A rare case of fluid overload-associated large B-cell lymphoma and antigen loss at relapse.

Fluid overload-associated large B-cell lymphoma (FO-LBCL) is a new addition to the list of large B-cell lymphomas in the 5th edition of the World Health Organization Classification of Haematolymphoid Tumours (WHO-HAEM5). This report presents an unusual case of FO-LBCL with partial B cell antigen loss at relapse and reviews the characteristics, treatment, and prognosis of these patients to enhance our understanding of this disease. Immunophenotyping was performed through immunohistochemistry and flow cytometry. Immunoglobulin gene rearrangements were analyzed using BIOMED-2 multiplex primers. MYC, BCL2, and BCL6 gene rearrangements were detected by fluorescent in situ hybridization (FISH). Cell-free DNA (cfDNA) from pleural effusion and peripheral blood was subjected to somatic mutation evaluation using next-generation sequencing (NGS). In 2020, the patient was initially diagnosed with tuberculous pleurisy and received anti-tuberculous drugs. Subsequent testing of the pleural effusion cell block revealed that the large cells to be positive for CD10, CD20, CD79a, PAX5, MUM1, Bcl-2, Bcl-6, and c-myc and negative for CD3, CD30, Cyclin D1, and HHV8. In situ hybridization for Epstein-Barr virus (EBV)-associated mRNA (EBER-ISH) was negative. Additionally, a clonal rearrangement of immunoglobulin heavy locus (IGH) FR2-JH was detected; the results of MYC, BCL2, and BCL6 gene rearrangements were all negative. Following pleural drainage treatment, the patient achieved symptom remission and was diagnosed with large B-cell lymphoma (HHV8-unrelated PEL-like lymphoma). In 2022, the patient was readmitted due to the recurrence of pleural effusion. The pleural effusion cell block revealed a distinct immunophenotype compared to previous findings, positivity for PAX5 and negativity for CD20, CD10, CD3, CD5, CD30, CD38, CD138, CD79a, MUM1, Bcl-2, Bcl-6, Cyclin D1, c-myc, ALK, and HHV8. Identical IGH FR2-JH clonal rearrangement suggested the recurrence of the original clone. CfDNA analysis showed mutations in CD79B, MYD88, CCND3, and DTX1. The patient was ultimately diagnosed with FO-LBCL based on the WHO-HAEM5 classification. Diagnosis of FO-LBCL should be differentiated from primary effusion lymphoma (PEL). The features of this case align with the description of "FO-LBCL" in WHO-HAEM5 and "HHV8 and EBV-negative primary effusion-based lymphoma" in International Consensus Classification (ICC). FO-LBCL patients generally have a more favorable prognosis compared to PEL. In this case, the patient exhibited a favorable prognosis for over 22 months without additional treatment apart from pleural drainage.