CD34-Positive Acral Chondromyxoid Mesenchymal Neoplasm Harboring a Novel TCF4::ERG Fusion.

Molecular testing has significantly transformed the field of anatomic pathology over the past several decades. Despite these advances, acral mesenchymal neoplasms remain diagnostically challenging, requiring careful integration of clinical presentation, histologic features, and molecular findings for accurate classification. Herein, we present a case of an acral chondromyxoid mesenchymal neoplasm harboring a novel in-frame TCF4::ERG fusion involving the right index finger of a 26-year-old female. Morphologically, this tumor consisted of nests and sheets of monotonous small round-to-ovoid cells embedded in a background of chondromyxoid stroma and hyalinized collagen. The tumor cells were diffusely CD34, ERG, and focally p63 reactive, while S100 protein, cytokeratin AE1/AE3, Pan-TRK, ALK, smooth muscle actin, and desmin were negative. Albeit short follow-up (3 months), the patient continues to do well without evidence of metastasis or local recurrence.