Chronic Lymphocytic Leukemia is Classified Under Both Leukemia and Lymphoma
CLL/SLL represents a single disease entity that is classified under leukemia when predominantly in the blood (≥5×10⁹/L B lymphocytes) and under lymphoma when predominantly in lymph nodes (<5×10⁹/L B lymphocytes with lymphadenopathy), but they are managed identically as the same disease. 1
The Dual Classification Explained
Why It's Called a Leukemia
- CLL is the most common adult leukemia in Western countries, representing approximately 25% of all adult leukemias 1
- The diagnosis of CLL requires ≥5×10⁹/L (5,000/μL) monoclonal B lymphocytes in peripheral blood sustained for at least 3 months 1
- The disease is characterized by progressive accumulation of small, mature-appearing lymphocytes primarily in the peripheral blood and bone marrow 1
Why It's Also Called a Lymphoma (SLL)
- The WHO classification explicitly considers small lymphocytic lymphoma (SLL) and CLL as a single entity 1
- When B lymphocytes in peripheral blood are <5×10⁹/L but lymphadenopathy and/or splenomegaly is present, the diagnosis is SLL instead of CLL 1
- SLL cells show the identical immunophenotype as CLL (CD5+, CD19+, CD20+, CD23+) 1
- CLL/SLL constitutes approximately 7% of newly diagnosed non-Hodgkin's lymphomas 1
The Critical Distinction: Location, Not Biology
The only meaningful difference between CLL and SLL is anatomic distribution, not biological behavior:
- CLL: Abnormal lymphocytes predominantly in blood and bone marrow (≥5×10⁹/L) 1
- SLL: Abnormal lymphocytes predominantly in lymph nodes and bone marrow (<5×10⁹/L in blood) 1
- Both are managed identically with the same treatment algorithms 1
Clinical Implications
For Diagnosis
- Flow cytometry of peripheral blood alone is usually sufficient for diagnosis 1
- Lymph node biopsy should be performed to confirm SLL when immunophenotyping shows the CLL phenotype but blood lymphocyte count is <5×10⁹/L 1
- The characteristic immunophenotype (CD5+, CD19+, CD20+ low, CD23+, surface Ig low) confirms the diagnosis regardless of whether it's called CLL or SLL 1
For Treatment Decisions
- The distinction between CLL and SLL does not affect treatment decisions 1
- Both use identical staging systems (Binet or Rai) 1
- Treatment indications are the same: active/symptomatic disease or advanced stage 1
- Targeted therapies (BTK inhibitors, venetoclax) are used identically for both 2, 3, 4
Common Pitfall to Avoid
Do not confuse this with monoclonal B-cell lymphocytosis (MBL), which has <5×10⁹/L monoclonal B lymphocytes without lymphadenopathy, organomegaly, or cytopenias—MBL is not yet a leukemia or lymphoma and requires only observation 1