Expanding the Range of Epithelial Marker and SMA Staining of Superficial CD34+ Fibroblastic Tumors: A Case Report and Review of the Literature.

Superficial CD34+ fibroblastic tumor (SCD34FT) is a relatively recently described borderline mesenchymal neoplasm. Owing to a relative lack of specificity in clinical presentation, radiopathologic findings, and immunohistochemical staining, the diagnoses of SCD34FT can be challenging. In this study, we present a case of a 55-year-old woman with an indolent painless nodule on the right shin. Histopathologic evaluation of the resected specimen showed a moderately cellular, subcutaneous lesion composed of spindled cells with mild pleomorphism arranged in sheets and fascicles. Immunohistochemical staining demonstrated diffuse positivity for CD34. Surprisingly, the tumor also showed diffuse expression of smooth muscle actin (SMA) and more than focal (ranging from subset to diffuse) expression of AE1/AE3 and CAM5.2. DNA and RNA next-generation sequencing revealed a t(X; 11)(q13; q24) MED12::PRDM10 fusion, confirming the diagnosis of SCD34FT. To the best of our knowledge, this is the first reported case highlighting SCD34FT with more extensive immunoreactivity to epithelial markers (AE1/AE3 and CAM5.2) and SMA compared with the focal staining reported in the existing literature. We hope that adding this case to the existing literature will raise awareness among pathologists to recognize the more variable staining pattern of epithelial markers and SMA when considering the diagnosis of SCD34FT.