Primary renal synovial sarcoma - Molecular and morphologic delineation of an entity previously included among embryonal sarcomas of the kidney

We report 15 primary renal neoplasms with morphologic, immunohistochemical, and molecular features identical to those of synovial sarcoma. These turne rs form a distinct subset of the entity previously designated as embryonal sarcoma of the kidney. Most were diagnosed between the ages of 20 and 50 ye ars. On gross examination, tumors are large, partially necrotic, and usuall y contain smooth-walled cysts. Microscopically, tumors are characterized by mitotically active, monomorphic plump spindle cells with indistinct cell b orders growing in short, intersecting fascicles. Grossly identified cysts a re lined by mitotically inactive polygonal eosinophilic cells with apically oriented nuclei ("hobnailed epithelium"). The spindle cells are immunoreac tive for vimentin, often immunoreactive for EMA, but typically non-immunore active for desmin, actin, S100, or cytokeratins, whereas the cyst epitheliu m is cytokeratin-positive. These findings are consistent with monophasic, s pindled synovial sarcoma encircling dilated native renal collecting ducts. The presence of an SYT-SSX gene fusion resulting from the t(X;18) character istic of synovial sarcoma was demonstrated by reverse transcriptase polymer ase chain reaction in three of three tumors in which adequate RNA could be obtained from paraffin blocks. An additional case demonstrated the characte ristic t(X;18) translocation on cytogenetic analysis, but adequate material to perform molecular studies was not available in this case or the remaini ng 11 cases. Primary renal synovial sarcoma is a distinctive clinicopatholo gic entity confirmed by molecular detection of SYT-SSX fusion transcripts.