Track

Case Reports

Abstract

Large B-cell lymphoma with IRF4 rearrangement (LBCL-IRF4) is a novel provisional entity in the WHO Classification of Tumors of Hematopoietic Neoplasms (revised 4th edition, 2017). LBCL-IRF4 is a rare type of large-B cell lymphoma that accounts for 0.05% of cases and is commonly reported in children and young adults with a favorable prognosis. In this population, LBCL-IRF4 has a predilection for the head and neck lymph nodes, tonsillar and mucosal-associated lymphoid tissue, and the bowel. IRF4 rearrangements, however, can also be present in adult-onset LBCL with a unique and poorly investigated clinicopathologic and immunophenotype profile. We report an unusual case of LBCL-IRF4 in a 59-year-old male who presented with a firm violaceous subcutaneous nodule on the posterior neck. Microscopic examination demonstrated a dense infiltrate of medium to large lymphocytes occupying the subcutaneous fat without significant involvement of the epidermis or dermis. The atypical lymphocytes were positive on immunohistochemistry for CD20, BCL2, BCL6, MUM1, and CD10 (focally), with a high proliferative index, suggesting diffuse large B-cell lymphoma (DLBCL) NOS or leg type and follicular large cell lymphoma. The clinical setting, in combination with the immunophenotype, prompted fluorescence in situ hybridization (FISH) analysis. The FISH analysis confirmed IRF4 rearrangement and was negative for MYC and BCL2 rearrangements, allowing for the diagnosis of the provisional entity LBCL-IRF4. This rare case of adult-onset LBCL-IRF4 with an atypical anatomic location and morphology broadens the spectrum of LBL-IRF4 presentation and highlights the importance of FISH investigation in adult cases of large B-cell lymphomas.