Clinical history: 48-year-old male presents with fever, night sweats and an enlarged lymph node in the left groin. An excisional biopsy was performed.
Microscopic description: In this lymph node there is effacement of the nodal architecture by numerous, closely packed follicles of variable sizes and shapes (Fig 1.1). The individual follicles have a poorly defined mantle zone (Fig 1.2) predominantly composed of centrocytes having scant cytoplasm and an irregular-shaped, indented nucleus with uniform chromatin and one to multiple small nucleoli (Fig 1.3). Interspersed within the centrocytes are occasional centroblasts with larger nuclei, vesicular chromatin and one to two large, paracentral nucleoli. The number of centroblasts varies from 1-2 to a maximum of 8 per high power field in some areas. The mantle zone of many of these follicles is further surrounded by a paler appearing rim of predominantly centrocyte-like cells that have more abundant cytoplasm.
Immunophenotyping shows all of the cells to be positive for CD20, CD10, PAX5, bcl2 (Fig 1.3) and bcl6. CD23 highlights the follicular dendritic cells while the infiltrating T cells are highlighted by CD3, CD5 and CD43. MUM1, p53 and cyclin D immunostains are negative. While some germinal centers show focally high proliferative index (~70%), the overall proliferation fraction with Ki67 is low (~20%).