor of the skin: histologic, immunohistochemical, and ultrastructural features in three cases and a review of the criteria for malignancy.

Three cases of cutaneous mixed tumor with atypical features are described. These biphasic tumors comprise cords and nests of often plasmacytoid cells with areas of tubule formation, set in a chondroid stroma. The tumor cells show immunohistochemical positivity for both CAM 5.2 and S-100 and ultrastructural features of myoepithelium. The atypical histological features of an infiltrative margin, satellite tumor nodules, and tumor necrosis, which were present in these cases, have been described in malignant chondroid syringoma; however, a review of the small number of previous case reports demonstrates that histological appearances did not always correspond with apparent malignant potential in this tumor. The term atypical mixed tumor is recommended for tumors in which there are histological features of malignancy, especially local invasion, without proven metastases. Complete excision of the tumor and careful follow-up are advised, as at present the malignant potential of these tumors cannot be reliably predicted from their histological appearance.