Immunohistochemical staining of CD10 in atypical fibroxanthomas

Background:  The histologic diagnosis of atypical fibroxanthoma (AFX) may be difficult at times with a differential diagnosis including spindle cell/desmoplastic melanoma and a poorly differentiated squamous cell carcinoma (SCC). While there are immunohistochemical stains that may be used to support a diagnosis of either melanoma or carcinoma, the diagnosis of AFX tends to be one of exclusion. Methods:  A total of 35 cases including 16 AFXs, 10 poorly differentiated SCCs and nine spindle cell/desmoplastic melanomas were stained with the antibody against CD10. Results:  There was strong (3+) diffuse expression of CD10 in 15/16 (94%) AFXs, weak (1+) to moderate (2+) patchy expression of CD10 in 5/10 (50%) SCCs and weak (1+) expression of CD10 in 3/9 (33%) spindle cell/desmoplastic melanomas. Conclusion:  CD10 expression is helpful in distinguishing between AFXs, carcinomas and melanomas and will be useful in a panel of antibodies when distinguishing between these entities.