Background: Programmed death-ligand 1 (PD-L1) contributes to immune escape and is a therapeutically relevant target in aggressive B-cell lymphomas. However, its distribution and clinicopathologic significance across primary mediastinal aggressive large B-cell lymphomas remain incompletely defined. Methods: We retrospectively reviewed 53 cases diagnosed between 2010 and 2025, including diffuse large B-cell lymphoma (DLBCL), primary mediastinal large B-cell lymphoma (PMBCL), mediastinal gray zone lymphoma (MGZL), and T-cell/histiocyte-rich large B-cell lymphoma (THRLBCL). PD-L1 immunohistochemistry (clone 22C3) was performed on formalin-fixed, paraffin-embedded tissue. Tumor cells and intratumoral inflammatory cells were assessed together. Membranous staining was scored as 0, 1+, 2+, or 3+, and the proportion of cells in each category was recorded. Results: PD-L1 staining was present in 40/53 cases (75.5%). Mean PD-L1-positive percentages were similar in DLBCL, PMBCL, and MGZL (38%, 36%, and 44%, respectively; p = 0.939), and no tested cutoff separated the diagnostic groups. In contrast, MGZL showed a higher mean 3+ component (31.11%) and more cases with 3+ staining in at least 50% of evaluated cells (44% vs. 4.3% in DLBCL and 4.7% in PMBCL; p = 0.007). PD-L1 extent was not associated with age, tumor size, LDH, beta-2 microglobulin, Ann Arbor stage, SUVmax, treatment response, or overall survival. Conclusions: PD-L1 expression is common in mediastinal aggressive large B-cell lymphomas. MGZL is distinguished by a strong-staining pattern, whereas combined PD-L1 extent was not prognostic in this cohort.

Background: Programmed death-ligand 1 (PD-L1) contributes to immune escape and is a therapeutically relevant target in aggressive B-cell lymphomas. However, its distribution and clinicopathologic significance across primary mediastinal aggressive large B-cell lymphomas remain incompletely defined. Methods: We retrospectively reviewed 53 cases diagnosed between 2010 and 2025, including diffuse large B-cell lymphoma (DLBCL), primary mediastinal large B-cell lymphoma (PMBCL), mediastinal gray zone lymphoma (MGZL), and T-cell/histiocyte-rich large B-cell lymphoma (THRLBCL). PD-L1 immunohistochemistry (clone 22C3) was performed on formalin-fixed, paraffin-embedded tissue. Tumor cells and intratumoral inflammatory cells were assessed together. Membranous staining was scored as 0, 1+, 2+, or 3+, and the proportion of cells in each category was recorded. Results: PD-L1 staining was present in 40/53 cases (75.5%; 95% CI, 61.7–86.2%). Mean PD-L1-positive percentages were similar in DLBCL, PMBCL, and MGZL (38.81%, 31.67%, and 43.89%, respectively; p = 0.726), and no tested cutoff separated the diagnostic groups. In contrast, MGZL showed a higher mean 3+ component (31.11%) and more cases with 3+ staining in at least 50% of evaluated cells (4/9, 44.4%; 95% CI, 13.7–78.8%) than DLBCL (1/21, 4.8%; 95% CI, 0.1–23.8%) or PMBCL (1/21, 4.8%; 95% CI, 0.1–23.8%); the overall comparison remained significant after Bonferroni correction (unadjusted p = 0.012; adjusted p = 0.049). PD-L1 extent was not associated with age, tumor size, LDH, beta-2 microglobulin, Ann Arbor stage, SUVmax, treatment response, or overall survival. Conclusions: PD-L1 expression is common in mediastinal aggressive large B-cell lymphomas. MGZL showed an exploratory strong-staining pattern that requires independent validation. No prognostic association was detected in this cohort.