Low-grade B-cell lymphoma with immunoglobulin (IG) and interferon regulatory factor 4 (IRF4) gene rearrangement is extremely rare, with only 4 cases being previously reported. In this article, we report one additional case that arises from the skull and review the literature. The patient was a 69-year-old man who presented with recurrent and disabling vertigo and was found to have a 5.0 × 1.7 cm lesion within the left posterior parietal bone. Histological examination revealed a bone lesion with diffuse lymphoid infiltrate comprising of mostly small lymphocytes with scant cytoplasm, slightly irregular nuclei and inconspicuous nucleoli, and scattered larger cells resembling prolymphocytes and paraimmunoblasts. Immunohistochemical studies showed that the neoplastic cells were positive for CD20, CD79a, PAX5, CD23, CD43, BCL-2, BCL-6, MUM-1, LEF-1, and IgM and negative for CD5, CD10, cyclinD1, SOX11, and IgD. Flow cytometric analysis identified CD5 negative and CD10 negative monoclonal B cells with lambda light chain restriction. Fluorescence in situ hybridization analysis revealed del(13q) abnormality, but was negative for IGH/BCL2, IGH/CCND1, and BIRC3/MALT1 translocations. Next-generation sequencing identified IGK-IRF4 rearrangement and BRD4 E1113 del abnormalities. Given a low clinical stage (IE) of the disease, the patient did not receive additional treatments and was free of disease at 1 year after the diagnosis.
Keywords: IGLK-IRF4 rearrangement; bromodomain containing protein 4; low-grade B-cell lymphoma; non-Hodgkin lymphoma.