The American Journal of Surgical Pathology 2021 August 19 [Link]
David B Chapel, Michelle S Hirsch
Primary peritoneal malignant mesothelioma (MM) can demonstrate morphologic overlap with low-grade and high-grade tubo-ovarian serous neoplasms; it is also biologically and prognostically distinct from benign mesothelial proliferations. Currently, there is no single biomarker that can definitively distinguish these neoplasms. Sex-determining region Y box 6 (SOX6) immunohistochemistry has been recently described to differentiate pleural epithelioid MM from lung adenocarcinoma, but it has not been evaluated in the peritoneum. SOX6 immunohistochemistry was performed on 43 peritoneal epithelioid MM, 7 peritoneal biphasic MM, 5 well-differentiated papillary mesotheliomas, 5 serous borderline tumors, 29 low-grade serous carcinomas (LGSCs), 20 high-grade serous carcinomas (HGSCs), and 25 cases of peritoneal reactive mesothelial hyperplasia. Quantitative SOX6 expression in epithelioid MM (median, 100% of tumor cells) was significantly greater than in LGSC/serous borderline tumor (median, 90%; P=0.004) and HGSC (median, 45%; P=0.0001). However, when SOX6 is expression is defined as ≥10% of tumor cells, there was no significant difference in the rate of SOX6 positivity between epithelioid MM (41/43, 95%), LGSC (28/29, 97%; P=1.0), and HGSC (17/20, 85%; P=0.16). Quantitative extent of SOX6 expression in epithelioid MM was significantly greater than in biphasic MM (median, 0%; P=0.0001), well-differentiated papillary mesothelioma (median, 20%; P=0.001), and reactive mesothelial hyperplasia (median, 20%; P=0.0001), but not significantly different from flat quiescent mesothelium (median, 90%; P=0.82). SOX6 immunohistochemistry is 95% sensitive for peritoneal epithelioid MM, but is also consistently expressed in LGSC and HGSC, negating its usefulness in this common differential diagnosis. SOX6 also shows variable expression across the spectrum of reactive, benign neoplastic, and malignant mesothelial lesions of the peritoneum, and does not appear to be diagnostically useful in distinguishing benign from malignant mesothelial proliferations.