Subtitle: EWSR1::SMAD3-positive fibroblastic tumour (WHO 5th edition): acral superficial site, zonation with hyalinised centre, nuclear ERG positivity, benign behaviour, differential diagnosis and 6 MCQs.
Author: PathologyMCQ Editorial Board
Category: Soft Tissue Pathology
Last Updated: October 2026
Read Time: 4 minutes
At-a-Glance
- Benign fibroblastic tumour named by its fusion: EWSR1::SMAD3 (WHO 2020, emerging entity).
- Acral, superficial (hands and feet); broad age range; female predominance.
- Zonation: cellular fascicles of bland spindle cells around a hypocellular hyalinised centre.
- Nuclear ERG positive; CD34, S100, desmin negative.
- May recur if incompletely excised; no metastasis reported.
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Morphology
The tumour is composed of intersecting cellular fascicles of cytologically bland, monomorphic spindle cells that alternate with hypocellular, hyalinised areas, often in a zonal pattern with a paucicellular centre. Mitoses are few and there is no significant atypia or necrosis.
IHC: strong nuclear ERG in tumour cells is the key clue. CD34, S100, SOX10, desmin and keratin are negative; SMA may be focal. Confirmation is by detecting the EWSR1::SMAD3 fusion (EWSR1 FISH or NGS).
Differential Diagnosis
| Lesion | Key features |
|---|---|
| EWSR1::SMAD3 fibroblastic tumour | Zonation, hyalinised centre, ERG+, CD34− |
| Superficial acral fibromyxoma | Myxoid/collagenous, CD34+, EMA and CD99 often +, often subungual |
| Palmar/plantar fibromatosis | Fascicles in aponeurosis, nuclear beta-catenin in some |
| Calcifying aponeurotic fibroma | Children, calcification with chondroid foci; FN1::EGF |
| Cellular fibrous histiocytoma | Storiform, overlying epidermal hyperplasia, factor XIIIa |
| Epithelioid sarcoma | Young adults, nodules with central necrosis; keratin+, INI1 lost |
Read more: EWSR1-rearranged tumours →
Test Yourself: 6 MCQs
EWSR1::SMAD3 fibroblastic tumour: 6 MCQs
Site, morphology, ERG and differentials, with instant explanations.
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Exam Pearls
- Acral + zonation + ERG positive = think EWSR1::SMAD3.
- ERG positivity does not always mean a vascular tumour.
- CD34 positive acral lesion: think superficial acral fibromyxoma instead.
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References
- WHO Classification of Tumours Editorial Board. Soft Tissue and Bone Tumours. 5th ed. IARC; 2020.
- Michal M, et al. EWSR1-SMAD3-rearranged fibroblastic tumor: an emerging entity. Am J Surg Pathol. 2018;42:1325–33.
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