Non-Hodgkin Lymphoma — Pathology

Chromosomal Translocations and Oncogenic Activation

• t(14;18) translocation in follicular lymphoma juxtaposes the BCL2 gene (chromosome 18) to the immunoglobulin heavy chain locus (chromosome 14), resulting in constitutive BCL2 overexpression. BCL2 protein inhibits the intrinsic apoptotic pathway by preventing mitochondrial outer membrane permeabilization and cytochrome c release, allowing malignant B cells to survive despite accumulation of additional genetic damage. This translocation is present in 85-90% of follicular lymphomas and explains their typically indolent behavior despite being incurable with conventional therapies.
• t(8;14) translocation in Burkitt lymphoma places the MYC oncogene (chromosome 8) under control of the immunoglobulin heavy chain enhancer, driving uncontrolled proliferation and a high Ki-67 proliferation index (>95%). MYC encodes a transcription factor regulating cell cycle progression, metabolic reprogramming, and ribosomal biogenesis, resulting in aggressive disease with rapid doubling time.
• t(11;14) translocation in mantle cell lymphoma activates CCND1 (cyclin D1), promoting G1/S cell cycle transition through CDK4/6 complex formation, resulting in intermediate-grade malignancy with poor prognosis.

Loss of Tumor Suppressors and Apoptotic Regulation

• TP53 mutations occur in 5-10% of NHLs overall but in up to 30% of diffuse large B-cell lymphomas (DLBCL), particularly germinal center B-cell (GCB) subtype. Loss of p53 function eliminates DNA damage checkpoints and apoptotic responses, conferring resistance to chemotherapy and radiation. TP53-mutated lymphomas manifest as chemorefractory disease with poor outcomes.
• PTEN loss in DLBCL and follicular lymphoma disrupts the PI3K/AKT/mTOR survival pathway, whereas FOXO1 mutations impair forkhead box protein O1-mediated apoptosis. These alterations enhance phosphatidylinositol 3-kinase (PI3K) signaling, promoting cell survival and proliferation.

B-Cell Receptor (BCR) Signaling Activation

• Constitutive BCR signaling drives proliferation and survival in lymphomas through activation of downstream effectors including SYK, BTK, and CARD11. Activated B-cell-like (ABC) subtype DLBCL exhibits chronic active BCR signaling (with or without oncogenic mutations in CARD11, MYD88, or CD79B), making these lymphomas responsive to BTK inhibitors like ibrutinib. This pathway contrasts with GCB-type DLBCL, which depends on PI3K/mTOR signaling.

Epigenetic Dysregulation

• CREBBP and EP300 mutations in follicular lymphoma and DLBCL disrupt histone acetylation and chromatin remodeling, altering gene expression patterns that favor survival. Mutations in histone methyltransferases (EZH2, KMT2D) further dysregulate developmental programs.

Microenvironmental Adaptation

• Malignant cells actively reshape the tumor microenvironment by recruiting follicular helper T cells (TFH), regulatory T cells (Tregs), and tumor-associated macrophages (TAMs) through chemokine secretion. These immune cells secrete IL-10 and TGF-β, creating an immunosuppressive niche. The stromal cells and extracellular matrix provide survival signals through CD40-CD40L interactions and additional cytokine signaling, protecting lymphoma cells from chemotherapy-induced apoptosis and accounting for variable treatment responses.

Virus-Associated Lymphomagenesis

• Epstein-Barr virus (EBV) drives lymphomas through LMP1 and EBNA1 proteins that activate NF-κB and PI3K/AKT pathways, promoting survival. EBV+ DLBCL and primary CNS lymphoma in HIV patients exemplify this mechanism.
• Human T-cell leukemia virus type 1 (HTLV-1) Tax protein directly activates NF-κB and inactivates TP53, driving adult T-cell leukemia/lymphoma (ATLL).
• Hepatitis C virus (HCV) causes chronic antigenic stimulation of B cells, leading to lymphoproliferation and marginal zone lymphoma development.

