Which single feature most reliably distinguishes Follicular Lymphoma from Marginal Zone Lymphoma (MZL)?

Explanation

## Distinguishing Follicular Lymphoma from Marginal Zone Lymphoma ### Cytogenetic Hallmark **Key Point:** The t(14;18)(q32;q21) translocation involving BCL2 and the immunoglobulin heavy chain (IGH) locus is present in 85–90% of follicular lymphomas and is virtually absent in marginal zone lymphomas. ### Comparative Table | Feature | Follicular Lymphoma | Marginal Zone Lymphoma | |---------|---------------------|------------------------| | **Cytogenetics** | t(14;18) BCL2/IGH (85–90%) | No t(14;18); may have t(11;18) or other | | **CD10 expression** | Positive (germinal center B-cell) | Negative | | **BCL2 protein** | Overexpressed (anti-apoptotic) | Normal | | **Morphology** | Follicular/nodular pattern | Marginal zone infiltration | | **Primary site** | Lymph nodes (nodal) | Extranodal (MALT, splenic, nodal) | | **Grade** | I–III (WHO) | Indolent (no grading) | | **Median survival** | 8–10 years (Grade I–II) | 5–10 years | ### Why t(14;18) is the Gold Standard Discriminator **High-Yield:** The t(14;18) translocation: 1. **Juxtaposes BCL2** (an anti-apoptotic gene on chromosome 18) to the IGH enhancer on chromosome 14. 2. **Causes constitutive BCL2 overexpression**, preventing apoptosis and enabling lymphoma development. 3. **Is detected by karyotyping, FISH, or PCR** and is part of WHO diagnostic criteria for follicular lymphoma. 4. **Is essentially absent in MZL** (which may harbor t(11;18) in MALT lymphomas, a different translocation). **Clinical Pearl:** Follicular lymphoma is the most common indolent lymphoma in Western countries. The presence of t(14;18) can be used for minimal residual disease (MRD) monitoring and predicts response to rituximab-based therapy. ### Why Other Features Are Not Discriminators **Warning:** Both follicular lymphoma and MZL are indolent, have prolonged survival, and may show CD10+ or CD10− phenotypes depending on subtype. Extranodal involvement is more typical of MZL but can occur in follicular lymphoma. The morphologic distinction (germinal center vs. marginal zone) requires expert histology and is not as objective as cytogenetics.