Immunoprofile of mesenchymal chondrosarcoma: aberrant desmin and EMA expression, retention of INI1, and negative estrogen receptor in 22 female-predominant central nervous system and musculoskeletal cases

Mesenchymal chondrosarcoma is a rare malignant tumor in the differential diagnosis of other small, round blue cell tumors, including atypical teratoid tumor in the central nervous system (CNS) and rhabdomyosarcoma in the musculoskeletal (MSK) locations. We reviewed the morphology of CNS and MSK cases and applied a panel of immunostains. Archival cases were pulled from our files. Immunohistochemistry and follow-up were obtained. Twenty-two cases included 5 CNS (all female; mean age, 30.2) and 17 MSK (11 female and 6 male; mean age, 31.1). Both CNS and MSK examples had similar round cells, staghorn vascular pattern, increased mitotic activity, and centrally located hyaline cartilage islands. The CNS examples demonstrated more spindling and the MSK cases more necrosis. INI1 was retained in all tumors studied. Epithelial membrane antigen (EMA) and desmin were expressed focally in 35% and 50% of cases, respectively. The round cells of all cases were negative for MyoD1, myogenin, smooth muscle actin (SMA), glial fibrillary acid protein (GFAP), keratins, and estrogen receptor, as well as a panel of other antiobodies. Eighty percent of patients with follow-up had pulmonary metastases and/or died within a mean of 5 years. The CNS and MSK mesenchymal chondrosarcoma predominantly affects adult females with poor prognosis. There are only subtle morphologic differences between the CNS and MSK groups. By immunohistochemistry, mesenchymal chondrosarcoma occasionally expresses aberrant desmin and EMA but is negative for SMA, myogenin MyoD1, GFAP, and keratins, refuting true smooth or skeletal muscle, epithelial, or meningothelial phenotype. Retained INI1 separates these tumors from atypical teratoid tumor. Despite marked female predominance in our series, estrogen receptor is negative in mesenchymal chondrosarcoma.