Extra-axial poorly differentiated chordoma of the knee masquerading as periprosthetic joint infection: a case report

Poorly differentiated chordoma (PDC) is a malignant neoplasm defined by the co-expression of brachyury and cytokeratin with loss of SMARCB1 (INI-1) expression. Extra-axial PDC is exceedingly rare, with only 10 cases reported to date. Here, we report an additional case of extra-axial PDC arising in the soft tissues surrounding the knee joint of a 69-year-old woman, which was initially misdiagnosed as periprosthetic joint infection following total knee arthroplasty. Histologically, the tumor comprised epithelioid to spindled cells with vacuolated, eosinophilic cytoplasm, focal rhabdoid areas, without myxoid stroma. Immunohistochemically, the tumor cells showed diffuse positivity for brachyury and cytokeratin, with loss of INI-1 expression. Next-generation sequencing identified a homozygous deletion of CDKN2A, but no SMARCB1 genomic alteration was detected despite INI-1 loss. This case shows that extra-axial PDC should be considered in the differential diagnosis of soft tissue tumors, irrespective of anatomical site or morphology.