Utility of cytokeratin subsets for distinguishing poorly differentiated synovial sarcoma from peripheral primitive...

Poorly differentiated synovial sarcoma (PDSS) is a round cell sarcoma that may be difficult to distinguish from other round cell sarcomas, such as peripheral primitive neuroectodermal tumour (pPNET), on histological examination alone. Furthermore, these two tumours may show overlapping immunophenotypes, as some cases of PDSS express CD99, and, on the other hand, pPNET may express epithelial markers. The goal of this study was to determine the utility of cytokeratin (CK) subsets in distinguishing between these two lesions.

We evaluated 13 cases of pPNET with RT-PCR detected EWS/FLI-1 fusion transcript and 21 cases of PDSS for the immunohistochemical expression of CK7 and CK19. All cases of PDSS had areas of recognizable monophasic or biphasic synovial sarcoma that expressed at least one epithelial marker. Thirteen of 21 (62%) PDSS stained with AE1/AE3. CK7 and 19 were expressed in 11 (52%) and nine (43%) cases, respectively. Although six of 19 (32%) PDSS demonstrated cytoplasmic staining for CD99, none showed a membranous pattern of immunoreactivity. In contrast, 12 of 13 (92%) pPNET showed strong membranous immunoreactivity for CD99. Four tumours (31%) showed focal staining with AE1/AE3, two of which (15%) stained for CK19. CK7 was not detected in any of the pPNETs.

Although AE1/AE3 may be found in up to 31% of cases of pPNET, the expression of CK7 makes this diagnosis less likely.