EWSR1-SMAD3-Positive Fibroblastic Tumor.

EWSR1-SMAD3-positive fibroblastic tumor is a recently characterized neoplasm with distinct clinicopathologic features and recurrent EWSR1-SMAD3 gene fusion. ESFT typically presents as a small, painless tumor in extremity subcutaneous tissues. Their behavior is benign, although they are prone to local recurrence. They typically comprise two components: intersecting fascicles of overlapping, uniform plump spindle cells, and less cellular hyalinized areas containing stippled calcifications. Immunohistochemically, the cells consistently show diffuse ERG nuclear expression, while other markers are negative. The morphology of this neoplasm can lead to histologic confusion with both benign and malignant soft tissue tumors, including monophasic synovial sarcoma, malignant peripheral nerve sheath tumor, and spindle cell sarcoma, not otherwise specified. Correct identification of ESFT is critical, most importantly to avoid unnecessary overtreatment as sarcoma.