Medical School · Year 2 · Hematology Oncology · includes a quiz and discussion video

Lecture 8: Acute Leukemias

Unit 2.9: Hematology and Oncology

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the classification of acute leukemias
2. Explain the pathophysiology and clinical presentation of AML
3. Describe the diagnosis and risk stratification of AML
4. Explain the pathophysiology and clinical presentation of ALL
5. Describe the treatment principles for acute leukemias
6. Explain prognosis and complications of acute leukemia therapy

Lecture Outline

I. Overview of Acute Leukemia

Acute leukemia represents a clonal malignancy of hematopoietic progenitor cells characterized by accumulation of immature cells called blasts in the bone marrow and peripheral blood, with failure of normal hematopoiesis resulting in life-threatening cytopenias. The defining hallmark of acute leukemia is the presence of 20 percent or more blasts in the bone marrow or peripheral blood, though certain genetic abnormalities can establish diagnosis even with lower blast percentages. Without treatment, acute leukemia follows a rapidly progressive course that is uniformly fatal within weeks to months due to infection, bleeding, or organ infiltration by leukemic cells. The disease originates from either myeloid progenitors giving rise to acute myeloid leukemia (AML) or lymphoid progenitors resulting in acute lymphoblastic leukemia (ALL), with rare mixed phenotype acute leukemias exhibiting characteristics of both lineages.

The classification of acute leukemia has evolved from morphology-based systems to current approaches integrating morphology, immunophenotype, cytogenetics, and molecular genetics to define biologically and prognostically distinct entities. Acute myeloid leukemia encompasses malignancies arising from myeloid progenitors capable of differentiating toward granulocytes, monocytes, erythroid cells, or megakaryocytes, each with distinct morphologic features. Acute lymphoblastic leukemia originates from lymphoid progenitors committed to either B-cell or T-cell lineage, with B-ALL comprising approximately 85 percent of cases and T-ALL the remaining 15 percent. Mixed phenotype acute leukemia (MPAL) demonstrates markers of both myeloid and lymphoid lineages and requires specialized diagnostic criteria, representing a challenging diagnostic and therapeutic entity.

The epidemiology of acute leukemia differs substantially between AML and ALL, reflecting distinct biologic origins and pathogenic mechanisms. Acute myeloid leukemia occurs predominantly in adults with median age at diagnosis of 65 to 70 years and represents the most common acute leukemia in adults, with incidence increasing progressively with age. Acute lymphoblastic leukemia demonstrates a bimodal age distribution with a peak incidence in children aged 2 to 5 years, where it represents the most common childhood malignancy, followed by a second smaller peak in adults over age 50. This age-related distribution reflects different genetic mechanisms driving leukemogenesis in pediatric versus adult populations, with childhood ALL characterized by favorable genetic abnormalities that are less common in adult disease.

The pathogenesis of acute leukemia involves acquisition of genetic mutations in hematopoietic progenitors that confer proliferative advantage, block normal differentiation, and enable clonal expansion at the expense of normal hematopoiesis. Driver mutations may occur in genes controlling cell proliferation such as FLT3 and RAS family members, genes regulating differentiation such as RUNX1 and CEBPA, or genes controlling epigenetic modifications including DNMT3A, TET2, and IDH1/2. The accumulation of mutations leads to clonal expansion of immature blasts that lack the ability to differentiate into functional blood cells, progressively replacing normal marrow elements. This maturation arrest results in accumulation of blasts incapable of carrying out immune defense, oxygen transport, or hemostasis, while normal blood cell production fails, explaining the characteristic clinical presentation with cytopenias.

