Clinicopathologic and Prognostic Implications of Programmed Death Ligand 1 Expression in Thymoma

Background. Programmed death ligand 1 (PD-L1) has been reported to be expressed in various malignancies and is considered to be a prognostic factor and an immunotherapeutic target. The aim of this study was to characterize PD-L1 expression in thymoma and determine statistical associations between this expression and clinical features. Methods. We reviewed formalin-fixed, paraffin embedded tissue specimens from 82 thymoma cases accumulated at Kurume University, the majority of which achieved surgical complete resection. Expression of PD L1 was evaluated by immunohistochemistry. Statistical associations between PD-L1 expression and clinicopathologic features were evaluated by using z2 test and Fisher's exact test. Disease-free survival and overall survival curves were established by the Kaplan-Meier method and compared using a log-rank test. Predictive factors for disease-free survival after complete resection were analyzed by using a Cox proportional hazards model in univariate and multivariate analysis. Results. Overall, 44 thymoma cases (54%) revealed high PD-L1 expression. High PD-L1 expression was statistically associated with Masaoka stage III/IV disease (p = 0.043) and World Health Organization type B2 or B3 thymoma (p = 0.044). Disease-free survival after complete resection in high PD-L1 expression was significantly worse than that in low PD-L1 expression (p = 0.021), although there was no significant difference in overall survival (p = 0.957). Multivariate analysis also revealed high PD-L1 expression as an independent risk factor for recurrence (p = 0.008). Conclusions. Characterization of PD-L1 expression in thymoma should enable more effective clinical approaches, including prognostic stratification of patients and potential use of anti-PD-L1 antibody immunotherapy. (C) 2016 by The Society of Thoracic Surgeons