Spatially and Clonally Multiple B‐/Plasma Cell Proliferations in T Follicular Helper Cell Lymphoma: A Case Report

T follicular helper cell lymphoma (TFHL) is a systemic T-cell lymphoma with a phenotype reminiscent of TFH cells and frequent RHOA p.G17V mutation. Here, we report a case of TFHL with spatially multiple and clonally independent B-/plasma cell proliferations (B/PC-Ps), which posed diagnostic difficulties. An 87-year-old female presented with a skin rash, and a skin biopsy from the chest was initially diagnosed as low-grade B-cell lymphoma with plasmacytic differentiation. Three months after rituximab monotherapy, bilateral pleural effusions developed. Cytology of the pleural effusion revealed abnormal lymphocytes and plasma cells. The abnormal T-cells were positive for CD3, CD4, CD10, and PD1, while the neoplastic plasma cells showed kappa-predominant light-chain restriction. Additional cutaneous biopsies from the left forearm and upper arm revealed Epstein-Barr virus-positive and -negative B/PC-Ps, respectively. Polymerase chain reaction identified RHOA p.G17V and the same monoclonal rearrangement of T-cell receptor gamma gene (TRG) in all the four specimens. In addition, three cutaneous lesions showed distinct monoclonal immunoglobulin heavy chain gene (IGH) rearrangements. This case demonstrates phenotypic and clonal diversity of monoclonal B/PC-Ps in TFHL, which can obscure the neoplastic TFH proliferation. In such cases, careful clinicopathological correlation and the integration of immunophenotypic and genetic analyses may aid in the diagnosis.