Immunohistochemical profile of synovial sarcoma with emphasis on the epithelial-type differentiation. A study of 49 primary tumours, recurrences and metastases.

The relationship between biphasic (BSS) and monophasic (MSS) subtypes of synovial sarcoma (SS) as well as the relationship between cells of solid/glandular areas and the spindle cells of BSS remain controversial. In order to further evaluate the immunohistochemical phenotype of SS we studied 34 primary tumours (15 BSS; 19 MSS), 7 recurrences (4 from primary BSS; 3 from primary MSS) and 8 metastases (7 BSS; one MSS), using several antibodies (EMA, CEA, keratins 1, 4, 5/6, 7, 8, 13, 18, 19, 20, vimentin, collagen IV and laminin) that work in paraffin-embedded material. Spindle cells outside solid/glandular areas of BSS and in MSS showed immunoreactivity for keratins 5/6, 7, 8, 18 and 19. The transition of solid/glandular areas to surrounding spindle cells also showed keratin staining and failed to show a distinct separation regarding the immunoreactivity for laminin and collagen IV. Peripheral cells of solid/glandular areas were immunoreactive for vimentin. No major differences were observed between immunophenotypical cell profiles of BSS and MSS, apart from the exclusive immunostaining of solid/glandular areas of BSS for keratin 13 and CEA. Downgrading of keratin and extracellular matrix antigens immunoreactivity was observed when primary tumours were compared to recurrent and/or metastatic tumours of both subtypes (MSS and BSS). We conclude that SS should be regarded as carcinomas of soft tissues with an immunohistochemical phenotype depending on the degree of epithelial differentiation: spindle cells (MSS and BSS) predominantly expressing simple keratins, and poorly differentiated (solid/glandular) as well as well-differentiated (glandular) areas (BSS) expressing, in addition, complex epithelial-type keratins.