Abstract

Superficial CD34-positive fibroblastic tumor (SCD34FT) is a rare, indolent soft tissue neoplasm that was recently recognized as a distinct entity in the current World Health Organization Classification of Soft Tissue and Bone Tumors after its initial recognition in 2014. We present the case of a 16-year-old patient with a 1cm forearm lesion showing gradual growth over one year and mild pain. Morphologically, the lesion appeared as a well-defined nodule in the subcutaneous adipose tissue, composed of spindle and epithelioid cells with prominent eosinophilic cytoplasm and moderate inflammatory cell infiltration. Prominent nucleoli and cytologic atypia were observed, while rare mitotic figures were present. Immunohistochemistry revealed diffuse strong positivity for CD34 and focal positivity for SMA, with no loss of INI-1 and SMARCA4. Notably, our case exhibited a unique feature compared to previous cases, showing diffuse positive pan-keratin (AE1/3) staining instead of patchy distribution. Desmin, SOX-10, S100, ALK, and EMA were negative in the spindle cells. Molecular testing by next-generation sequencing showed a PRDM10 rearrangement and recent literature has suggested that PRDM1-rearranged soft tissue tumor and SCD34FT represent the same entity. Both SCD34FT and PRDM10-STT exhibit nuclear pleomorphism, low mitotic count, mixed inflammatory cells, and diffuse CD34 expression. This case represents one of the youngest reported instances of SCD34FT, contributing to the expanding case pool of SCD34FT and PRDM10-STT.