Primary desmoplastic small round cell tumor of bone: report of a case with cytogenetic confirmation.

We report a case of desmoplastic small round cell tumor occurring in the right ilium of a 13-year-old boy. Morphologically, the neoplasm consisted of small round cells of primitive appearance with a diffuse growth pattern replacing marrow space and eroding bone. Immunohistochemical staining was positive for vimentin, synapsin, CD99 (MIC2 protein), and FLI-1, prompting an initial diagnosis of Ewing sarcoma/primitive neuroectodermal tumor. However, a diagnosis of desmoplastic small round cell tumor was rendered after the detection by cytogenetic analysis of the reciprocal chromosomal translocation, t(11;22)(p13;q12), which is uniquely associated with this tumor. This is the first documented instance of desmoplastic small round cell tumor arising in bone.