CD99 reactivity in mesenchymal chondrosarcoma.

CD99 reactivity has been reported to be a sensitive and specific marker for Ewing's sarcoma (ES) and primitive neuroectodermal tumor (PNET). However, within the group of "small round blue cell tumors," the specificity has not been extensively examined. We investigated the immunoreactivity of mesenchymal chondrosarcoma to CD99 using the O13 antibody and found reactivity in 11 of 11 (100%) of cases, specifically in the "small round blue cell" component. All cases showed strong reactivity involving more than 50% of the small round blue cells with a distinct membrane pattern of staining. Staining was either absent or focal and weak in cartilaginous areas. We conclude that mesenchymal chondrosarcoma cannot be distinguished from ES/PNET on the basis of CD99 immunoreactivity.