Accuracy of pathologic diagnosis for thymic epithelial tumors: a brief report from an Italian reference Center

Objectives - Rare tumors are diagnostic challenges for pathologists. Consultation or referral to Centers with expertise is crucial for the right diagnosis. This is particularly true for Thymic Epithelial Tumors (TETs), whose treatment strategies vary according to histological subgroup. We aimed at evaluating the accuracy of TET pathologic characterization in an Italian reference Center. Materials and methods - All the cases with diagnosis or suspicion of TETs, which underwent a pathological second opinion at Istituto Nazionale dei Tumori (INT), Milan, between 2015 and 2019 were retrospectively reviewed. All cases had been pathologically characterized through immunohistochemistry (IHC). Descriptive statistics were used for qualitative variables. Concordance was estimated through Cohen's kappa (k). Results - Out of 278 cases of TETs diagnosed in INT, 72 were referred to INT for a pathologic revision. The INT revision changed the diagnosis in 41 cases (56.9%), with a potential therapeutic shift in 32 (44.4%). In particular, 20 cases of thymoma were reviewed as a different subtype of thymoma (19/20) or lymphoma (1/20); nine cases of thymic carcinoma were reviewed as thymoma. On the other hand, three cases of lung carcinoma were reviewed as thymic carcinoma (2/3) or thymoma (1); eight cases of carcinoma Not Otherwise Specified were reviewed as thymic carcinoma; one case of lymphoma was reviewed as thymoma. Concordance between pathologists was moderate for thymoma (74.7%, k 0.447), inferior for thymic carcinoma (60.5%, k 0.139). Conclusion - A significant proportion of cases referred to INT for a presumptive TET received a different characterization. A potential shift in therapeutic indication was not rare. This underlines the importance for TETs to get a second pathological diagnosis by an expert pathologist and supports the need for networks on rare cancers.