EWSR1-SMAD3 fibroblastic tumour: emerging cutaneous soft tissue neoplasm

EWSR1-SMAD3 fibroblastic tumour represents a novel cutaneous soft tissue neoplasm, and further expands the spectrum of EWSR1-rearranged tumours. Most commonly arising in acral sites, the tumour classically features a well-circumscribed, nodular architecture. The tumour consists of uniform, bland, fibroblastic spindle cells set within a collagenous to myxoid stroma – in which areas of abrupt hyalinisation can be present. Variant patterns include collagen rosette formation and myopericytomatous growth. The tumour is usually based in the dermis, but may abut the epidermis and/or infiltrate the subcutis. By immunohistochemistry, the tumour cells are diffusely and strongly positive for ERG but negative for CD34 - findings that should prompt confirmation of the EWSR1-SMAD3 gene fusion. The differential diagnosis includes benign (e.g., superficial acral fibromyxoma) and malignant (e.g., low-grade fibromyxoid sarcoma) entities. The tumour is biologically benign, but can recur locally if incompletely excised. Reference: Habeeb O, Korty KE, Azzato EM, et al. EWSR1-SMAD3 rearranged fibroblastic tumor: Case series and review. J Cutan Pathol 2021; 48: 25562.