Molecular analyses, morphology and immunohistochemistry together differentiate pleural synovial sarcomas from mesotheliomas: clinical implications.

Background: Only recently pleural synovial sarcomas (SS) have been definitively identified because of the presence of a characteristic X;18 translocation. SS are sarcomatoid or biphasic malignancies morphologically almost indistinguishable from sarcomatoid or biphasic malignant mesotheliomas (MM).

Patient And Methods: We demonstrated a primary pleural biphasic SS in a patient referred to us has having a biphasic MM.

Results: Histology showed a spindle cell tumor with focal epithelioid differentiation, microcalcifications, and a hemangiopericytomatous vascular pattern. Ultrastructurally, the epithelioid tumor cells had few blunt microvilli and occasional intercellular junctions. Immunohistochemically the tumor cells were positive for BCL-2, CD99, CD56, and focally for BerEp4, Pancytokeratin and cytokeratin 5/6. These findings suggested the possibility of SS rather than MM. Detection of the t(X:18) translocation using RT-PCR, Southern blot, and DNA sequencing definitively confirmed the diagnosis of SS.

Conclusion: The differential between pleural SS and MM requires a high degree of suspicion and molecular analyses because morphology (histology, immunohistochemistry and electron microscopy) is insufficient to definitively distinguish between these two malignancies. This differential is critical because patients with pleural SS can be susceptible to chemotherapy, and accordingly are treated, while patients with sarcomatoid MM are resistant to chemotherapy and accordingly are not treated.