<image>Panel A: Blast morphology comparing myeloblasts with granules and Auer rods versus lymphoblasts with scant cytoplasm. Panel B: Lineage determination flowchart using immunophenotyping markers for AML, B-ALL, and T-ALL. Panel C: Age distribution curves showing pediatric predominance of ALL and adult predominance of AML. Panel D: Pathogenic model of leukemogenesis showing mutation accumulation, clonal expansion, and marrow failure.</image>

II. Acute Myeloid Leukemia - Classification

The World Health Organization 2022 classification of acute myeloid leukemia integrates morphology, immunophenotype, and genetic features to define disease entities with distinct biology, prognosis, and treatment implications. The major categories include AML with recurrent genetic abnormalities, AML with myelodysplasia-related changes, therapy-related AML, and AML not otherwise specified (NOS), each carrying different prognostic and therapeutic implications. This classification system recognizes that genetic abnormalities are the primary determinants of disease behavior, prognosis, and response to treatment, making molecular characterization essential for optimal patient care. The identification of specific genetic abnormalities guides risk stratification, treatment selection, and monitoring strategies throughout the disease course.

AML with recurrent genetic abnormalities represents a category defined by specific cytogenetic translocations or molecular mutations that establish the diagnosis and determine prognosis. Favorable-risk abnormalities include t(15;17) creating the PML-RARA fusion defining acute promyelocytic leukemia, t(8;21) producing RUNX1-RUNX1T1 fusion, inv(16) or t(16;16) creating CBFB-MYH11 fusion characteristic of core binding factor AML, and NPM1 mutation without FLT3-ITD. These favorable-risk abnormalities are associated with higher complete remission rates and better long-term survival with standard chemotherapy. Adverse-risk abnormalities include complex karyotype with three or more chromosomal abnormalities, monosomal karyotype, TP53 mutation which confers very poor prognosis, and FLT3-ITD especially with high allelic ratio, all indicating need for more intensive approaches including allogeneic transplantation.

The French-American-British (FAB) classification, while largely supplanted by the WHO system, remains useful for understanding morphologic subtypes and their clinical associations. FAB M0 through M7 subtypes classify AML based on degree and type of differentiation: M0 is undifferentiated, M1 shows minimal differentiation, M2 demonstrates maturation and is often associated with t(8;21), M3 is acute promyelocytic leukemia with characteristic hypergranular promyelocytes. M4 represents acute myelomonocytic leukemia often associated with gingival hyperplasia, M5 is acute monocytic leukemia frequently presenting with extramedullary disease, M6 is acute erythroid leukemia, and M7 is acute megakaryocytic leukemia associated with Down syndrome in children. These morphologic features provide initial diagnostic clues and may suggest specific genetic abnormalities requiring confirmatory testing.

Secondary AML encompasses both therapy-related AML arising after prior chemotherapy or radiation exposure and AML evolving from antecedent hematologic disorders such as myelodysplastic syndrome or myeloproliferative neoplasms. Therapy-related AML following alkylating agent exposure typically develops 5 to 7 years after exposure and frequently demonstrates MDS-like features with complex cytogenetics and chromosome 5 or 7 abnormalities. Therapy-related AML following topoisomerase II inhibitor exposure develops earlier at 1 to 3 years and often shows balanced translocations involving KMT2A (MLL) gene at 11q23. Secondary AML arising from prior MDS, MPN, or other marrow disorders carries the genetic abnormalities of the preceding condition and generally demonstrates worse prognosis than de novo AML. Recognition of secondary etiology is essential because these patients typically respond poorly to standard chemotherapy and require consideration of alternative strategies including allogeneic transplantation.

<image>Panel A: WHO 2022 classification hierarchy showing AML with recurrent genetic abnormalities, myelodysplasia-related changes, therapy-related, and NOS categories. Panel B: Genetic abnormalities with associated prognosis demonstrating favorable, intermediate, and adverse risk categories. Panel C: FAB classification M0-M7 with morphologic features and clinical associations. Panel D: Timeline of therapy-related AML development following alkylating agents versus topoisomerase II inhibitors.</image>

III. AML - Clinical Presentation

The clinical presentation of acute myeloid leukemia reflects bone marrow failure from replacement of normal hematopoietic elements by leukemic blasts, resulting in deficiencies of all three major blood cell lines. Anemia from decreased red blood cell production manifests as fatigue, dyspnea on exertion, pallor, and decreased exercise tolerance, with severity depending on the degree and rapidity of hemoglobin decline. Thrombocytopenia from impaired platelet production causes bleeding manifestations ranging from petechiae and ecchymoses to mucosal bleeding from gums and nose, with severe hemorrhage including gastrointestinal and intracranial bleeding in profound thrombocytopenia. Neutropenia from lack of mature granulocyte production renders patients susceptible to serious bacterial and fungal infections, which may be the presenting manifestation of disease and can progress rapidly to sepsis and death.

