Leukemia

Leukemia – High‑Yield Study Guide for Medical Students

Definition

Leukemia is a group of hematologic malignancies characterized by clonal proliferation of abnormal hematopoietic cells (usually white blood cell precursors) in the bone marrow, peripheral blood, and sometimes other tissues. It leads to bone marrow failure with anemia, thrombocytopenia, and immunodeficiency, and can be classified by tempo (acute vs. chronic) and cell lineage (myeloid vs. lymphoid).

Major categories:

• Acute lymphoblastic leukemia (ALL) – malignant proliferation of lymphoid precursors (lymphoblasts), most common childhood leukemia.
• Acute myeloid leukemia (AML) – malignant proliferation of myeloid precursors (myeloblasts), more common in adults and associated with various cytogenetic abnormalities.
• Chronic lymphocytic leukemia (CLL) – clonal expansion of mature-appearing but functionally incompetent B lymphocytes, usually in older adults.
• Chronic myeloid leukemia (CML) – clonal myeloproliferative neoplasm driven in most cases by BCR-ABL1 fusion (Philadelphia chromosome), with overproduction of mature and maturing granulocytes.

Epidemiology

Hematologic malignancies as a group represent a substantial proportion of cancer morbidity and mortality globally, with leukemia being one of the most frequent forms, particularly in children. Heterogeneity in disease biology and response to treatment contributes to variable outcomes across subtypes.¹

• ALL
  • Most common childhood cancer; peak incidence between 2–5 years.
  • Less common in adults, but prognosis is generally poorer with increasing age.
• AML
  • Most common acute leukemia in adults; incidence rises with age (median ~65–70 years).
  • Often related to prior myelodysplastic syndrome, chemotherapy, or radiation.
• CLL
  • Most common adult leukemia in Western countries.
  • Primarily affects older adults (>60 years); male predominance.
• CML
  • Accounts for a smaller proportion of leukemias but is a prototype of targeted therapy success.
  • Median age at diagnosis ~50–60 years.

Pathophysiology

Leukemia pathogenesis involves acquired genetic and epigenetic alterations in hematopoietic stem or progenitor cells that confer growth advantage, impaired differentiation, and resistance to apoptosis. The resultant clonal expansion fills the bone marrow, suppressing normal hematopoiesis and leading to cytopenias and systemic manifestations.¹

General Mechanisms

• Genetic lesions
  • Balanced translocations (e.g., t(9;22) BCR-ABL1 in CML; t(15;17) PML-RARA in acute promyelocytic leukemia, a subtype of AML).
  • Aneuploidy and copy number changes (e.g., hyperdiploidy in pediatric ALL).
  • Mutations in transcription factors, epigenetic regulators, tumor suppressors, and signaling pathways.
• Clonal expansion and marrow failure
  • Leukemic blasts or mature clonal cells overcrowd the marrow niche.
  • Normal erythropoiesis, myelopoiesis, and megakaryopoiesis are suppressed, producing anemia, neutropenia, and thrombocytopenia.
• Immune dysfunction
  • Abnormal immune cell function leads to increased susceptibility to infections.
  • Autoimmune phenomena may occur, particularly in CLL.
• Organ infiltration
  • Leukemic cells may infiltrate liver, spleen, lymph nodes, CNS, gums, skin, and other tissues.
  • In acute leukemias, high circulating blast counts can lead to leukostasis, especially in pulmonary and CNS vasculature.

Subtype-Specific Highlights

• ALL
  • Arises from precursor B or T lymphoblasts.
  • Commonly associated with aberrant expression of lymphoid markers (e.g., CD10, CD19, TdT for B-ALL; CD3, CD7 for T-ALL).
  • Frequent CNS involvement; sanctuary sites can harbor disease despite systemic chemotherapy.
• AML
  • Results from arrested differentiation of myeloid precursors with accumulation of blasts.
  • Acute promyelocytic leukemia (APL) is driven by PML-RARA fusion and is associated with coagulopathy (DIC) but is highly curable with differentiation therapy (ATRA and arsenic trioxide).
• CLL
  • Clonal expansion of mature CD5+ B cells in blood, bone marrow, and lymphoid tissues.
  • Often indolent but can transform to aggressive lymphoma (Richter transformation).
• CML
  • Driven by BCR-ABL1 tyrosine kinase, which activates multiple signaling pathways (RAS, PI3K/AKT, JAK/STAT), promoting proliferation and survival.
  • Classically progresses through chronic, accelerated, and blast crisis phases if untreated.

