Clinicopathologic Features of Thymoma With the Expression of Programmed Death Ligand 1

Programmed death ligand 1 (PD-L1) is reportedly expressed in various malignancies and is considered a prognostic factor. We attempted to reveal the usefulness of the PD-L1 expression as a prognostic factor in patients with thymoma.

Eighty-one patients with thymoma who underwent surgical resection between 2004 and 2015 were retrospectively reviewed. The PD-L1 expression was evaluated by immunohistochemistry and stratified by the proportion of positive tumor cells. Strong membranous reactivity of the PD-L1 antibody in 1% or more of tumor cells was considered "positive." The association between the PD-L1 expression and the clinicopathologic features was investigated.

The PD-L1 expression was positive in 22 patients (27%) and negative in 59 patients (73%). The PD-L1 positivity was significantly associated with type B2 and B3 thymoma (p < 0.001) and stage III and IV disease (p = 0.048). In addition, PD-L1 positive tumors showed a significantly higher maximum standardized uptake value than PD-L1 negative tumors (p = 0.026). The 5-year disease-free survival rate was 82% in PD-L1 positive patients and 88% in PD-L1 negative patients, showing no significant difference (p = 0.57). Furthermore, PD-L1 positivity was not an independent prognostic factor for the disease-free survival on a Cox proportional hazards analysis (p = 0.59).

A strong expression of PD-L1 in thymoma was significantly associated with type B2 and B3 and higher pathologic stages. In addition, PD-L1 positivity was associated with an increased maximum standardized uptake value of the tumor. However, patients with PD-L1 positive thymomas did not show a significantly worse prognosis than patients with PD-L1 negative tumors.