Clinicopathologic study of deciduoid mesothelioma using SMARCB1/INI1 immunohistochemistry and fluorescence in situ hybridization

Deciduoid mesothelioma is a rare variant of epithelioid mesothelioma. Malignant rhabdoid tumors, renal medullary carcinoma, and some synovial sarcomas show a loss of SMARCB1/INI1 protein, a member of the SWI/SNF chromatin-remodeling complex. All of those tumors are known to have rhabdoid cells. Some mesothelioma cases, such as those of the deciduoid type, have also been reported to possess such rhabdoid features. Since this topic has not been studied in malignant mesothelioma, we analyzed the immunohistochemical expression of SMARCB1/INI1 in malignant mesotheliomas [45 epithelioid type (including 9 deciduoid type), 12 biphasic type, and 17 sarcomatoid type]. We employed (a) SMARCB1/INI1 immunohistochemistry, using an antibody to the INI1 gene product and (b) Fisher exact test, logistic regression analysis, the Kaplan-Meier method, and the Wilcoxon test for survival analysis for prognostic factor evaluation (SAS 9.4; SAS Institute, Cary, NC). The results showed that 17 of 74 (23%) malignant mesothelioma cases (epithelioid: 24%; biphasic; 8%; sarcomatoid; 29%) had reduced SMARCB1/INI1 expression. Reduced SMARCB1/INI1 expression appeared to be more frequent in the deciduoid type (67%), of which there were admittedly only a few cases, than in either the epithelioid type (14%) or biphasic type (8%), whether or not rhabdoid cells were present, but not different between the deciduoid and sarcomatoid types. However, there was no statistically significant difference in prognosis between malignant mesotheliomas with reduced versus preserved SMARCB1/INI1 protein expression. The results suggest that in differential diagnosis, cases with reduced SMARCB1/INI1 protein expression should not be excluded from a diagnosis of malignant mesothelioma.