ORIGINAL ARTICLE The Importance of EZH2 and MOC-31 Expression in the Differential Diagnosis of Benign and Malignant Effusions Canan Sadullahoglu, 1 * Deniz Nart, 2 and Ali Veral 2 Background: All malignant tumors may spread throughout the pleural, peritoneal, and pericardial cavities. The presence of tumor cells in serosal fluid is a poor prognostic indicator. It may be difficult to differentiate nuclear atypia of mesothelial cells due to injury of serosal surfaces from mesothelioma or malignant epithelial tumor cells. Epithelial and mesothelial immunohistochemical markers can be used in such conditions. The aim of this study was to evaluate the expression of two immunohistochemical markers (MOC-31 and EZH2) in serosal effusions. Methods: The study included a total of 142 patients diagnosed with benign or malignant cytology between January 2012 and April 2014. MOC-31 and EZH2 were applied to the cell blocks of 53 patients with benign cytology and 89 patients with malig- nant cytology determined based on the clinical, radiological data, histopathology diagnosis, and clinical follow-up in the absence of any surgical material of the patient in the hospital archive system. Results: None of the benign cases showed MOC-31 and EZH2 expression, although these markers were positive in 96 and 93% respectively of the malignant cases. Conclusion: In conclusion, it could be considered cost-effective to use a double immunohistochemical antibody kit for these two markers, MOC-31 membranous and EZH2 nuclear staining, in the diagnosis of malignant effusions. Diagn. Cytopathol. 2016;00:000–000. V C 2016 Wiley Periodicals, Inc. Key Words: malignant effusion; MOC-31; EZH2; reactive mesothelial cell Introduction All malignant tumors, especially carcinomas, may extend into serosal spaces. 1–5 Malignant pleural effusion is often caused by lung carcinoma in men and breast carcinoma in women while in the peritoneal cavity, lymphoma/leu- kemia and gastrointestinal tract carcinoma in men and ovarian carcinoma in women are the leading causes. In children, Non-Hodgkin lymphoma is the most common cause in both pleural and peritoneal malignant effu- sions. 1,2 The presence of malignant cells in the pleural, peritoneal and pericardial effusions reflects poor progno- sis, with a mean survival time shorter than 6 months. 1–5 Their presence is detected in acid or peritoneal washing fluid in order to determine the stage of certain gynecolog- ical and nongynecological tumors. 1–3 Mesothelial cells paving the serosal surface have oval- round paracentric nuclei with smooth contours and small nucleoli. 1–5 The cytoplasm is dense in the center, and lacy in the periphery. The nuclear/cytoplasmic ratio varies. Reactive atypia of mesothelial cells may be seen due to injury of the serosal surfaces and the differential diagnosis of mesothelial atypia from cases of suspected malignancy may be challenging. 1–4 If the malignant effusion is biphasic, neoplastic cells would show contrast with existing benign mesothelial cells. However, sometimes all cells are tumoral, and may not exhibit any contrast enhancement. Furthermore, meta- static carcinoma of serosal surfaces usually presents with 1 Department of Pathology, Antalya Education and Research Hospital, Antalya, Turkey 2 Department of Pathology, Ege University Faculty of medicine, Bornova, _ Izmir, Turkey Disclosures: All the authors declare that they have no conflict of inter- est and financial disclosure. *Correspondence: Canan Sadullahoglu, M.D.; Department of Pathology, Antalya Education and Research Hospital, Antalya, Turkey. E-mail: canan-rana@hotmail.com Received 23 March 2016; Revised 19 November 2016; Accepted 22 November 2016 DOI: 10.1002/dc.23653 Published online 00 Month 2016 in Wiley Online Library (wileyonli- nelibrary.com). V C 2016 WILEY PERIODICALS, INC. Diagnostic Cytopathology, Vol. 00, No 00 1