Abstract

Atypical fibroxanthoma (AFX) and pleomorphic dermal sarcoma (PDS) are pleomorphic spindle cell neoplasms with identical cytomorphology. Their diagnosis relies on excluding differentiation towards other dermal neoplasms, including melanocytic, smooth muscle, epithelial (squamous), and vascular differentiation. This is accomplished via immunohistochemistry and usually includes the utilization of melanocyte markers like S100 and sox10, desmin, cytokeratins and/or P63/P40, and vascular markers. We report two cases of AFX, which at first glance showed extensive S100 staining. However, close examination revealed the positivity was in relatively small spindled and dendritic cells, favored to be background Langerhans cells. This suspicion was confirmed with a CD1a stain in both cases, which showed identical staining to the S100. Both cases were entirely negative for Sox10. The CD1a staining confirmed a prominent background population of Langerhans cells, and the tumors were both confirmed to be AFX. The reason why certain AFX/PDS tumors show this background population of Langerhans cells while most are entirely negative for S100 is unknown. Both of the tumors reported herein showed broad ulceration, but further investigation is required to determine if any correlation exists. These cases highlight a very important diagnostic pitfall in the use of S100 in pleomorphic spindle cell neoplasms in the skin, and most importantly the utilization of CD1a to avoid a misdiagnosis of melanoma.

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