Squamous cell carcinoma following fluorouracil-responsive 'keratoacanthoma'.

To the Editor.— The treatment of keratoacanthoma (KA) with fluorouracil cream administered by the intralesional1,2and topical routes at concentrations of 20%3and 5%4,5has been reported. When treating these lesions without histologic confirmation, the following criteria are suggested to preclude the misdiagnosis of squamous cell carcinoma: (1) characteristic morphology of the keratoacanthoma, (2) history of growth of the lesion to its ultimate size within four to six weeks, and (3) prompt response of the lesion to fluorouracil treatment with at least 60% to 70% involution during a two-week treatment period.4We describe a patient with such a lesion that initially responded to fluorouracil therapy only to recur three weeks after completion of therapy. An excisional biopsy specimen at that time revealed a poorly differentiated squamous cell carcinoma. Report of a Case.— A 76-year-old man presented with a three- to four-week history of a rapidly growing