Superficial CD34 fibroblastic tumors and PRDM10-rearranged soft tissue neoplasm: An evolving diagnosis.

Superficial CD34 fibroblastic tumors (SCD34FT) and PRDM10-rearranged tumors (PRTs) are mesenchymal tumors that have recently received increased scientific attention due to their irrefutable similarities yet debatable relationship. A 74-year-old male presented to the dermatology clinic with a violaceous, well-defined nodule on the left medial knee of 2-year duration. Shave biopsy demonstrated spindle cells arranged in a vaguely storiform pattern forming fascicles. Immunohistochemical stains were positive for vimentin, CD68, CD10, and CD34 diffusely. ERG, S-100, HMB45, and SOX-10 were negative. Molecular studies identified a mediator complex subunit 12 (MED12)-PR/SET Domain 10 (PRDM10) gene fusion thus favoring confirming the diagnosis of a PRT. Our patient underwent wide local excision with negative margins and had no complications. This case aims to provide context for considering SCD34FT and PRT as intersecting entities and to discuss a diagnostic approach when encountering these tumors.