Indolent monoclonal proliferation of mature CD5+ B lymphocytes — most common adult leukemia in Western countries.
Also known as: CLL, chronic lymphocytic leukemia, small lymphocytic lymphoma, SLL
Overview
Mature B-cell neoplasm characterized by clonal proliferation and accumulation of small CD5+ CD19+ CD23+ B lymphocytes in peripheral blood, bone marrow, and lymphoid tissues. CLL and small lymphocytic lymphoma (SLL) are the same disease distinguished by site (CLL has ≥5,000 monoclonal B cells/μL in peripheral blood; SLL is tissue-predominant).
Epidemiology
Most common adult leukemia in the United States and Europe (rare in Asia). Median age at diagnosis ~70 years. Male predominance (~2:1). Annual US incidence ~5 per 100,000. Often asymptomatic and discovered on routine CBC.
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Question 1HematologyMedium
A 68-year-old man presents with fatigue. Physical examination reveals diffuse, painless lymphadenopathy. A complete blood count shows a WBC of 85,000/µL with a predominance of mature small lymphocytes, and the peripheral smear shows numerous smudge cells. Flow cytometry of the peripheral blood demonstrates a clonal B-cell population that is CD5-positive, CD19-positive, and CD23-positive. Which of the following is the most likely diagnosis?
AChronic lymphocytic leukemia
BHairy cell leukemia
CAcute lymphoblastic leukemia
DMantle cell lymphoma
EFollicular lymphoma
Reveal answer & full explanation
Correct answer: A — Chronic lymphocytic leukemia
AChronic lymphocytic leukemia✓
BHairy cell leukemia
CAcute lymphoblastic leukemia
DMantle cell lymphoma
EFollicular lymphoma
Why Chronic lymphocytic leukemia is correct
Chronic lymphocytic leukemia (CLL) is the most common adult leukemia and typically presents after age 60 with fatigue and painless lymphadenopathy
The peripheral smear shows mature small lymphocytes with smudge cells, fragile leukemic cells that rupture during slide preparation
The clone carries the characteristic immunophenotype CD5+, CD19+, and CD23+, confirming a clonal mature B-cell process
Marked lymphocytosis (WBC 85,000/µL) of mature-appearing lymphocytes supports CLL over an acute or aggressive process
Why the others are wrong
Hairy cell leukemia — typically causes pancytopenia and massive splenomegaly with fine cytoplasmic projections and is CD23-negative; the marked lymphocytosis and CD23 positivity argue against it (confused-with another indolent B-cell neoplasm)
Mantle cell lymphoma — also CD5-positive but is characteristically CD23-NEGATIVE and more aggressive; the CD23 positivity here defeats it (buzzword-matching CD5 without integrating CD23)
Acute lymphoblastic leukemia — presents with circulating blasts rather than mature lymphocytes and is far more common in children; the mature lymphocyte morphology and smudge cells rule it out (anchoring on the high WBC)
Follicular lymphoma — the most common indolent non-Hodgkin lymphoma also causes painless, diffuse lymphadenopathy in older adults, but its clone is typically CD10-positive and CD5-negative with t(14;18) BCL2 overexpression, and it does not usually present with a marked lymphocytosis of mature lymphocytes and smudge cells (anchoring on painless adenopathy in an older adult)
Question 2HematologyMedium
A 65-year-old male with chronic lymphocytic leukemia (CLL) (Rai Stage III, TP53 deletion, unmutated IGHV) requires first-line treatment. He has a good performance status and no significant comorbidities. Which of the following is the preferred first-line treatment?
