Diagnosis and Classification of Follicular Lymphoma and Related Entities

Follicular lymphoma (FL) is a mature B cell neoplasm classically characterized by B cells harboring the t(14;18) IGH::BCL2 leading to the overexpression of BCL2 in most cases. Conventional FL occurs in lymph nodes and typically shows a follicular B-cell proliferation expressing at least one germinal center marker. Two early lesions closely related to conventional FL are recognized as variants, namely in situ follicular neoplasia (ISFN), and duodenal-type follicular lymphoma (DTFL). FL lacking BCL2 rearrangement ( BCL2 -R negative) accounts for around 10% to 15% of FLs and constitutes a heterogeneous group of FLs. Most of these alternative forms of FL are considered as distinct entities separate from conventional FL in the 2022 International Consensus Classification. This review aims to summarize the key pathologic and diagnostic features of FL conventional and its alternative forms as well as further emphasize the increasing role of molecular studies in the diagnostic work-up.