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An 86-year-old-woman was diagnosed with mucosa-associated lymphoid tissue (MALT) lymphoma involving the stomach in 2014 and treated with bendamustine-rituximab therapy. She presented in 2022 with right renal lesion and subcarinal lymphadenopathy (SLN). The hematoxylin and eosin (panel A; total magnification ×200) and immunostains (immunohistochemistry [IHC]) from renal biopsy (RB) showed recurrent MALT lymphoma positive for CD20 and BCL-2, and negative for CD5. Flow cytometry immunophenotyping (FCI) from RB showed a large population of CD10⁺κ⁺ B cells (panel B) with immunophenotype (CD5^–CD10⁺CD19⁺CD20⁺CD22⁺CD43^–), identical to concurrent SLN, except CD10 was negative in the latter (panel C). In contrast to FCI, CD10 IHC was strongly positive in renal tubules (panel D; total magnification ×400); however, lymphoma cells were negative.

CD10 is an important marker for determining cell of origin (COO) in B-cell lymphomas. Unexpected CD10 positivity by FCI in RB created a diagnostic conundrum regarding classification. CD10/neprilysin is normally expressed on renal tubules, an endopeptidase released from cell surface by ectodomain shedding. CD10 shedding by renal tubules and its detection on lymphoma cells by FCI, and not by IHC, contributed to false-positive (FP) result. CD10 FP expression in background polytypic B cells (panel B; blue arrow) and T cells (not shown) by FCI, CD10-negative IHC in lymphoma cells, negative IGH::BCL2 fluorescence in situ hybridization, concurrent SLN FCI result, and history helped to establish a correct diagnosis. Awareness of this diagnostic pitfall can prevent incorrect COO assignment in lymphomas involving kidney.

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