Effusion-based lymphoma (EBL) is a rare but distinct entity of large B-cell lymphoma in effusion without association with human herpes virus-8 (HHV-8). Spontaneous regression after pleurocentesis has been observed; but to our knowledge, there are no reports on the morphological and molecular features of subsequent aspirations in regressing cases. Here, we report the case of a 92-year-old male with chronic obstructive pulmonary disease, who presented with right pleural effusion. He had no human immunodeficiency virus, hepatitis B virus, or hepatitis C virus infection, and CT scans revealed no mass lesion. The first pleural effusion aspiration cytology revealed large lymphoma cells with vesicular nuclei, irregular nuclear contours, and prominent nucleoli, consistent with EBL. The second aspiration cytology showed a few slightly enlarged lymphocytes in a background of small lymphocytes. Immunohistochemical study on cell block of the second aspiration revealed equal amounts of CD3-positive and CD20-positive cells. All these cells on the section tested negative for HHV-8 through immunohistochemistry and Epstein-Barr virus by in situ hybridization. Our initial impression was EBL in regression. However, flow cytometric immunophenotyping showed monotypic light chain expression of the gated B-cells. B-cell receptor gene rearrangement study showed a clonal result. Furthermore, fluorescence in situ hybridization revealed rearrangement of IGH gene. The diagnosis of the second aspiration was EBL with morphological regression but retained clonal genetic features. The patient passed away one month after diagnosis without chemotherapy. This case illustrated the importance of ancillary studies in confirming the clonal nature of a morphologically regressing EBL.