AFCR chemoimmunotherapy
BChlorambucil plus rituximab
CAlemtuzumab monotherapy
DIdelalisib plus rituximab
EIbrutinib monotherapy
Reveal answer & full explanation
Correct answer: E — Ibrutinib monotherapy
AFCR chemoimmunotherapy
BChlorambucil plus rituximab
CAlemtuzumab monotherapy
DIdelalisib plus rituximab
EIbrutinib monotherapy✓
Why Ibrutinib monotherapy is correct
TP53 deletion/mutation in chronic lymphocytic leukemia (CLL) disrupts the TP53 apoptosis pathway that chemotherapy depends on, making chemotherapy largely ineffective
Bruton tyrosine kinase (BTK) inhibitors (ibrutinib, acalabrutinib, zanubrutinib) act independently of p53 and remain highly effective regardless of TP53 status
Continuous BTK inhibitor therapy is a preferred first-line approach for TP53-deleted/mutated CLL, and ibrutinib is the only BTK inhibitor offered here; current NCCN guidance favors the second-generation agents acalabrutinib or zanubrutinib because they cause less atrial fibrillation and hypertension
Why the others are wrong
Idelalisib plus rituximab — the PI3K-delta inhibitor idelalisib is active in TP53-deleted disease, but idelalisib-rituximab is approved only for relapsed CLL; front-line trials were halted because of excess deaths from infection along with hepatotoxicity, colitis, and pneumonitis, so it is never a first-line choice
Chlorambucil plus rituximab — chlorambucil-based regimens are chemoimmunotherapy and are ineffective in TP53-deleted CLL; also inferior even in non-TP53 CLL compared to targeted agents
FCR chemoimmunotherapy — fludarabine-cyclophosphamide-rituximab (FCR) is now reserved for selected young, fit IGHV-mutated patients, and is NOT appropriate for TP53 deletion due to chemotherapy resistance
Alemtuzumab monotherapy — the anti-CD52 antibody alemtuzumab acts independently of p53 and was once used for TP53-deleted CLL, but it causes profound immunosuppression with CMV reactivation and opportunistic infections and has been replaced by BTK inhibitors and venetoclax-based regimens; it is not a current first-line option
Additional high-yield points
Venetoclax plus obinutuzumab (CLL14 trial): fixed-duration regimen achieving deep remissions and measurable residual disease (MRD) negativity; preferred for patients wanting fixed-duration therapy
Ibrutinib plus venetoclax (GLOW trial): showed superior outcomes compared to chlorambucil-based therapy
Unmutated IGHV (as in this patient) is an adverse prognostic factor independent of TP53 status
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Family history (~8x increased risk in first-degree relatives — strongest familial predisposition of any hematologic malignancy)
Monoclonal B-cell lymphocytosis (MBL) — precursor state with <5,000/μL clonal cells; ~1-2% annual progression risk to CLL
No clear environmental cause; Agent Orange exposure (recognized by VA)
Pathophysiology
Clonal CD5+ B cells accumulate due to defective apoptosis (BCL-2 overexpression) and chronic B-cell receptor (BCR) signaling. Antigen drive and microenvironmental signals (BTK, PI3K, NF-κB pathways) sustain the clone. The malignant cells are functionally incompetent, producing hypogammaglobulinemia and immune dysfunction. Common cytogenetic abnormalities (FISH panel): del(13q) (favorable, most common), trisomy 12 (intermediate), del(11q) (ATM, adverse), del(17p) (TP53, very adverse).
Clinical presentation
Symptoms
Often asymptomatic — discovered on incidental lymphocytosis (60-70% of cases)
TP53 sequencing for mutations (treatment selection)
IGHV mutational status — mutated (favorable) vs unmutated (unfavorable)
Direct antiglobulin test (DAT) for autoimmune hemolysis screen
Quantitative immunoglobulins — hypogammaglobulinemia common
Beta-2 microglobulin (prognostic)
Bone marrow biopsy not required for diagnosis; reserved for unexplained cytopenia evaluation
Rai or Binet staging based on lymphocytosis, lymphadenopathy, organomegaly, anemia, thrombocytopenia
Imaging
CT chest/abdomen/pelvis is not routinely needed at diagnosis or for staging (iwCLL 2018); obtain it when clinically indicated (e.g., symptomatic or bulky adenopathy), before starting venetoclax to assess tumor lysis risk, or in clinical trials
PET/CT if Richter transformation suspected (sudden enlargement of one node or B symptoms)
Diagnostic algorithm
Rai Stage
Findings
Median Survival (untreated, pre-targeted era)
0 (Low risk)
Lymphocytosis only
>10 years
I (Intermediate)
Lymphocytosis + lymphadenopathy
~8 years
II (Intermediate)
Lymphocytosis + hepatomegaly or splenomegaly (± LAD)
~6 years
III (High)
Lymphocytosis + anemia (Hb <11)
~2-4 years
IV (High)
Lymphocytosis + thrombocytopenia (Plt <100K)
~2-4 years
Rai staging for CLL — modern targeted therapy markedly improves survival across all stages.
