The role of CD10 in distinguishing atypical fibroxanthoma from sarcomatoid (spindle cell) squamous cell carcinoma

Background: The role of CD10 needs clarification in a broader immunohistochemical battery for distinguishing atypical fibroxanthoma (AFX) from spindle cell squamous cell carcinoma (sSCC). Methods: We retrospectively reviewed 23 cutaneous spindle cell tumors previously classified as AFX (n = 11) or as sSCC (n = 12). Each tumor was stained with CD10, S-100, p63 and two or more cytokeratin stains. Defining AFX as a diagnosis of exclusion based on multiple negative cytokeratin stains and negative p63 staining, we reclassified four squamous cell carcinomas (SCCs) as AFX. CD10 staining was reviewed and graded in all tumors. Results: Fifteen tumors were classified as AFX. Strongly positive CD10 staining was observed in all 15 AFXs, as well as four (50%) of the eight SCCs. Expression of p63 was seen in six sSCCs (75%). Conclusions: CD10 is consistently expressed by AFX. However, CD10 is also often strongly expressed by sSCC. Positive staining with p63 favors a diagnosis of sSCC. An immunohistochemical battery useful for distinguishing AFX from sSCC may include CD10, p63 and two cytokeratin markers. However, CD10 alone should not be relied upon in the distinction of these entities.