Osseous myxochondroid sarcoma: a detailed study of 5 cases of extraskeletal myxoid chondrosarcoma of the bone.

Extraskeletal myxoid chondrosarcoma (EMC) is a rare mesenchymal neoplasm with a characteristic translocation usually involving NR4A3 and EWSR1. EMC has rarely been reported in the bone and may be confused with conventional chondrosarcoma with myxoid features or various small round cell sarcomas. We present 5 cases of molecularly confirmed EMC arising primarily in the bone. Patients included 4 men and 1 woman, aged 38 to 77 years (median 54 y). Tumors arose in the ilium (2 cases), manubrium, rib, and humerus. Four tumors extensively infiltrated and destroyed preexisting bone with cortical breakthrough and associated soft tissue extension; 1 case demonstrated only focal cortical breakthrough. Microscopically, 2 cases had small round cell features; 1 of these was hypercellular, whereas the other was hypocellular with abundant myxochondroid matrix. Three cases were composed of eosinophilic spindled cells with variable fascicular to corded or wreath-like growth patterns. Fluorescence in situ hybridization was positive for both EWSR1 and NR4A3 translocation in 3 cases; rearrangement for EWSR1 or NR4A3, but not both, was seen in 2 tumors. After definitive therapy, 1 patient experienced multiple local recurrences at 36 months and died of disease at 61 months. Two patients developed lung metastases at 26 and 74 months and are alive with disease at 44 and 74 months, respectively. Two patients are disease free at 5 and 24 months. EMC of the bone is a diagnostic dilemma and requires molecular confirmation. We propose to classify tumors with the appropriate phenotype and molecularly confirmed NR4A3/EWSR1 rearrangements as myxochondroid sarcoma, either osseous or extraskeletal variants.