A series of extraskeletal myxoid chondrosarcomas with rare morphological and molecular variations

AIMS: Extraskeletal myxoid chondrosarcoma (EMC) is a rare soft tissue sarcoma characterised by myxoid matrix, multilobular architecture, eosinophilic ovoid to short spindle cells arranged in cords, clusters or reticular patterns and NR4A3 gene rearrangement. However, some EMCs show morphological variations or non-EWSR1::NR4A3 fusion. We described herein five cases of variant EMCs. METHODS AND RESULTS: The five patients included two females and three males, aged 24-59 years (median: 49 years). The tumours were located in the dorsal region, buttock, thigh, paravertebral region and elbow, respectively. Grossly, the tumour sizes ranged from 4.0 to 16.0 cm (median: 6.5 cm) in greatest dimension. Morphological examination revealed tumours with varied growth patterns, including solid variants with eosinophilic proteinaceous fluid (n = 2), classic EMC morphology with rhabdoid cells (n = 1), a biphasic variant, composed of fibroblastic/myofibroblastic-like cells and oval to short spindle-shaped cells (n = 1), and a spindle-cell morphology with a myxoid stroma and prominent haemorrhagic cystic spaces (n = 1). Immunohistochemically, all cases showed variable expression of CD117. Next-generation sequencing (NGS) identified an EWSR1::NR4A3 fusion, a novel FUS::NR4A2 fusion, a novel ACTB::NR4A3 fusion, and two FUS::NR4A3 fusions. Follow-up for all five patients showed no signs of local recurrence or distant metastasis. CONCLUSION: These five cases of EMC highlight the continuous morphological spectrum of this tumour, demonstrating significantly greater histological diversity than classically described. The identification of novel fusion partners further expands its genetic landscape.