Atypical fibroxanthoma in a patient with xeroderma pigmentosum.

A 3-year-old boy with xeroderma pigmentosum developed a papule on the right ala nasi that had the light-microscopic, immunohistochemical, and ultrastructural features of atypical fibroxanthoma. This lesion most commonly arises on the sun-exposed skin of older adults. The occurrence of an atypical fibroxanthoma in such a patient adds weight to the concept that this tumor of "fibrohistiocytic" origin is induced by ultraviolet exposure. Mechanisms of tumor formation in a patient with xeroderma pigmentosum may be similar to those occurring in the more typical situation in adults.