Atypical fibroxanthoma (AFX) is a rare skin tumor that generally pursues an indolent course despite its alarming histological appearances. It is important for the pathologist to distinguish this neoplasm from more aggressive lesions that may show very similar histological features. Recently, it has been suggested that demonstration of CD117 is of value in identifying AFX. To test this hypothesis, 50 cases of AFX were stained immunohistochemically for CD117 to determine the diagnostic value of this antibody. Cases were also stained for tryptase to identify mast cells, which are CD117 positive. In addition, S100 and CD1a stains were performed to assess any possible contribution of melanocytes or Langerhans cells to CD117 staining. Only 1 of 50 AFXs (2%) showed CD117 positivity in the neoplastic cells, but all tumors demonstrated included CD117- and tryptase-positive mast cells in similar distribution. CD117 is only rarely stainable in the neoplastic cells of AFX and is therefore not useful in identifying these tumors. Mast cells are also CD117 positive, frequently present in AFX, and can lead to misinterpretation. Using immunohistochemistry for mast cell tryptase may be of value where there is doubt as to the nature of CD117-positive cells in neoplasms.
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