Pathologic features of mediastinal large-cell lymphoma. (A) H&E histologic section of a MLBL tumor showing the characteristic large-cell infiltrate with intervening bands of compartmentalizing fibrosis. The tumor cells form individual cells and small groups (400× magnification). (B) CD20 immunoperoxidase staining demonstrating the mature MLBL infiltrate (1000× magnification). (C) IRF4/MUM 1 staining of a MLBL tumor showing characteristic nuclear staining (100× magnification; Olympus BX40 microscope).