Assessment of BCL ‐2 expression in bone marrow by flow cytometry to aid in the diagnosis and differential diagnosis in patients with follicular lymphoma

Follicular lymphoma (FL) is a common indolent non-Hodgkin's lymphoma (NHL), with 40%-70% of newly diagnosed patients presenting bone marrow infiltration (BMI). Accurate diagnosis and differential diagnosis based on bone marrow are particularly important. High BCL-2 expression is a characteristic feature of FL, but the utility of flow cytometry (FCM) for evaluating BCL-2 expression in bone marrow aspirates has not been well established. Two hundred and sixty-five FL patients were enrolled and divided into BD cohort and BECKMAN cohort (new diagnosed: n = 230; relapsed/refractory: n = 35). BCL-2 expression was investigated using the BCL-2 mean fluorescence intensity (MFI), BCL-2 percentage, and BCL-2:CD200 ratio. Patients with FCM-detectable clonal B cells (FCM (+)) were compared with those without detectable involvement (FCM (-)) for immunophenotypic features and progression-free survival (PFS). FCM (+) patients showed a characteristic immunophenotype, including CD10 (+), CD19 (dim+), CD20 (dim+), CD22 (dim+), CD200 (-). Compared with FCM (-) patients, FCM (+) patients had higher BCL-2 MFI, BCL-2 percentage, and BCL-2:CD200 ratio, and significantly shorter PFS. Among the three BCL-2-related parameters, BCL-2 percentage showed the highest area under the curve for discriminating FCM (+) from FCM (-) cases, followed by the BCL-2:CD200 ratio and BCL-2 MFI. Notably, among FCM (+) patients, higher BCL-2 expression was associated with longer PFS. Among FCM (+) patients, BCL-2 expression was higher in FL than other indolent NHLs (CLL: n = 6, MZL: n = 4, and MCL: n = 3). The results were all consistent between the BD cohort and BECKMAN cohort. We confirmed that detecting BCL-2 expression levels in the bone marrow aspirate of FL patients by FCM can aid in the diagnosis of FL, potentially assisting in distinguishing FL from other indolent NHLs, and exploratory assessment of PFS.