Primary mediastinal large B-cell lymphoma (photos courtesy of Andrew Evans, Department of Pathology, University of Rochester, Rochester, NY). The tumor was a 5.2 cm isolated anterior mediastinal mass from a 23 year old female. (A-B) Tumor cells are medium-to-large in size, with round-to-irregular nuclei, open chromatin, and abundant pale cytoplasm. As commonly seen, tumor cells are infiltrating through a background of compartmentalizing alveolar or “packeted” fibrosis. (C) Membranous CD20 is abundantly and strongly expressed. (D) Remnant thymic epithelium (positive for cytokeratin) is commonly seen at the tumor edge infiltrated by malignant cells.