Extraskeletal myxoid chondrosarcoma in young children.

Two extraskeletal myxoid chondrosarcomas with a solid soft tissue mass occurred on the right upper arm of a 4-year-old boy and on the chest wall of a 1-year-old boy. Microscopically, both tumors were characterized by lobular configuration and were sparsely cellular with a background of myxoid matrix. The cells were small and round, and appeared undifferentiated, sometimes with a narrow eosinophilic cytoplasm. They grew in nests or strands and sometimes in a single file. They were strongly positive for S-100 protein and vimentin. Ultrastructural features suggested that the cells had a poorly differentiated mesenchymal nature with chondrocytic differentiation. These are the sixth and seventh reported cases of extraskeletal myxoid chondrosarcoma occurring in children. There are definite differences between this tumor with immature features and the extraskeletal myxoid chondrosarcoma in adults. Problems of differential diagnoses from other small round cell sarcomas also are discussed.