Unusual Variants of Follicular Lymphoma

Follicular lymphoma (FL) is one of the most frequently diagnosed lymphomas in the United States and Europe. The definition of and basic approach to diagnosis and grading of FL is essentially unchanged in the recently updated revision of the World Health Organization (WHO) classification. FL is a biologically and histopathologically heterogeneous disease. Although there is an improved understanding of some FL variants and specific subtypes, there are cases whose recognition is particularly challenging, either because they have unusual features or represent examples of new or rare variants. Herein, we share a series of unusual and difficult to recognize FLs with the goal of increasing awareness of the expanding histopathologic variability in FL. Unusual FL discussed here include: FL with Castleman-like changes, FL with plasmacytic differentiation, and immunoglobulin G4-positive plasma cells in the setting of immunoglobulin G4-related disease, FL with marginal zone differentiation and involving mucosa-associated lymphoid tissue sites, diffuse FL variant expressing CD23 with STAT6 mutation, large B-cell lymphoma with IRF4 rearrangement, CD10-negative and MUM1-positive aggressive FL, and Epstein-Barr virus–positive FL.