SPARC Is a Novel Positive Immunohistochemical Marker of Epithelioid Mesothelioma to Differentiate It From Lung Adenocarcinoma and/or Squamous Cell Carcinoma

Epithelioid mesothelioma with a solid histologic pattern (solid epithelioid mesothelioma) is difficult to distinguish from a poorly differentiated squamous cell lung carcinoma and/or solid lung adenocarcinoma. Thus, immunohistochemical markers are essential for diagnosis; however, the sensitivity and specificity of pre-existing mesothelial markers are suboptimal, particularly for differentiation from squamous cell carcinoma. Using a cancer-dependency map, we analyzed gene expression data of pleural mesothelioma and non-small cell lung cancer cell lines (squamous cell carcinoma and adenocarcinoma) and identified secreted protein acidic and cysteine-rich (SPARC) as a promising candidate for the differential diagnosis of epithelioid mesothelioma from lung squamous cell carcinoma and/or lung adenocarcinoma. SPARC expression in mesothelioma and lung cancer cell lines was validated using reverse-transcription polymerase chain reaction, western blotting, and immunohistochemistry. Immunohistochemical staining was performed using anti-SPARC antibodies against solid epithelioid mesothelioma, solid lung adenocarcinoma, and poorly differentiated lung squamous cell carcinoma. SPARC positivity was seen in 42/45 (93.3%) of solid epithelioid mesothelioma, 2/40 (5%) solid lung adenocarcinoma, and 2/45 (4.5%) of lung squamous cell carcinomas. The sensitivity, specificity, and diagnostic accuracy for differentiating solid epithelioid mesothelioma from lung cancer (solid lung adenocarcinoma and poorly differentiated lung squamous cell carcinoma) were 93.3, 95.2, and 94.6%, respectively. In conclusion, SPARC is a novel mesothelial marker that can be used to differentiate epithelioid mesothelioma from squamous cell carcinoma and lung adenocarcinoma.