Leukemic infiltration of tissues outside the bone marrow produces characteristic clinical findings that may suggest specific AML subtypes. Gingival hyperplasia from leukemic infiltration of gum tissue is particularly common with monocytic differentiation seen in FAB M4 and M5 subtypes, appearing as swollen, purple gums that may bleed easily. Leukemia cutis describes skin infiltration manifesting as violaceous papules, nodules, or plaques that may be mistaken for other dermatologic conditions. Central nervous system involvement is less common in AML than ALL but can occur, presenting with headache, cranial nerve palsies, or signs of increased intracranial pressure. Bone pain from marrow expansion and periosteal infiltration may be present, and mild splenomegaly occurs in some patients though massive organomegaly is uncommon in AML compared to chronic leukemias.

Oncologic emergencies may complicate the initial presentation of AML and require immediate recognition and management. Leukostasis occurs when very high white blood cell counts, typically exceeding 100,000 per microliter, cause symptomatic hyperviscosity with pulmonary manifestations including dyspnea, hypoxia, and diffuse pulmonary infiltrates, and neurologic symptoms including confusion, headache, and visual changes from retinal vessel sludging. Disseminated intravascular coagulation is particularly characteristic of acute promyelocytic leukemia where granules from malignant promyelocytes trigger the coagulation cascade, but can occur with any AML subtype presenting with combined bleeding and thrombotic complications. Tumor lysis syndrome from rapid cell turnover or treatment-induced death of leukemic cells causes hyperuricemia, hyperkalemia, and hyperphosphatemia with secondary hypocalcemia, risking renal failure and cardiac arrhythmias.

Leukostasis deserves particular attention as a medical emergency that can cause rapid deterioration and death if not promptly addressed. The pathophysiology involves increased blood viscosity from extremely elevated white cell counts combined with the large, sticky nature of leukemic blasts that adhere to vascular endothelium and obstruct microcirculation. Clinical manifestations primarily affect the lungs and central nervous system where microvascular obstruction has the most severe consequences, with patients presenting with respiratory distress, hypoxia, altered mental status, and sometimes stroke-like symptoms. Management requires urgent cytoreduction through leukapheresis to mechanically remove circulating blasts, hydroxyurea for rapid chemical cytoreduction, and initiation of definitive chemotherapy while carefully avoiding red blood cell transfusion that would further increase blood viscosity.

<image>Panel A: Clinical manifestations of bone marrow failure showing anemia symptoms, bleeding from thrombocytopenia, and infection from neutropenia. Panel B: Gingival hyperplasia and leukemia cutis demonstrating extramedullary infiltration. Panel C: Leukostasis pathophysiology showing microvascular sludging in pulmonary and cerebral circulation. Panel D: DIC mechanism in acute promyelocytic leukemia with procoagulant granule release.</image>

IV. AML - Diagnosis and Workup

The diagnosis of acute myeloid leukemia requires demonstration of 20 percent or more myeloid blasts in the bone marrow or peripheral blood, with the important exception that certain recurring cytogenetic abnormalities establish the diagnosis regardless of blast percentage. The diagnostic threshold of 20 percent distinguishes AML from myelodysplastic syndrome, which demonstrates less than 20 percent blasts and follows a more indolent course, though both may exist on a continuum with MDS progressing to AML over time. The diagnosis further requires establishing myeloid lineage through morphology, cytochemistry, and immunophenotyping to distinguish AML from acute lymphoblastic leukemia, which requires entirely different treatment approaches. Cases meeting blast threshold with t(8;21), inv(16), or t(15;17) translocations are diagnosed as AML regardless of blast percentage, recognizing the defining nature of these genetic abnormalities.

