CD163 is not a sensitive marker for identification of atypical fibroxanthoma

The histopathological features of atypical fibroxanthoma (AFX) overlap with those of poorly differentiated carcinoma, melanoma and leiomyosarcoma in the skin. As there are no specific stains to identify AFX, the diagnosis is essentially one of exclusion and requires completion of a panel of immunostains. Recently, it has been suggested that the macrophage/monocyte-specific marker CD163 is of value in identifying AFX. To investigate this claim, 57 AFX were stained for CD163. Only 21 of 57 (37%) of AFX stained positively, and intratumoral macrophages confounded interpretation of the stain at times. In four cases, it was not possible to definitively interpret the tumor staining reaction because of this effect. While a lack of stainable CD163 antigenicity may indicate that AFX is not of histiocytic lineage, it is conceivable that expression of the antigen has been lost for some reason in cells that are in fact of macrophage lineage. In summary, CD163 only stains a minority of AFX and staining results can be difficult to interpret. CD163 is therefore of very limited value in the diagnosis of AFX. Beer TW. CD163 is not a sensitive marker for identification of atypical fibroxanthoma.