Abstract
Flow cytometry is widely used in the diagnosis of lymphoma. In B-cell lymphoma, light chain restriction (LCR) is especially useful for distinguishing B-cell malignancy from a reactive lymphoid proliferation. Usually the neoplastic B-cell population predominates, and monoclonality is conspicuous as demonstrated by distinct LCR. However, if the specimen contains a large reactive component, especially polyclonal B-cells, a small number of neoplastic B-cells will be buried in the background, thus creating a diagnostic challenge. We retrospectively analyzed three/four-color flow cytometry by examining 12 cases of B-cell lymphoma with a small proportion of neoplastic B-cells hidden in polyclonal B-cells and other intense reactive components. These 12 cases (9 FNA, 1 BF, 2 tissues) comprised 7% of specimens diagnostic or suspicious for lymphoma by flow cytometry at our institution. Eight cases were diffuse large B-cell lymphoma (DLBCL) (18.6% of DLBCL and 27.6% of DLBCL in FNA/BF) while four were lymphoma of other types (3% of non-DLBCL). In all the cases, LCR was obscured by polyclonal B-cells (8/12) or absence of surface immunoglobulin (sIg) in neoplastic B-cells (4/12), and morphology resembled a reactive picture. The clues by flow cytometry prompting further analysis included increased forward angle light scatter(FSC)/side angle light scatter(SSC) (12/12), brighter CD20 (6/12), dim CD20 (1/12), absence of sIg (4/12), and a distinct population of B-cells co-expressing CD10 (2/12), CD5 (1/12), CD2 (1/12), CD23 (4/12) or CD38 (1/12). By backgating/regating suspected subpopulations, the concealed neoplastic B-cells were demonstrated by enriched LCR (5 to10 fold) or tight clustering on FSC/SSC. In conclusion, cryptic neoplastic B-cells, especially diffuse large B-cell lymphoma in FNA/BF, can be extracted, according to their altered immunophenotypic features, from background polyclonal B-cells and reactive T-cells via manipulation of gating strategies.
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