Immunosuppression and HIV Infection

Chronic Antigenic Stimulation

Viral Infections

Genetic Predisposition and Familial Risk

Environmental Exposures

Inflammatory Conditions and Medications

B Symptoms and Constitutional Signs

Lymphadenopathy

Abdominal and Mediastinal Involvement

Extranodal Involvement

• CNS involvement (leptomeningeal or parenchymal) in 5-10% overall, more common in high-grade lymphomas and HIV+ patients, presenting with headache, altered mental status, cranial nerve palsies, or myelopathy
• Gastrointestinal involvement (10-15%, especially MALT lymphoma, Burkitt lymphoma) causes abdominal pain, nausea, obstruction, or hemorrhage
• Bone involvement causes lytic or sclerotic lesions with bone pain and fracture risk
• Testicular involvement in 2-5% (often bilateral at diagnosis or relapse in 25% of cases) presents with scrotal mass or enlargement
• Cutaneous involvement in mycosis fungoides and other cutaneous T-cell lymphomas manifests as patches, plaques, or tumors progressing through patch, plaque, and tumor stages

Laboratory Manifestations

• Anemia (Hgb <10 g/dL in 25-30%) results from bone marrow involvement, chronic disease, hemolysis (autoimmune hemolytic anemia occurs in ~5%), or chemotherapy
• Thrombocytopenia indicates marrow infiltration and correlates with advanced disease
• Lymphocytosis or circulating lymphoma cells in peripheral blood, particularly in lymphocytic lymphomas and leukemias
• Elevated LDH (>normal in 30-50% of patients) reflects tumor burden and cell turnover; LDH >3× upper limit of normal indicates very high disease burden
• Hypercalcemia (5-10% of cases) from osteoclast-activating factor secretion or calcitriol production in granulomatous lymphomas
• Hyperuricemia and hyperphosphatemia from rapid cell turnover, particularly in highly proliferative lymphomas

Disease-Specific Presentations

• Waldeyer's ring involvement (palatine, pharyngeal, lingual tonsils) causes throat pain, dysphagia, and obstructive sleep apnea
• Burkitt lymphoma presents acutely with rapidly enlarging abdominal mass, often with obstruction or perforation; can rapidly evolve to leukemia
• Lymphoblastic lymphoma presents with mediastinal mass, dyspnea, and can rapidly progress to acute lymphoblastic leukemia
• Cutaneous T-cell lymphoma (mycosis fungoides) initially mimics eczema or dermatitis, evolving from patches to plaques over months to years
• Angioimmunoblastic T-cell lymphoma presents with rash, hepatosplenomegaly, and severe autoimmune phenomena (hemolytic anemia, thrombocytopenia)
• Primary CNS lymphoma in HIV presents with focal neurologic deficits, confusion, or seizures

Physical Examination Findings

• Rubbery, matted lymphadenopathy with possible fixed nodes suggesting extra-nodal extension
• Hepatomegaly and splenomegaly (concordantly enlarged in advanced disease)
• Skin involvement: patches, plaques, or nodules in cutaneous lymphomas
• Testicular mass or enlargement (often asymmetric)
• Signs of superior vena cava syndrome: facial plethora, venous distension, arm edema with mediastinal masses
• Absence of tender nodes or lymph node mobility with antecedent infection (distinguishing feature from reactive adenopathy)

Clinical Suspicion and Diagnostic Approach

Tissue Diagnosis (Excisional Lymph Node Biopsy)

