Immunohistochemistry in Peritoneal Mesothelioma: A Single-Center Experience of 244 Cases.

Context: - Diagnosis of malignant mesothelioma is more common in the chest than it is in the abdomen. Most published immunohistochemistry data are more applicable to pleural than to peritoneal mesothelioma.

Objective: - To clarify the practical utility of 17 immunohistochemistry markers in the differential diagnosis of peritoneal mesothelioma with an emphasis on stains for which there is either contradictory information or a paucity of literature.

Design: - Consultation files of peritoneal mesothelioma diagnoses rendered from 1999 to 2014 were reviewed; 244 cases were identified. The results of immunohistochemistry markers performed were tabulated.

Results: - Immunohistochemistry markers positive in peritoneal mesothelioma in order of sensitivity were calretinin (244 of 244; 100%), WT1 (205 of 218; 94%), CK5/6 (173 of 194; 89%), mesothelin (132 of 150; 88%), and D2-40 (78 of 97; 80%). Markers used to differentiate carcinoma from mesothelioma showed immunoreactivity in peritoneal mesothelioma: estrogen receptor (2 of 84; 2%), B72.3 (6 of 196; 3%), CK20 (5 of 116; 4%), CD15 (7 of 192; 4%), p63 (3 of 62; 5%), carcinoembryonic antigen (9 of 199; 5%), PAX8 (12 of 191; 6%), progesterone receptor (5 of 71; 7%), Ber-EP4 (17 of 209; 8%), and CD138 (9 of 91; 10%). BAP1 loss, increasingly used in the differential diagnosis of benign versus malignant mesothelial proliferation, occured in 55% (99 of 181) of peritoneal mesothelioma cases.

Conclusions: - The results support the experience that there is no definitive marker to rule out malignant mesothelioma, including PAX8, estrogen receptor, progesterone receptor, and p63 immunoreactivity. The high rate of immunoreactivity for mesothelin may have a role as a predictive marker for immune targeting. BAP1 loss of 55% in this cohort of peritoneal mesothelioma confirms published observations, and BAP1 retention is seen in a significant proportion of neoplastic cases.