Synovial sarcoma and malignant mesothelioma of the pleura: Review, differential diagnosis and possible role of apoptosis

Synovial sarcoma of the pleural cavity is exceptionally rare and may be con fused, both clinically and histologically, with malignant mesothelioma, wit h subsequent inappropriate therapy. To address this dilemma, four biphasic synovial sarcomas (BSSs) and four biphasic malignant mesotheliomas (BMMs) w ere studied with a panel of mucin and immunohistochemical stains to determi ne if they would allow one to distinguish between the two. The BMMs were al l pleural-based. The BSSs were extrapleural. The mucin and immunohistochemi cal stains were all performed on formalin-fixed, paraffin-embedded tissue u sing standard techniques, with appropriate positive and negative controls. Mucin present in BSS is, in general, mucicarmine-positive and resistant to both hyaluronidase and diastase. Of the immune markers evaluated, only calr etinin, Ber-Ep4 and bcl-2 were of limited discriminatory value. Subsets of cytokeratins, CEA and CD 34 were not helpful. With the exception of bcl-2, the apoptotic markers p53, bax and cpp32 (caspase) also were not useful. Ho wever, when the apoptotic stains were viewed collectively, variations in ex pression between the two tumours raised the possibility that alterations in apoptotic activity might be responsible for their pathogenesis and behavio r. The diagnosis of BSS or BMM of the pleural should be made only after tot al consideration of clinical, radiological, histochemical and immunohistoch emical findings. Although mucin stains are useful in differential diagnosis , reliance solely on immunohistochemical markers, with the possible excepti on of calretinin, Ber-Ep4 and bcl-2, is not dependable. The role of apoptos is in the pathogenesis of these tumours needs to be explored with a much la rger series.