Rare Sinonasal Case of <i>IRF4</i> -Rearranged Large B-Cell Lymphoma Mimicking Chronic Sinusitis

Large B-cell lymphoma (LBCL) with <i>IRF4/</i>MUM1 rearrangement is a rare and newly recognized entity predominantly involving Waldeyer's ring of lymphoid tissue in the oropharynx. Unusual clinical presentations can result in delayed diagnosis of LBCL with <i>IRF4</i> rearrangement, resulting in increased patient morbidity. We present a case involving a pregnant woman with a frontal sinus swelling and clinical diagnosis of chronic sinusitis and possible Pott's puffy tumor. An endoscopic procedure and subsequent histologic examination revealed diffuse LBCL with diffuse <i>IRF4</i>/MUM1 staining, supported by fluorescence in situ hybridization analysis showing an <i>IRF4</i> locus rearrangement at 6p25.3. Subsequent imaging revealed metastatic disease to cervical, thoracic, and lumbar spinal regions. This case highlights the importance of consideration of lymphomas in patients who may clinically appear to have non-neoplastic disease, especially in the head and neck regions.