Dermatofibroma with monster cells.

Nineteen cases of dermatofibroma associated with monster cells are reported. The term "monster" (an animal with a strange or terrifying shape, one unusually large for its kind) implies a strikingly atypical cell with an extremely large nucleus. Except for monster cells, these 19 lesions had all of the typical histopathological findings of dermatofibroma. The clinical diagnosis for 16 of these lesions was dermatofibroma (or histiocytoma). Three lesions were submitted without any clinical diagnosis. Eighteen of 19 lesions occurred on the extremities. One was on the back. Monster cells are seen in the early, histiocytic stage of dermatofibroma when lipophages and/or siderophages are usually present in large numbers. Only rarely were mitotic figures seen in dermatofibromas with monster cells, and they were neither numerous nor atypical. It is important for histopathologists to distinguish dermatofibroma with monster cells from cutaneous malignant fibrous histiocytoma and radiation sarcoma. The criteria for differentiation concern primarily the architectural pattern of the lesion rather than its cytological features.