Evaluation of primary mediastinal large B cell lymphoma by flow cytometry

Background Primary mediastinal large B cell lymphoma (PMLBCL) is a B cell non‐Hodgkin lymphoma (B‐NHL) that shows morphologic, immunophenotypic, and genetic similarities to classical Hodgkin lymphoma (CHL). We evaluated the neoplastic and reactive infiltrate of PMLBCL by flow cytometry (FC). Methods A total of 24 cases of PMLBCL were retrospectively characterized using FC combinations for B‐NHL, T‐NHL, and CHL. Results The CHL assay identified the neoplastic population (NP) in all cases, while the B‐NHL assay identified the NP in 18 of 24 cases. In four cases, the neoplastic population was retrospectively identified in the B‐NHL tube, given the CHL tube‐derived immunophenotype. Neoplastic cells of PMLBCL displayed a B cell immunophenotype (CD19/CD20+), with CD40 consistently and brightly expressed. Most cases lacked immunoglobulin light chains, CD10, and CD15; CD30 was frequently expressed. All cases showed expression of CD71 and CD95. The reactive infiltrate in PMLBCL showed increases in T cells relative to reactive tissues. Lower CD4/CD8 ratios in PMLBCL relative to CHL and reactive tissues were also observed. Although not seen as commonly as with CHL, 41% of cases showed a reactive CD3/CD4/CD7bright/CD45bright+ T cell population. Reactive populations seen in PMLBCL were similar to that seen in diffuse large B cell lymphoma, not otherwise specified. Conclusions FC can detect the NP of PMLBCL and a FC assay for CHL performed better than a B‐NHL assay in this regard. The neoplastic cells showed a B cell immunophenotype with over‐expression of CD40. The reactive infiltrate in PMLBCL shows unique features including a prominent CD8+ T cell infiltrate. © 2017 International Clinical Cytometry Society