EZH2 and POU2F3 Can Aid in the Distinction of Thymic Carcinoma from Thymoma

SIMPLE SUMMARY: The morphologic distinction of thymic carcinomas from thymomas can be challenging but is important as thymic carcinomas tend to be associated with worse patient outcomes. We assessed whether immunohistochemistry for two proteins, EZH2 and POU2F3, could distinguish thymic carcinoma and thymoma. We compared our results with markers previously proposed for this purpose, including CD117 and CD5. POU2F3 (≥10% hotspot staining), CD117, and CD5 staining was seen in 51%, 86%, and 35% of carcinomas, respectively, and in none of the type A thymomas or micronodular thymomas with lymphoid stroma that were tested. EZH2 staining in ≥80% of tumor cells was present in most carcinomas and in only one subtype (type B3) of thymoma. All thymic carcinomas had EZH2 staining in >10% of tumor cells, such that EZH2 staining in ≤10% of tumor cells could help exclude carcinoma. EZH2 and POU2F3 immunohistochemistry may therefore be useful for distinguishing thymic carcinoma from thymoma. ABSTRACT: Thymic carcinoma is an aggressive malignancy that can be challenging to distinguish from thymoma using histomorphology. We assessed two emerging markers for these entities, EZH2 and POU2F3, and compared them with conventional immunostains. Whole slide sections of 37 thymic carcinomas, 23 type A thymomas, 13 type B3 thymomas, and 8 micronodular thymomas with lymphoid stroma (MNTLS) were immunostained for EZH2, POU2F3, CD117, CD5, TdT, BAP1, and MTAP. POU2F3 (≥10% hotspot staining), CD117, and CD5 showed 100% specificity for thymic carcinoma versus thymoma with 51%, 86%, and 35% sensitivity, respectively, for thymic carcinoma. All POU2F3 positive cases were also positive for CD117. All thymic carcinomas showed >10% EZH2 staining. EZH2 (≥80% staining) had a sensitivity of 81% for thymic carcinoma and a specificity of 100% for thymic carcinoma versus type A thymoma and MNTLS but had poor specificity (46%) for thymic carcinoma versus B3 thymoma. Adding EZH2 to a panel of CD117, TdT, BAP1, and MTAP increased cases with informative results from 67/81 (83%) to 77/81 (95%). Overall, absent EZH2 staining may be useful for excluding thymic carcinoma, diffuse EZH2 staining may help to exclude type A thymoma and MNTLS, and ≥10% POU2F3 staining has excellent specificity for thymic carcinoma versus thymoma.