## Why CD20 positivity confirms B-cell origin and is the primary target for rituximab-based therapy (R-CHOP), which is the first-line treatment for DLBCL is right CD20 is a pan-B-cell antigen expressed on the surface of B-lymphocytes and is a hallmark of B-cell lymphomas including DLBCL. The CD20+ immunoprofile (along with CD19+, CD79a+, and PAX5+) confirms B-cell lineage origin, which is essential for diagnosis. Critically, CD20 positivity is the molecular basis for rituximab, a chimeric monoclonal antibody against CD20, which forms the backbone of R-CHOP (rituximab + cyclophosphamide + doxorubicin + vincristine + prednisone). R-CHOP is the standard first-line treatment for DLBCL and achieves cure rates of 60–70% overall, making CD20 both a diagnostic marker and a therapeutic target. (Harrison 21e, Ch 105) ## Why each distractor is wrong - **CD20 positivity indicates a germinal center B-cell (GCB) subtype with inherently better prognosis and lower risk of CNS involvement**: CD20 is expressed in both GCB and ABC (activated B-cell) subtypes; CD20 positivity alone does not distinguish cell of origin. GCB vs. ABC classification requires gene expression profiling or additional IHC markers (e.g., CD10, BCL6, IRF4) and has prognostic implications, but CD20+ status does not determine subtype or CNS risk. - **CD20 positivity alone is sufficient to diagnose DLBCL without the need for additional FISH studies for MYC/BCL2 rearrangements**: CD20 positivity is necessary but not sufficient for DLBCL diagnosis. FISH for MYC, BCL2, and BCL6 rearrangements is mandatory to identify double-hit or triple-hit lymphomas, which are highly aggressive and require intensified treatment (e.g., DA-EPOCH-R or POLA-RCHP). Omitting FISH risks undertreatment of high-risk disease. - **CD20 positivity predicts resistance to anthracycline-based chemotherapy and mandates immediate CAR-T cell therapy as first-line treatment**: CD20 positivity is the basis for responsiveness to rituximab and anthracycline-based R-CHOP, not resistance. CAR-T cell therapy (axicabtagene, lisocabtagene, tisagenlecleucel) is reserved for relapsed or refractory DLBCL after failure of first-line therapy, not as initial treatment. **High-Yield:** CD20+ B-cell lymphomas are rituximab-sensitive; R-CHOP is standard first-line for DLBCL, but FISH for MYC/BCL2 rearrangements must always be performed to stratify risk and guide intensification. [cite: Harrison 21e, Ch 105]
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