The initial laboratory workup begins with complete blood count with differential revealing variable presentations from marked leukocytosis with circulating blasts to leukopenia with few or no blasts visible in peripheral blood. Peripheral blood smear examination provides crucial morphologic information, with AML blasts typically appearing as large cells with round or oval nuclei, fine chromatin, prominent nucleoli, and moderate amounts of cytoplasm that may contain granules or Auer rods. Auer rods are crystallized azurophilic granules that form elongated pink or red rod-shaped inclusions in the cytoplasm and are pathognomonic for myeloid lineage, immediately confirming AML when present. Bone marrow aspiration and biopsy provide material for comprehensive evaluation including morphology with blast enumeration, flow cytometry for immunophenotyping, cytogenetics, and molecular testing essential for classification and risk stratification.

Flow cytometry analysis detects myeloid-specific surface and intracellular markers that confirm lineage and may suggest specific subtypes. Core myeloid markers include CD13, CD33, and CD117 expressed on myeloid blasts, and cytoplasmic myeloperoxidase (MPO) which is highly specific for myeloid differentiation. CD34 expression indicates primitive blasts and is variably present across subtypes, while HLA-DR is typically positive except in acute promyelocytic leukemia where its absence provides a diagnostic clue. Monocytic differentiation is suggested by CD14 and CD64 expression, while aberrant lymphoid marker expression occasionally occurs in AML without indicating mixed phenotype disease. The immunophenotypic profile helps guide classification, provides markers for monitoring minimal residual disease, and occasionally reveals unexpected findings requiring diagnostic revision.

Comprehensive genetic characterization is essential for AML classification, risk stratification, treatment selection, and monitoring, requiring both cytogenetic and molecular testing on diagnostic specimens. Conventional karyotyping identifies chromosomal translocations, deletions, and numerical abnormalities that define disease entities and predict prognosis. Fluorescence in situ hybridization (FISH) provides rapid turnaround for specific abnormalities including PML-RARA in suspected APL where immediate treatment decisions depend on results. Molecular testing using polymerase chain reaction and next-generation sequencing identifies mutations in genes including NPM1, FLT3, CEBPA, IDH1, IDH2, RUNX1, ASXL1, and TP53 that inform risk stratification and increasingly guide targeted therapy selection. Quantitative PCR for specific fusion transcripts enables sensitive monitoring of minimal residual disease during and after treatment.

<image>Panel A: Blast morphology demonstrating myeloblast features with nuclear characteristics, cytoplasmic granules, and Auer rod identification. Panel B: Flow cytometry dot plots showing myeloid marker expression pattern with CD13, CD33, CD117, and MPO positivity. Panel C: Karyotype showing t(8;21) translocation and FISH demonstrating PML-RARA fusion. Panel D: Diagnostic algorithm integrating morphology, flow cytometry, cytogenetics, and molecular testing.</image>

V. Acute Promyelocytic Leukemia (APL)

Acute promyelocytic leukemia represents a distinct subtype of AML defined by the t(15;17) translocation creating the PML-RARA fusion gene, and is classified as FAB M3 based on morphology showing hypergranular promyelocytes as the predominant cell population. The PML-RARA fusion protein acts as an aberrant transcription factor that blocks normal myeloid differentiation at the promyelocyte stage, leading to accumulation of malignant promyelocytes with characteristic morphologic features. The distinctive biology of APL has led to development of targeted therapies making it the most curable subtype of AML in adults, with cure rates exceeding 90 percent with modern treatment. Recognition of APL is critical because it requires immediate initiation of specific therapy and carries unique risks of early death from coagulopathy if not promptly treated.

