Large B-cell lymphoma with IRF4 rearrangement: a multi-centric study with focus on potential misleading phenotypes

: Large B-cell lymphoma with IRF4 rearrangement (LBCL-IRF4) is a rare lymphoid neoplasm, usually occurring in the pediatric/young-adult age. Despite this, subsets of cases occur in elderly patients and express CD5, possibly entering the differential diagnosis with adult aggressive lymphomas, such as blastoid/pleomorphic mantle cell lymphoma (MCL-B/P). To better characterize the clinical-pathological features and differential diagnosis of LBCL-IRF4, we conducted a multi-centric study on 12 cases, focusing on CD5, Cyclin D1, and SOX11 expression. While most cases had typical presentation, adult-to-elderly age at diagnosis and unusual anatomic locations were reported in 3/12 (25.0%) and 2/12 (16.7%) patients, respectively. Histologically, CD5 was positive in 4/12 (33.3%) cases, Cyclin D1 was invariably negative, and SOX11 was weakly/partially expressed in 1/12 (8.3%) case. In conclusion, LBCL-IRF4 can have unconventional clinical presentations that may challenge its recognition. Although CD5 is frequently expressed, negativity for Cyclin D1 and SOX11 contributes to the differential diagnosis with MCL-B/P.