Track: Case Reports

Abstract:

We report a 65-year-old male with a two-week history of a tender 5 mm pearly pink papule on the right upper vermillion border. Histopathology showed a well-demarcated dermal nodular spindle cell proliferation arranged in fascicles, with cytologic pleomorphism and mitotic figures. Initial immunohistochemistry was negative for SOX10, CK5/6, SMA, CD31, and CD34, but positive for factor XIIIa, CD68, and CD10, which was suggestive of a fibrohistiocytic tumor. Additional stains including MITF, NKI/C3, and ALK-1 were negative, while PGP 9.5 was strongly and diffusely positive. To exclude a metastatic neuroendocrine tumor with spindle cell morphology, CAM and chromogranin were performed and were both negative. The histopathologic and immunophenotypic profile supported a diagnosis of a rare PGP 9.5 positive spindle-cell atypical fibroxanthoma (AFX). AFX is an uncommon, indolent mesenchymal neoplasm, typically a well-circumscribed dermal tumor with marked pleomorphism and spindled or epithelioid cells. Diagnosis relies on exclusion, as it can mimic other dermal neoplasms. In this case, AFX was favored based on its morphology, CD10 positivity, and absence of cytokeratin, myofibroblastic, melanocytic, and vascular markers. Diagnostic complexity arose from diffuse PGP 9.5 expression, which is a nonspecific marker usually seen in neural, neuroendocrine, or fibroblastic tumors. Notably, PGP 9.5 positivity in AFX has been reported in only nine cases in the literature. We present this case to highlight an exceedingly rare spindle-cell AFX variant with an atypical immunohistochemical profile, underscoring the importance of comprehensive staining and careful exclusion of histologic mimics.