Mediastinal Gray Zone Lymphomas: Diagnostic Challenges, Clinicopathologic Overlap, and Emerging Management Strategies

Background: Mediastinal gray zone lymphoma (MGZL) is a rare B-cell lymphoma characterized by overlapping clinicopathologic and molecular features of primary mediastinal B-cell lymphoma (PMBL) and classical Hodgkin lymphoma (CHL). Under current WHO-HEMA5 and International Consensus Classification (ICC) frameworks, MGZL is restricted to EBV-negative lymphomas arising in the mediastinum. Methods: This review summarizes current evidence on epidemiology, clinical presentation, pathology, molecular characteristics, diagnostic challenges, and therapeutic approaches to MGZL, with data derived from retrospective series, limited prospective cohorts, and recent molecular studies. Results: MGZL predominantly affects young adults and commonly presents with bulky mediastinal disease. Diagnosis is challenging due to transitional morphology, pleomorphic Reed–Sternberg-like cells, and variable expression of B-cell and activation markers. Molecular studies demonstrate shared alterations with PMBL and CHL, including 9p24.1 (JAK2/PD-L1/PD-L2) gains, while additional reported features such as HOXA5 hypomethylation and MYC copy number gains support its biological distinctiveness, although evidence remains limited. Frontline treatment commonly involves intensive chemoimmunotherapy regimens such as DA-EPOCH-R; however, outcomes remain inferior to PMBL and CHL, with 5-year overall survival rates of approximately 40–60%. Relapsed or refractory disease frequently requires salvage chemotherapy and autologous stem cell transplantation. Immune-based therapies, including brentuximab vedotin and PD-1 inhibitors, have shown promising activity, particularly in combination. Conclusions: MGZL remains a diagnostically challenging and therapeutically complex lymphoma with inferior outcomes compared with related mediastinal lymphomas. Advances in molecular profiling and immunotherapy offer promising avenues toward more personalized treatment; however, prospective clinical trials and international collaboration are urgently needed to establish evidence-based management strategies for this rare entity.