• Routine histopathology with H&E staining to assess lymphoid architecture, growth pattern (diffuse vs. nodular), and cytologic grade
• Flow cytometry to identify abnormal populations (e.g., CD5+/CD23+ B cells in CLL, CD10+/BCL6+ in follicular lymphoma)
• Immunohistochemistry (IHC) with antibodies against B-cell markers (CD20, CD79a), T-cell markers (CD3, CD4, CD8), light chains (kappa, lambda), proliferation marker Ki-67, and subtype-specific markers (CD10, BCL2, BCL6 for GCB vs. ABC classification in DLBCL)
• Cytogenetics/FISH for t(14;18), t(8;14), t(11;14), and other translocations when appropriate
• Molecular studies: clonality assessment by PCR of immunoglobulin heavy chain (IGH) or T-cell receptor (TCR) genes; mutational studies (TP53, NOTCH1, SF3B1, etc.) in selected cases
• EBV status (by in situ hybridization for EBER or immunostaining for LMP1) in aggressive lymphomas

Stabilise first — treat the emergency, not the histology

• Tumor lysis syndrome (TLS) prophylaxis: aggressive IV isotonic fluids plus a xanthine oxidase inhibitor (allopurinol) for standard risk, or recombinant urate oxidase (rasburicase) for high tumor burden (Burkitt, lymphoblastic, bulky DLBCL, LDH >3× ULN). NCCN B-Cell Lymphomas guidance ties prophylaxis intensity to proliferation rate and baseline renal function. Avoid urine alkalinisation, which precipitates calcium phosphate.
• Airway compromise, SVC syndrome, or cord compression: obtain tissue before steroids if at all safe, since corticosteroids are lympholytic and can render the biopsy non-diagnostic. If the patient is unstable, treat first.

First-line therapy by subtype (NCCN)

• DLBCL: anti-CD20 monoclonal antibody + anthracycline-based chemoimmunotherapy — R-CHOP (rituximab, cyclophosphamide, doxorubicin, vincristine, prednisone) for six cycles. Polatuzumab vedotin-containing regimens (Pola-R-CHP) are an option for higher-risk IPI disease. CNS prophylaxis with high-dose methotrexate is considered for high CNS-IPI, testicular, or paravertebral involvement.
• Follicular lymphoma: observation for asymptomatic low-burden disease — it is indolent and incurable, so early treatment does not prolong survival. Treat for bulk, cytopenias, or symptoms with rituximab monotherapy, bendamustine-rituximab, or R-CHOP; rituximab maintenance may follow.
• Burkitt lymphoma: dose-intensive regimens with mandatory intrathecal CNS prophylaxis (DA-EPOCH-R or CODOX-M/IVAC). R-CHOP alone is inadequate.
• Gastric MALT lymphoma: ***H. pylori* eradication** with a proton pump inhibitor–based regimen (ACG guidance) induces remission in most localised cases — antibiotics as cancer therapy.

Escalation

• Relapsed/refractory DLBCL: platinum-based salvage (R-ICE, R-DHAP) with autologous stem cell transplant if chemosensitive; CD19-directed CAR T-cell therapy (axicabtagene ciloleucel) is now used in the second line for early relapse or primary refractory disease. Bispecific T-cell engagers and BTK inhibitors are later options.

Contraindicated/avoid

• Surgery is diagnostic, not curative — no debulking resection.
• Screen HBsAg/anti-HBc before rituximab; unscreened use risks fatal reactivation.
• Anthracyclines in significantly reduced LVEF; live vaccines during anti-CD20 therapy.

