Abstract

We report a case of a PDRM10-rearranged soft tissue tumor that presented in a 12-year-old female as a subcutaneous plaque with a tent sign on the thigh. The clinical differential diagnosis included epidermoid cyst and pilomatricoma. Histologic sections showed a multinodular mass composed of fascicles of spindled cells with scattered pleomorphic and multinucleated cells in a collagenous stroma. Scattered mitotic figures were present as were lymphoid aggregates. By immunohistochemistry, the tumor was positive for CD34 and showed some focal immunoreactivity for CD68 and SMA. The tumor was negative for desmin, calponin, S100, MSA, STAT6, keratin, and EMA. Next generation sequencing to detect gene fusions revealed a MED12-PRDM10 fusion, confirming the diagnosis of PDRM10-rearranged soft tissue tumor. This is a recently described entity defined by fusions involving the PRDM10 as the 3’-partner, and either MED12 or CITED2 as the 5’-partner. Histologically, they are composed of fascicles of atypical, pleomorphic spindled cells within a collagenous to myxoid stroma and an associated inflammatory infiltrate. They are consistently positive for CD34 and often show some immunoreactivity for cytokeratin. There is significant histologic overlap with CD34-positive superficial fibroblastic tumor, and these may be related entities. PDRM10-rearranged soft tissue tumors have a risk of local recurrence, but no or only limited risk a risk of metastasis. It is important to recognize this entity to avoid misdiagnosis as a more aggressive sarcoma.

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