Mesenchymal Chondrosarcoma Presenting as a Calcified Mass Extending Into the Inferior Vena Cava.

A 50-year-old man presented with lower abdominal pain. Imaging revealed a pelvic mass extending from the right internal iliac vein into the inferior vena cava. Fluorodeoxyglucose-positron emission tomography revealed intense uptake (SUVmax 13.97) without metastasis. Biopsy revealed a biphasic tumor with cartilaginous matrix and staghorn vasculature. Immunohistochemistry indicated positive for SMA, desmin, MyoD1, and NKX3.1. Moreover, genomic analysis identified a HEY1::NCOA2 fusion, confirming mesenchymal chondrosarcoma (MCS). Given the extent of the tumor, chemoradiotherapy was initiated. MCS is a rare, high-grade sarcoma with limited treatment options. Targeted therapies against HEY1::NCOA2-associated pathways are promising and are currently under investigation.