Bone marrow infiltration by flow cytometry at diffuse large B-cell lymphoma NOS diagnosis implies worse prognosis without considering bone marrow histology.

Background: The significance of discrepant findings between histology (BMB) and flow cytometry (FC) in bone marrow (BM) examination at diffuse large B-cell lymphoma (DLBCL) diagnosis is uncertain.

Methods: We performed a 5-year retrospective single-center study of patients diagnosed by DLBCL not otherwise specified (n = 82), divided into three groups according to BM infiltration at diagnosis: BMB-/FC- (75.6%), BMB+/FC+ (13.4%), and BMB-/FC+ (11%).

Results: Median infiltration by FC analysis of the BMB-/FC+ group was 0.8% and if we considered BM infiltration as positive in all cases, 4/9 would be upstaged. Median follow was 33 months. Event-free survival (EFS) after 18 months was 82, 23, and 27% for BMB-/FC-, BMB-/FC+, and BMB+/FC+, respectively (p < .001). After 18 months of observation, OS was 87, 46, and 55% for BMB-/FC-, BMB-/FC+, and BMB+/FC+, respectively (p = .001). In multivariate analysis (BM infiltration vs. cell-of-origin according to Hans algorithm and standard IPI), BM infiltration was independently associated with EFS (HR: 1.94, 95% CI: 1.3-2.9) and overall survival (HR: 1.69, 95% CI: 1.1-2.7).

Conclusion: In summary, minimal BM infiltration, detected by FC but not by BMB, has same prognostic implications than overt BM infiltration and should be considered as extranodal involvement regardless the infiltration quantity.