Intramuscular myxoma with chondroid features: two cases expanding the morphological spectrum.

Intramuscular myxoma is a benign mesenchymal tumour typically composed of bland spindle to stellate cells in abundant myxoid stroma and usually characterized by GNAS mutations. Chondroid matrix has not been previously reported in intramuscular myxoma. We describe two intramuscular myxomas with chondroid-type matrix and review the relevant radiological, histological, immunohistochemical and molecular findings, with emphasis on differential diagnostic considerations. Case 1 occurred in the triceps of a 64-year-old woman and showed focal cartilaginous differentiation, including chondrocytes within lacunar spaces, in an otherwise classic intramuscular myxoma background. The lesional cells were negative for MUC4, S100, cytokeratin, SMA, desmin, CD34, ERG, STAT6 and EMA, and targeted sequencing detected no pathogenic alteration. Case 2 occurred in the vastus medialis of a 48-year-old woman and showed bland spindle to stellate cells in myxoid to collagenous stroma with foci of chondromyxoid matrix, patchy lacunar-type spaces, and territorial basophilia. The lesional cells were positive for CD34, focally positive for SMA and negative for S100, desmin, cytokeratin and SOX10. Whole transcriptome sequencing detected no gene fusion, and targeted sequencing identified GNAS c.602G>A (p.R201H). Both tumours were deep intramuscular and markedly T2 hyperintense and showed peripheral and septal enhancement on MRI. These cases expand the morphological spectrum of intramuscular myxoma by documenting chondroid and chondromyxoid matrix, likely representing cartilaginous metaplasia. Awareness of this variant is important to avoid misclassification as other chondromyxoid neoplasms.