Clinical Presentation

Presentation varies by leukemia type and tempo. Acute leukemias typically present over weeks with signs of bone marrow failure and systemic illness, while chronic leukemias may be incidentally discovered on routine blood tests or present with constitutional symptoms and organomegaly.

Common General Features

• Symptoms of bone marrow failure
  • Anemia: fatigue, pallor, dyspnea on exertion, tachycardia.
  • Neutropenia: recurrent or severe infections, poor wound healing.
  • Thrombocytopenia: easy bruising, petechiae, epistaxis, mucosal bleeding, menorrhagia.
• Constitutional symptoms
  • Fever, night sweats, unintentional weight loss, malaise.
• Organ infiltration
  • Lymphadenopathy and hepatosplenomegaly.
  • Bone pain or tenderness due to marrow expansion (especially in children with ALL).
  • Gingival hypertrophy and skin lesions (more typical in some AML subtypes).
  • CNS manifestations: headaches, cranial nerve palsies, confusion, especially with leukostasis or CNS involvement.
• Leukostasis (usually in acute leukemia with very high WBC count)
  • Dyspnea, hypoxia, pulmonary infiltrates.
  • Neurologic symptoms: visual changes, confusion, stroke-like features.

Subtype-Specific Presentations

• ALL
  • Children with fatigue, fever, infections, bone pain, and bleeding.
  • Mediastinal mass in T-ALL causing respiratory symptoms or SVC syndrome.
  • CNS involvement: headaches, vomiting, cranial nerve deficits.
• AML
  • Rapid onset cytopenic symptoms, gingival hypertrophy, skin infiltrates (leukemia cutis).
  • APL: bleeding, bruising, and signs of DIC (e.g., oozing, prolonged coagulation tests).
• CLL
  • Often asymptomatic; discovered via lymphocytosis on CBC.
  • Painless lymphadenopathy, splenomegaly, recurrent infections, autoimmune hemolytic anemia or thrombocytopenia.
• CML
  • Fatigue, weight loss, night sweats, abdominal fullness due to splenomegaly.
  • Marked leukocytosis with left-shifted granulocytosis; sometimes asymptomatic.

Diagnosis

Diagnosis of leukemia integrates clinical findings, complete blood count (CBC), peripheral smear, bone marrow examination, immunophenotyping, cytogenetic and molecular testing. A precise classification is critical because modern therapies increasingly target specific molecular lesions.¹

Initial Laboratory Evaluation

• CBC with differential
  • May show leukocytosis, leukopenia, or normal total WBC with abnormal differential.
  • Anemia and thrombocytopenia common in acute leukemias.
  • Lymphocytosis in CLL; marked neutrophilic leukocytosis with basophilia in CML.
• Peripheral blood smear
  • Blasts in acute leukemias (Auer rods in some AML types).
  • Smudge cells in CLL.
  • Left-shifted myeloid series in CML with all maturation stages present.
• Basic labs
  • LDH and uric acid may be elevated (high cell turnover).
  • Coagulation profile, especially in suspected APL (risk of DIC).
  • Electrolytes, renal and hepatic function to assess tumor lysis risk and therapy readiness.

