Mesenchymal tumours with RREB1–MRTFB fusion involving the mediastinum: extra‐glossal ectomesenchymal chondromyxoid tumours?

AimsEctomesenchymal chondromyxoid tumour (ECT) is a rare benign intraoral tumour which almost exclusively presents as a small mass of the anterior dorsal tongue. Recently, the RREB1–MRTFB (previously known as MKL2) fusion gene has been identified in 90% of ECTs, all located in the tongue, emphasising its genetic distinctiveness. Here, we report two mesenchymal tumours involving the superior mediastinum of adult women with RREB1–MRTFB fusions.Methods and resultsBoth tumours presented as well‐circumscribed paravertebral masses that were clinically suspected to be schwannoma. After fragmented resection, recurrence was not observed at 27 and 18 months. Although tumours were originally unclassifiable, next‐generation sequencing detected identical RREB1 (exon 8)–MRTFB (exon 11) fusion transcripts, which were validated by reverse transcriptase–polymerase chain reaction, Sanger sequencing, and fluorescence in‐situ hybridisation. Both tumours shared hyalinised areas with round cells embedded in a cord or reticular manner. The tumour cells showed mild nuclear atypia of possible degenerative type with very low mitotic activity, and were at least focally positive for S100, glial fibrillary acidic protein, smooth muscle actin and epithelial membrane antigen. Overall, these findings suggest that they may represent previously undescribed extra‐glossal ECT involving the mediastinum. However, the histology was not classic for ECT, because that in case 2 was predominated by storiform growth of spindle cells, whereas the tumour in case 1 lacked myxoid change.ConclusionsWe have provided the first evidence that RREB1–MRTFB fusion is not limited to tumours in the head region, and whether such tumours represent extra‐glossal ECTs requires further research.