Primary extra‐axial, para‐articular chordoma of the knee. A case report and the review of literature

was carried out using the OGT CytoSure ISCA oligoarray set (Oxford Gene Technology, Oxford, UK), containing 180k DNA oligonucleotides with a minimum resolution of 200 kb. Microarray hybridisation and copy number variant (CNV) analysis were performed according to the manufacturer’s instructions. All genome coordinates were according to NCBI human genome build 37 (hg19, February 2009). aCGH analysis revealed only a few copy number aberrations, as follows: arr[GRCh37] 2q24.1q31.1 (156893603-170587701)x1, 6q22.31q25.1(12540 6258-151703036)x1, 13q12.12q12.13(2355333226335543)x1 and 13q12.2q34(27940855-1150 93155)x1 (Figure 3C). Of note, there was no overlap of these numerical changes with the genomic copy number alterations reported recently in similar tumours by Bale et al. Thus, these might be secondary genetic alterations of as-yet unknown biological significance. Within the group of mesenchymal tumours with EWSR1–CREB gene family fusions, this newly described entity, of which we document the ninth case, shows histological overlap with the primary pulmonary myxoid sarcoma and myxoid angiomatoid fibrous histiocytoma. However, the first tumour presents as endobronchial lesions in adult patients and, for the latter, a fibrous pseudocapsule, peritumoral lymphoplasmacytic infiltrates and blood-filled cystic spaces were lacking. There are two other myxoid mesenchymal tumours with EWSR1 involvement: extraskeletal myxoid chondrosarcoma (EMC) and myoepithelial tumours. EMC occurs typically in the deep soft tissue of the limb in middle-aged adults. and in the vast majority of cases NR4A3 is the translocation partner. Myoepitheliomas also show histological overlap, but combine expression of keratin and/or EMA with S100 protein and/or GFAP. Moreover, various translocation partners have been described so far: POU5F1, PBX1, PBX3, ZNF444 and KLF17. Only one myoepithelial tumour in the pelvis of an adult was reported to have a EWSR1–ATF1 fusion, but the immunophenotype was typical for myoepithelioma. Within the cranium, only three primary myoepitheliomas have been described, with typical expression of keratin, S100 protein and GFAP, but no documented EWSR1 rearrangement. Our case confirms the observation of recurrent genetic alteration involving EWSR1 gene in myxoid mesenchymal intracranial tumours by Kao et al. (2017) and Bale et al. (2017), the second case carrying the EWSR1–ATF1 fusion. Interestingly, amianthoid-like fibres are lacking in the two cases with the EWSR1– ATF1 fusion, but are reported in the six cases with the other fusions. The potential significance of this is not clear. A combined approach using immunohistochemistry and molecular analysis is recommended to separate this rare entity from its mimics. The potential relation with myxoid angiomatoid fibrous histiocytoma awaits further study.