Disease-related — emergencies first

• Tumor lysis syndrome (EMERGENCY): massive cell turnover releases intracellular contents — hyperkalemia, hyperphosphatemia, hyperuricemia, and hypocalcemia (phosphate binds calcium). Signalled by rising creatinine and potassium within 12–72 hours of starting therapy; can occur spontaneously in Burkitt lymphoma. Arrhythmia and urate/calcium-phosphate nephropathy are the killers.
• Spinal cord compression (EMERGENCY): epidural or paravertebral mass; back pain preceding weakness, sensory level, urinary retention. MRI and dexamethasone immediately.
• Superior vena cava syndrome: mediastinal mass obstructs venous return — facial plethora, distended neck veins, orthopnea. Becomes an emergency with airway or cerebral edema; general anesthesia in a large anterior mediastinal mass can cause airway collapse.
• Bowel obstruction or perforation: transmural GI involvement, especially Burkitt; perforation may occur after chemotherapy as the mass necroses.
• Histologic transformation: indolent follicular lymphoma acquiring MYC/TP53 hits converts to aggressive DLBCL — signalled by a rapidly enlarging discordant nodal site, new B symptoms, and abrupt LDH rise.
• Autoimmune cytopenias: hemolytic anemia (positive direct antiglobulin test) and immune thrombocytopenia from dysregulated B-cell clones.

Treatment-related

• Febrile neutropenia (EMERGENCY): chemotherapy myelosuppression; a single fever with ANC <500 mandates blood cultures and empiric antipseudomonal beta-lactam within an hour per IDSA guidance.
• Anthracycline cardiotoxicity: doxorubicin-induced topoisomerase-IIβ/oxidative myocardial injury, cumulative-dose dependent; falling LVEF on surveillance echocardiography, later HFrEF.
• Vincristine neurotoxicity: microtubule disruption of axonal transport — loss of ankle reflexes, stocking-glove neuropathy, constipation/ileus. Intrathecal vincristine is uniformly fatal.
• Cyclophosphamide: acrolein-mediated hemorrhagic cystitis (hematuria; prevented with mesna and hydration), infertility, later MDS/AML.
• Rituximab: infusion reactions, hepatitis B reactivation (fulminant hepatitis), and rare progressive multifocal leukoencephalopathy from JC virus.
• CAR T-cell therapy (EMERGENCY): cytokine release syndrome (fever, hypotension, hypoxia; IL-6 driven, treated with tocilizumab) and ICANS (aphasia, tremor, encephalopathy; treated with corticosteroids), graded by ASTCT consensus criteria.

• Excisional lymph node biopsy is the single best next step: a stem describing persistent painless adenopathy wants whole-node architecture. Fine-needle aspiration is the classic distractor — it cannot distinguish follicular from diffuse growth pattern and cannot grade.
• Do not give corticosteroids before tissue is obtained: steroids are lympholytic and can dissolve the diagnosis. The exception is genuine cord compression or airway compromise.
• Translocation triad: t(14;18) → BCL2 → follicular (anti-apoptotic, indolent, incurable); t(8;14) → MYC → Burkitt (starry sky macrophages, Ki-67 approaching 100%); t(11;14) → cyclin D1 → mantle cell (CD5+ but CD23−, distinguishing it from CLL/SLL).
• TLS electrolytes: high potassium, high phosphate, high uric acid, low calcium. If the stem gives high calcium, think hypercalcemia of malignancy or ATLL, not TLS. Rasburicase is contraindicated in G6PD deficiency — hydrogen peroxide generated during urate breakdown causes hemolysis and methemoglobinemia.
• Gastric MALT lymphoma is treated with antibiotics: H. pylori eradication alone induces remission in most localised cases (ACG). *t(11;18)*-positive tumors are the ones that fail eradication and need lymphoma-directed therapy.
• Screen for hepatitis B before rituximab (AASLD): anti-CD20 depletion permits viral reactivation and fulminant hepatitis. This is a favorite one-line safety question.
• The four-drug backbone for DLBCL is R-CHOP, and cure is the goal; for asymptomatic low-burden follicular lymphoma the correct answer is often observation, since treating early does not improve survival. Aggressive lymphomas are curable; indolent ones generally are not — an inversion examiners love.
• Extranodal and non-contiguous spread favors NHL; contiguous nodal spread with Reed–Sternberg cells and bimodal age distribution favors Hodgkin lymphoma.
• Testicular DLBCL has a high rate of CNS relapse and mandates CNS-directed prophylaxis plus contralateral scrotal radiation.