Definitive Diagnostic Workup

• Bone marrow aspiration and biopsy
  • Acute leukemias: usually hypercellular marrow with ≥20% blasts (per WHO criteria for most AML/ALL entities).
  • Chronic leukemias: hypercellular marrow with increased mature or maturing lineage-specific cells.
• Immunophenotyping (flow cytometry)
  • Defines lineage (myeloid vs. B vs. T) and maturation stage.
  • Examples: CD34, TdT, CD19, CD20, CD3, CD13, CD33, MPO.
• Cytogenetics and molecular studies
  • Karyotyping for structural and numerical abnormalities.
  • Fluorescence in situ hybridization (FISH) for specific translocations (e.g., BCR-ABL1, PML-RARA).
  • RT-PCR or next-generation sequencing for fusion transcripts and somatic mutations.
  • These abnormalities guide prognosis and targeted therapy selection.
• Additional tests
  • Lumbar puncture to evaluate CNS involvement (especially in ALL and high-risk AML) when safe.
  • Imaging (e.g., chest X-ray, CT) for mediastinal mass or organomegaly.

Management

Management strategies are tailored to leukemia type, genetic risk, patient age, comorbidities, and disease stage. Modern therapy combines intensive chemotherapy, targeted agents, immunotherapy, and hematopoietic stem cell transplantation. Emerging therapies and precision medicine approaches continue to expand options, particularly for relapsed or refractory disease.¹

General Principles

• Initial stabilization
  • Treat infections empirically in neutropenic patients.
  • Manage tumor lysis risk with hydration, allopurinol or rasburicase, and close monitoring.
  • Transfuse red cells and platelets as needed; avoid platelet transfusions prior to diagnostic marrow only if clinically safe.
  • Address coagulopathy, particularly in APL (DIC management).
• Risk stratification
  • Use cytogenetic and molecular features to classify patients into favorable, intermediate, or adverse risk categories.
  • Risk category helps determine intensity of chemotherapy and need for allogeneic stem cell transplantation.

Acute Lymphoblastic Leukemia (ALL)

• Induction therapy
  • Multi-agent chemotherapy (e.g., vincristine, corticosteroids, anthracycline, ± asparaginase) aimed at achieving complete remission.
  • CNS prophylaxis with intrathecal chemotherapy (e.g., methotrexate, cytarabine) and sometimes cranial irradiation.
• Consolidation/intensification
  • Additional high-intensity regimens to eradicate residual disease.
  • Allogeneic hematopoietic stem cell transplantation considered in high-risk cases (e.g., poor cytogenetics, persistent minimal residual disease).
• Maintenance therapy
  • Prolonged low-intensity oral chemotherapy (e.g., mercaptopurine, methotrexate) for several years in pediatric protocols.
• Targeted and immunotherapies
  • Tyrosine kinase inhibitors (e.g., imatinib, dasatinib) for Philadelphia chromosome–positive ALL.
  • Monoclonal antibodies (e.g., rituximab for CD20+ disease, blinatumomab, inotuzumab ozogamicin).
  • Chimeric antigen receptor (CAR) T-cell therapies for relapsed/refractory B-ALL.

Acute Myeloid Leukemia (AML)

• Induction chemotherapy
  • Standard “7+3” regimen: cytarabine continuous infusion for 7 days plus an anthracycline (daunorubicin or idarubicin) for 3 days.
  • Goal is complete remission with <5% blasts in marrow and recovery of blood counts.
• Consolidation therapy
  • High-dose cytarabine or other regimens depending on risk category.
  • Allogeneic stem cell transplant for intermediate and high-risk disease or in relapsed AML.
• APL-specific therapy
  • All-trans retinoic acid (ATRA) plus arsenic trioxide (± anthracycline) to induce differentiation and remission.
  • Aggressive management of DIC and coagulopathy.
• Targeted therapies
  • FLT3 inhibitors, IDH1/2 inhibitors, BCL-2 inhibitors (e.g., venetoclax) in selected molecular subgroups.
  • Emerging antibody-drug conjugates and immune-based approaches as described in evolving therapeutic landscapes for hematologic malignancies.¹

Chronic Lymphocytic Leukemia (CLL)

