## Diffuse Large B-Cell Lymphoma (DLBCL): Comprehensive Overview ### Definition & Epidemiology **Key Point:** DLBCL is the most common type of non-Hodgkin lymphoma, accounting for ~30–40% of all NHL cases. It is an aggressive B-cell lymphoma with a median age of presentation around 60 years but can occur at any age. ### Histopathology & Immunophenotype **High-Yield:** DLBCL shows diffuse infiltration of large neoplastic B cells with abundant cytoplasm and prominent nucleoli. Immunophenotype: CD19+, CD20+, CD79a+, surface immunoglobulin+, CD5-negative (typically), CD23-negative (typically). **Clinical Pearl:** CD30 positivity can occur in DLBCL but does NOT indicate transformation from Hodgkin lymphoma. CD30 is a marker of activation and can be seen in various aggressive lymphomas. The presence of CD30 in DLBCL does not have the same prognostic or diagnostic implications as in primary cutaneous lymphomas or Hodgkin lymphoma. ### Molecular Subtypes: GCB vs. ABC **Mnemonic:** **GCB vs ABC** — Gene expression profiling divides DLBCL into two subtypes: - **Germinal Center B-cell (GCB) type**: Derived from germinal center B cells; better prognosis; associated with t(14;18) and BCL2 rearrangement - **Activated B-cell (ABC) type**: Derived from post-germinal center B cells; worse prognosis; associated with NF-κB pathway activation and mutations in CARD11, MYD88, and CD79B This classification is based on gene expression profiling (Hans algorithm or other methods) and has prognostic significance. ### Genetic Alterations **Key Point:** Common genetic abnormalities in DLBCL include: - **TP53 mutations** (~25% of cases): Associated with poor prognosis - **MYC rearrangements** (~10–15% of cases): Can occur alone or with BCL2/BCL6 rearrangements ("double-hit" or "triple-hit" lymphomas) - **BCL2 rearrangements** (~20% of cases): Often associated with GCB subtype - **BCL6 rearrangements** (~10% of cases) ### Primary Mediastinal DLBCL (PMBL) **High-Yield:** PMBL is a distinct subtype of DLBCL arising in the mediastinum, more common in young women. Key features: - **REL gene amplification** and **PD-L1/PD-L2 overexpression** are characteristic - Often presents with a large mediastinal mass - May have sclerosis mimicking classical Hodgkin lymphoma - Better prognosis than other DLBCL subtypes with modern chemotherapy ### Prognostic Factors **Clinical Pearl:** International Prognostic Index (IPI) score incorporates: - Age > 60 years - Elevated LDH - ECOG performance status ≥ 2 - Stage III–IV disease - Extranodal involvement > 1 site Gene expression profiling (GCB vs. ABC) and presence of MYC/BCL2 rearrangements also influence prognosis. ### Summary Table: Key Features of DLBCL Subtypes | Feature | GCB-DLBCL | ABC-DLBCL | PMBL | | --- | --- | --- | --- | | **Origin** | Germinal center B cells | Post-GC B cells | Mediastinal B cells | | **Prognosis** | Better | Worse | Intermediate–good | | **Common alterations** | BCL2, BCL6 | MYD88, CD79B | REL, PD-L1/L2 | | **NF-κB activation** | Low | High | Moderate | | **Presentation** | Nodal/systemic | Nodal/systemic | Mediastinal mass | ## Why Option 3 (CD30 Positivity Indicates Hodgkin Transformation) Is Wrong **Warning:** CD30 positivity in DLBCL does NOT indicate transformation from Hodgkin lymphoma. CD30 is an activation marker that can be expressed in various aggressive lymphomas, including some cases of DLBCL. Transformation from Hodgkin lymphoma to DLBCL is a rare event and is diagnosed by clinical history and morphologic evidence of both diseases, not by CD30 positivity alone. CD30 expression in DLBCL is not a marker of poor prognosis in the same way that it is in primary cutaneous lymphomas. The presence of CD30 in this patient's DLBCL is simply a marker of B-cell activation and does not change the diagnosis or prognostic implications compared to CD30-negative DLBCL.
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