A 38-year-old woman presents with a 2-month history of painless lymphadenopathy involving cervical and axillary nodes. Physical examination reveals firm, rubbery, non-tender lymph nodes. Complete blood count shows mild lymphocytosis. Chest X-ray reveals mediastinal widening. Which investigation is most appropriate to establish a definitive diagnosis?

## Diagnosis of Non-Hodgkin Lymphoma: Investigation of Choice ### Clinical Context The patient presents with constitutional features suggestive of lymphoma: painless lymphadenopathy, mediastinal involvement, and systemic symptoms. While imaging and laboratory tests provide supportive evidence, **tissue diagnosis is mandatory** before initiating treatment. ### Why Excisional Lymph Node Biopsy is the Gold Standard **Key Point:** Excisional (complete) lymph node biopsy with immunohistochemistry is the investigation of choice for definitive diagnosis of non-Hodgkin lymphoma (NHL). Excisional biopsy offers: 1. **Architectural preservation** — allows assessment of nodal architecture, which is critical for NHL classification (follicular vs. diffuse patterns) 2. **Immunophenotyping** — flow cytometry and immunohistochemistry identify B-cell or T-cell lineage and specific markers (CD20, CD5, CD10, etc.) 3. **Cytomorphology** — permits accurate histological grading and subtyping (e.g., follicular lymphoma grade, DLBCL vs. Burkitt) 4. **Clonality assessment** — identifies monoclonal lymphoid populations ### Role of Other Investigations | Investigation | Role | Limitation | |---|---|---| | Flow cytometry (peripheral blood) | Identifies immunophenotype; useful for leukemic phase | Cannot assess nodal architecture; misses tissue-based diagnosis | | CT chest/abdomen | Staging after diagnosis confirmed | Does not establish diagnosis; used for staging (TNM) | | Serum LDH, β2-microglobulin | Prognostic markers; assess tumor burden | Not diagnostic; supportive only | | Core needle biopsy | Acceptable if excisional biopsy not feasible | Lower diagnostic yield than excisional; less architectural detail | **Clinical Pearl:** Fine-needle aspiration (FNA) is inadequate for NHL diagnosis because it cannot preserve nodal architecture and does not allow adequate immunophenotyping. **High-Yield:** The WHO classification of NHL requires integration of morphology, immunophenotype, and clinical features — all best obtained from excisional biopsy. ### Staging After Diagnosis Once NHL is confirmed histologically, staging investigations (CT, PET-CT, bone marrow biopsy) are performed to determine extent of disease and guide therapy. [cite:Robbins 10e Ch 13]