• Watchful waiting
  • Many early-stage, asymptomatic patients are observed without immediate therapy.
  • Treatment is initiated based on progressive disease, symptomatic lymphadenopathy, cytopenias, or systemic symptoms.
• Systemic therapy
  • Chemoimmunotherapy regimens (e.g., bendamustine plus rituximab, fludarabine-based combinations) historically used.
  • Bruton tyrosine kinase (BTK) inhibitors (e.g., ibrutinib, acalabrutinib) and BCL-2 inhibitor venetoclax now central in many frontline and relapsed settings.
  • Anti-CD20 monoclonal antibodies (rituximab, obinutuzumab) used in combination regimens.
• Supportive care
  • Vaccination, infection prophylaxis, monitoring for autoimmune cytopenias.
  • Management of secondary malignancy risk and long-term toxicities.

Chronic Myeloid Leukemia (CML)

• Tyrosine kinase inhibitor (TKI) therapy
  • First-line treatment for most patients is a BCR-ABL1 TKI (e.g., imatinib, dasatinib, nilotinib, bosutinib).
  • TKIs induce high rates of hematologic, cytogenetic, and molecular remission and have transformed CML into a chronic condition for many patients.
• Monitoring
  • Serial quantitative PCR for BCR-ABL1 transcripts to assess molecular response.
  • Adjustment of therapy or switch to second/third-generation TKIs in case of inadequate response or intolerance.
• Stem cell transplantation
  • Reserved for patients with TKI resistance, advanced-phase disease, or specific high-risk scenarios.

Key Clinical Pearls and Exam Tips

• Classification
  • Remember the main division: acute vs. chronic, myeloid vs. lymphoid (ALL, AML, CLL, CML).
  • Acute leukemias: rapid onset, high blast count, urgent management; chronic leukemias: often insidious with mature-appearing cells.
• High-yield associations
  • Down syndrome is associated with increased risk of ALL and a transient myeloproliferative disorder that can evolve to AML.
  • Prior chemotherapy (alkylating agents, topoisomerase II inhibitors) and radiation are risk factors for secondary AML.
  • HTLV-1 infection is associated with adult T-cell leukemia/lymphoma, a distinct T-cell neoplasm with prominent neurologic and systemic features related to HTLV-1–driven disease.²
  • CML is strongly linked to the BCR-ABL1 fusion and responds dramatically to TKIs.
• Laboratory clues
  • Auer rods are pathognomonic for myeloid blasts (AML).
  • Smudge cells on smear suggest CLL.
  • Marked basophilia and very elevated WBC with left shift suggest CML.
  • Very high blast counts with neurologic and respiratory symptoms raise concern for leukostasis.
• Therapeutic pearls
  • Always think of APL in a patient with acute leukemia and DIC; prompt ATRA initiation is life-saving.
  • ALL management always includes CNS prophylaxis, even without overt CNS disease.
  • CML should trigger immediate evaluation for BCR-ABL1 and early TKI therapy.
  • Advances in targeted and cellular therapies (TKIs, monoclonal antibodies, CAR T cells) have significantly improved outcomes in many leukemias and are core exam topics.¹
• Complications to remember
  • Tumor lysis syndrome, particularly after initiation of therapy in highly proliferative leukemias.
  • Infections due to neutropenia and functional immune defects; opportunistic infections in heavily treated patients.
  • Secondary malignancies and late effects of chemotherapy and radiation in long-term survivors.

Summary for Exams

Leukemia encompasses a heterogeneous group of clonal hematologic malignancies with distinct genetic drivers, clinical presentations, and therapeutic approaches. For examinations, focus on differentiating acute vs. chronic and myeloid vs. lymphoid forms, recognize hallmark laboratory and clinical features, and understand the role of key molecular lesions (e.g., BCR-ABL1, PML-RARA) in both pathogenesis and targeted therapy. In clinical practice, accurate classification supported by cytogenetic and molecular testing is essential for risk-adapted treatment and for integrating novel targeted and immune-based therapies that are reshaping outcomes in hematologic malignancies.^1,2