Abstract

Undifferentiated pleomorphic sarcoma (UPS), formerly malignant fibrous histiocytoma (MFH), is a rare high-grade sarcoma defined by the lack of lineage-specific morphologic features and immunophenotype. We report a case of a 79-year-old female with prolonged history of right knee pyoderma gangrenosum, presented with a 2.5 x 1.5 cm exophytic nodule on the lateral right knee overlying the area of pyoderma gangrenosum for which multiple biopsies were performed due to the diagnostic ambiguity and positive margins. An initial biopsy showed intact, acanthotic epidermal layer with spongiosis. The underlying dermis showed fibrosis, hyalinized ectatic blood vessels, a mixed inflammatory infiltrate and scattered large atypical hyperchromatic stromal cells. The histologic findings were reminiscent of pleomorphic hyalinizing angiectatic tumor (PHAT); however, the lesion was more superficial and lacked the characteristic precursor lesion (hemosiderotic fibrolipomatous tumor) seen in PHAT. Also, the immunohistochemistry was negative for CD34, which is strongly expressed in PHAT. Other IHC markers including SMA, desmin, ERG, pancytokeratin, multiplex Sox10/Mart1, p40/CK5/6, and S100 were negative. NGS mutational profiling revealed an ARID2 R1677 mutation with absent t(1;10)(p22;q24) translocation or TGFBR3/MGEA5 rearrangement described in cases of HFLT/PHAT. With the subsequent excisions, given the size of the lesion and the morphologic findings, it was classified as superficial undifferentiated pleomorphic sarcoma (UPS). The histologic features of PHAT have been described in other high-grade sarcomas including myxoinflammatory fibroblastic sarcoma and myxofibrosarcoma. However, to the best of our knowledge, cases of UPS mimicking PHAT have not been reported. This case also highlights the wide morphologic range of UPS.

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