An extra-glossal ectomesenchymal chondromyxoid tumor with RREB1::MRTFB fusion-positive: A case report of a rare tumor at a rare site

Background: Ectomesenchymal chondromyxoid tumor (ECT) is a rare benign intraoral tumor 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, emphasizing its genetic distinctiveness. Herein we report one mesenchymal tumor involving the bottom of the left foot of young women with RREB1–MRTFB fusions. Case presentation: The tumor presented as well-circumscribed masse. After complete excision recurrence was not observed at 6 months. Although tumor were originally unclassifiable, targeted RNA sequencing revealed RREB1 (exon 8)–MRTFB (exon 11) fusion transcripts. The tumor cells showed mild nuclear atypical with very low mitotic activity. Immunohistochemistry staining show S100, glial fibrillary acidic protein, CD56 diffuse positive, variable expression of SMA, and negative for SOX10、P63. Overall, these findings suggest that they may represent previously undescribed extra-glossal ECT involving the plantar fascia. However, it has brought many obstacles to our diagnosis because of its unclassical morphology and uncommon site. Conclusions: We report for the first time a mesenchymal chondromyxoid tumor with RREB1-MTRFB gene fusion occurring in the foot. This case furthers awareness of the extra-glossal distribution of ECT. By carefully observing morphology and combining immunohistochemistry and molecular techniques, we can effectively avoid misdiagnosis and missed diagnosis of the diseases.