Undifferentiated Pleomorphic Sarcoma — SCE Dermatology MCQ

Correct answer: B — Undifferentiated pleomorphic sarcoma (formerly MFH)

The correct answer is B, undifferentiated pleomorphic sarcoma (formerly MFH). This elderly patient has a rapidly enlarging sun-exposed ear tumour with a pleomorphic spindle cell proliferation showing brisk mitoses and necrosis, and an immunophenotype negative for S100 (excludes melanoma), cytokeratins (excludes squamous or Merkel cell carcinoma) and CD34 (excludes dermatofibrosarcoma protuberans), but positive for CD68 and FXIIIA, markers seen in fibrohistiocytic tumours including atypical fibroxanthoma and its deep counterpart. The histology and immunoprofile are essentially identical to atypical fibroxanthoma (AFX), but the tumour extends into subcutaneous fat with necrosis present; once such a pleomorphic fibrohistiocytic tumour breaches the dermis into subcutis, particularly with necrosis or vascular/perineural invasion, it is reclassified as pleomorphic dermal sarcoma or undifferentiated pleomorphic sarcoma rather than AFX, reflecting a genuinely higher risk of recurrence and metastasis. This remains a diagnosis of exclusion made after ruling out melanoma, carcinoma and other sarcomas by immunohistochemistry, exactly as done here. Why the other options are wrong: D. Dermatofibrosarcoma protuberans: this is CD34 positive with a storiform dermal pattern; CD34 negativity here excludes it. A. Atypical fibroxanthoma: shares the identical immunophenotype (CD68, FXIIIA positive; S100, cytokeratin, CD34 negative) but by definition is confined to the dermis without subcutaneous invasion or necrosis, both present in this case. C. Leiomyosarcoma: arises from smooth muscle and stains positive for desmin and smooth muscle actin, not CD68 or FXIIIA. E. Merkel cell carcinoma: is a neuroendocrine carcinoma that stains positive for cytokeratins (classically CK20 in a perinuclear dot pattern) and neuroendocrine markers, which this tumour lacks. Key point: a fibrohistiocytic pleomorphic tumour with the AFX immunoprofile is only termed AFX if confined to the dermis; subcutaneous extension, necrosis, or vascular/perineural invasion reclassifies it as undifferentiated pleomorphic sarcoma (pleomorphic dermal sarcoma).

Reference: PCDS (Primary Care Dermatology Society), Atypical Fibroxanthoma clinical guidance, https://www.pcds.org.uk/clinical-guidance/atypical-fibroxanthoma