Highlights of the 14th international mesothelioma interest group meeting: Pathologic separation of benign from malignant mesothelial proliferations and histologic/molecular analysis of malignant mesothelioma subtypes.

OBJECTIVES: The separation of benign from malignant mesothelial proliferations and exact subclassification of mesothelioma subtypes is crucial to determining patient care and prognosis but morphologically can be very difficult.

METHODS: This session of the 2018 IMIG meeting addressed these problems.

RESULTS: A new immunohistochemical marker, methylthioadenosine phosphorylase, was shown to correlate well with CDKN2A FISH and is cheaper and faster to run. A 117 gene expression panel also provided good separation on both tissue biopsy and cytology samples. Review of a series of mesotheliomas thought to be biphasic produced only a moderate level of agreement among expert pathologists with some cases being classified as purely epithelioid or sarcomatoid; these classifications had prognostic significance. The entity called transitional mesothelioma was found to behave exactly like sarcomatoid mesothelioma. RNA-seq analysis of a large series of mesotheliomas from a public database showed that, genetically, the morphologic breakdown into epithelioid, sarcomatoid, or biphasic mesotheliomas is artificial because there is a continuous spectrum of genomic changes. There are now criteria for the diagnosis of mesothelioma in situ and this is potentially important, since such cases might be curable.

CONCLUSIONS: This session documented new morphological and molecular approaches to separating benign from malignant mesothelial proliferations and to subclassifying malignant mesoteheliomas in clinical relevant ways.