Diagnostic utility of low-affinity nerve growth factor receptor (p 75) immunostaining in atypical fibroxanthoma

Background: Atypical fibroxanthoma (AFX) is a locally destructive, dermal-based, fibrohistiocytic, mesenchymal tumor. Immunohistochemistry helps to differentiate AFX from squamous cell carcinoma and spindle cell melanoma. Immunomarkers against p75 yield positive stains in spindled cell melanomas and negative stains in AFX, suggesting that these may be useful in differentiating these two entities. However, a recent study concluded that p75 is not a specific marker of neuroectodermal tumors; furthermore, p75 staining in AFX has only been evaluated in a few cases. Methods: We stained 20 AFXs for p75 and various other markers. Results: Reactivity was noted for vimentin (20 of 20 cases), CD10 (17/20), CD68 (14/20), CD99 (13/20), D2-40 (10/20) and p75 (1/20). Conclusions: We confirmed that CD99 and CD10 are frequently expressed in AFX (65 and 85%, respectively) and that CD31 rarely stains positive (5%). The 50% positivity rate of D2-40, in contrast with published evidence for its absence in melanoma, suggests that D2-40 may be useful for distinguishing AFX from melanoma. Furthermore, because only one sample was positive for p75, we confirm that p75 is useful in differentiating AFX from spindle cell melanoma. We advocate adding p75 and D2-40 to assist in differentiating AFX from melanoma.