Large B-cell Lymphoma With Interferon Regulatory Factor 4 Rearrangement Presenting as a Primary Central Nervous System Lymphoma in the Brain

Large B-cell lymphoma (LBCL) with interferon regulatory factor 4 (<i>IRF4</i>)<i> </i>rearrangement (LBCL-<i>IRF4</i>r) is a rare type of lymphoma with an overall favorable prognosis, and has been included as a distinct entity in the 2022 revision of the World Health Organization Classification of Lymphoid Neoplasms. More common in pediatric/young adult populations and rare in older adults, it typically presents in the head and neck, specifically, the Waldeyer ring, and rarely in other sites. To our knowledge, it has not been reported as a primary central nervous system (CNS) lymphoma in the English language medical literature. Although <i>IRF4</i> rearrangement can be seen in other hematolymphoid neoplasms, their histology and immunophenotype differ greatly. Familiarity with such unusual entities is important for pathologists in the work-up and diagnosis of hematolymphoid neoplasms of the CNS and is especially critical with this entity due to overall better prognosis and possible targeted treatments in the future. Here, we present a case of LBCL-<i>IRF4</i>r presenting as a primary CNS lymphoma in the brain.