Treatment
First-line
Indications for treatment (iwCLL criteria) — DO NOT TREAT asymptomatic early-stage disease (Binet A or Rai 0-II without symptoms) — observation is standard; treatment indicated for symptomatic/progressive disease, threatening lymphadenopathy or organomegaly, rapid lymphocyte doubling time (<6 months), B symptoms, autoimmune cytopenias unresponsive to steroids, marrow failure (Hb <10, plt <100)
BTK inhibitors — ibrutinib, acalabrutinib, zanubrutinib — first-line for most CLL patients including del(17p)/TP53 mutated disease; second-generation BTKis (acala, zanu) have improved cardiac safety (less afib, hypertension)
Venetoclax (BCL-2 inhibitor) + obinutuzumab (anti-CD20) — fixed-duration 12-month regimen, deep responses including MRD negativity (CLL14 trial)
Chemoimmunotherapy (FCR fludarabine + cyclophosphamide + rituximab; BR bendamustine + rituximab) — historical first-line; now reserved for selected younger fit IGHV-mutated patients without del(17p)/TP53
PI3K inhibitors — idelalisib, duvelisib — less commonly used due to toxicity (colitis, pneumonitis, hepatotoxicity, infection); reserved for multiply relapsed
Pirtobrutinib — non-covalent BTK inhibitor for BTK C481-mutated resistance
Supportive care: IVIG for recurrent infection with hypogammaglobulinemia, vaccinations (avoid live vaccines), PJP prophylaxis with chemoimmunotherapy or PI3Ki
Manage autoimmune cytopenias: prednisone first, rituximab second
Complications
Recurrent bacterial infections (encapsulated organisms — pneumococcus, H. flu); opportunistic infections (PJP, fungal) in heavily treated
Autoimmune cytopenias (warm AIHA, ITP, pure red cell aplasia)
Hypogammaglobulinemia — major contributor to morbidity and mortality
Richter transformation (~5-10%) — aggressive diffuse large B-cell lymphoma; sudden node enlargement, B symptoms, elevated LDH; poor prognosis
Second malignancies (skin cancer especially, increased rates)
Tumor lysis syndrome with venetoclax (dose ramp-up over 5 weeks)
BTK inhibitor toxicities: atrial fibrillation, hypertension, bleeding, diarrhea, infections; acalabrutinib and zanubrutinib have improved profile vs ibrutinib
Smudge cells (basket cells) on peripheral smear in an older adult with absolute lymphocytosis are nearly diagnostic of CLL.
CLL is the only major leukemia where observation ('watch and wait') is the standard of care for early-stage asymptomatic disease — early treatment does not improve survival.
del(17p) / TP53 mutation: very adverse prognosis with chemoimmunotherapy; standard care is BTK inhibitor or venetoclax-based therapy (NOT chemoimmunotherapy).
IGHV mutational status is a major prognostic factor — mutated favorable, unmutated unfavorable.
Hypogammaglobulinemia → recurrent encapsulated organism infections; vaccinate against pneumococcus, influenza (inactivated), zoster (recombinant) and COVID-19, avoid live vaccines, and consider IVIG for recurrent serious infections with IgG <500 mg/dL.
Richter transformation — sudden development of aggressive lymphoma (DLBCL most commonly) in CLL; clue is rapid node growth and B symptoms; biopsy diagnostic; PET shows high SUV.
Venetoclax requires tumor lysis prophylaxis with stepwise dose escalation over 5 weeks plus hydration, allopurinol/rasburicase, and frequent labs.
Ibrutinib causes atrial fibrillation and bleeding (especially with anticoagulants); newer BTKis (acalabrutinib, zanubrutinib) have lower rates.
References
iwCLL 2018 — iwCLL guidelines for diagnosis, indications for treatment, response assessment, and supportive management of CLL (Hallek et al., Blood 2018)
CLL14 — Venetoclax-Obinutuzumab for Previously Untreated Chronic Lymphocytic Leukemia (Fischer et al., NEJM 2019)
RESONATE — Ibrutinib versus Ofatumumab in Previously Treated CLL (Byrd et al., NEJM 2014)
MURANO — Venetoclax-Rituximab in Relapsed or Refractory CLL (Seymour et al., NEJM 2018)
ELEVATE-TN — Acalabrutinib with or without obinutuzumab versus obinutuzumab + chlorambucil in treatment-naive CLL (Sharman et al., Lancet 2020)
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