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A 51-year-old woman with Castleman disease presented with fatigue, dizziness, lymphadenopathy, and pancytopenia. A bone marrow smear and flow cytometry showed no abnormalities. Histopathologic examination of the left cervical and inguinal lymph nodes was consistent with multicentric Castleman disease with interfollicular infiltration of small-to-medium lymphocytes without atypia (panel A-B; hematoxylin and eosin stain of the inguinal lymph node biopsy; 20× objective for panel A; 40× objective for panel B). Immunohistochemistry revealed that these lymphocytes expressed CD3 (panel C; 40× objective), CD5, CD4, CD8, partial CD99× (panel D; 40× objective), and partial terminal deoxynucleotidyl transferase (panel E; 40× objective). Flow cytometric evaluation of the biopsy samples revealed ∼40% immature T lymphoblasts that exhibited a cortical thymocyte-like immunophenotype (cyCD3⁺sCD3partial⁺CD4⁺CD8⁺CD1a⁺CD5dim⁺CD7dim⁺CD2⁺CD99⁺). Polytypic expression of surface (s) T-cell receptor β chain constant region 1 (TRBC1)/sTRBC2 and cytoplasmic (cy) TRBC1/cyTRBC2 were observed in sCD3-positive lymphoblasts and all cyCD3-positive lymphoblasts, respectively (panel F). No clonal T-cell receptor gene rearrangement was detected by polymerase chain reaction. Collectively, these findings established a diagnosis of indolent T-lymphoblastic proliferation (iT-LBP) associated with multicentric Castleman disease. The patient received siltuximab-based therapy for more than 1 year without disease progression.

iT-LBP is a rare entity with immunophenotypes that resemble an intermediate stage of thymocyte development and mimic cortical T-lymphoblastic leukemia/lymphoma. To date, no previous studies have documented the expression pattern of TRBC1/TRBC2 in iT-LBP. This case highlights the use of flow cytometry–based evaluation of TRBC1/TRBC2 to confirm T-cell polyclonality and identify iT-LBP.

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