The morphologic appearance of APL is highly distinctive, with malignant promyelocytes containing abundant large granules that fill the cytoplasm and may obscure the nucleus. Auer rods are characteristic and often multiple, with bundles of Auer rods forming "faggot cells" that are virtually pathognomonic for APL. The microgranular variant demonstrates cells with abundant small granules that may be difficult to appreciate, sometimes causing diagnostic confusion, but the bilobed or monocytoid nuclear appearance provides a clue. The immunophenotype typically shows absence of HLA-DR and CD34, distinguishing APL from most other AML subtypes, along with characteristic CD33 and CD13 positivity. The diagnosis should be suspected based on morphology and immunophenotype and confirmed with FISH or RT-PCR demonstrating PML-RARA, with treatment initiated on clinical suspicion before confirmatory results are available.

Disseminated intravascular coagulation represents the defining clinical challenge in APL, with the coagulopathy causing both bleeding and thrombotic complications that may be fatal before or shortly after treatment initiation. The granules of malignant promyelocytes contain procoagulant substances including tissue factor and cancer procoagulant that activate the coagulation cascade, while fibrinolytic activity and proteases released from granules further compound the hemostatic derangement. Bleeding is the predominant clinical manifestation, ranging from mucosal bleeding and ecchymoses to life-threatening pulmonary or intracranial hemorrhage that causes approximately 10 percent early mortality. Laboratory findings demonstrate prolonged PT and PTT, decreased fibrinogen, elevated D-dimer, and thrombocytopenia, requiring aggressive supportive care with transfusion of platelets, cryoprecipitate to maintain fibrinogen above 150 mg/dL, and fresh frozen plasma.

Treatment of APL represents a landmark success of targeted therapy, using agents that overcome the differentiation block caused by PML-RARA rather than relying solely on cytotoxic chemotherapy. All-trans retinoic acid (ATRA) binds to the RARA portion of the fusion protein and induces terminal differentiation of malignant promyelocytes into mature granulocytes. Arsenic trioxide (ATO) promotes degradation of the PML-RARA fusion protein and also induces differentiation and apoptosis. The combination of ATRA plus arsenic trioxide achieves cure rates exceeding 90 percent in low-risk APL without need for conventional chemotherapy, while high-risk patients receive additional anthracycline. Differentiation syndrome is a potentially fatal complication occurring when differentiating promyelocytes release cytokines causing fever, dyspnea, weight gain, pulmonary infiltrates, and hypotension, requiring prompt recognition and treatment with dexamethasone.

<image>Panel A: APL morphology showing hypergranular promyelocytes with abundant large granules and faggot cells with bundled Auer rods. Panel B: DIC pathophysiology in APL illustrating granule release, coagulation activation, and clinical bleeding. Panel C: ATRA mechanism of action showing binding to PML-RARA and induction of terminal differentiation. Panel D: Differentiation syndrome presentation with pulmonary infiltrates, fever, and weight gain.</image>

VI. AML - Treatment

The treatment of acute myeloid leukemia follows a structured approach beginning with induction chemotherapy to achieve complete remission, followed by consolidation therapy to eliminate residual disease and prevent relapse. Complete remission is defined as bone marrow with less than 5 percent blasts, recovery of normal blood counts with absolute neutrophil count exceeding 1,000 and platelets exceeding 100,000, and absence of extramedullary disease. The goal of induction is rapid reduction of leukemic burden to allow restoration of normal hematopoiesis, while consolidation aims to eradicate residual leukemic cells below the detection threshold of standard methods. Treatment intensity and selection of consolidation strategy depend on patient fitness, disease risk stratification, and increasingly on molecular features that guide targeted therapy integration.

The standard induction regimen for fit patients, known as "7+3," consists of continuous infusion cytarabine for 7 days combined with an anthracycline such as daunorubicin or idarubicin for 3 days. This regimen achieves complete remission in 60 to 80 percent of younger patients but has lower response rates in older adults due to both patient factors and the higher frequency of adverse-risk disease biology in elderly populations. The intensive nature of treatment causes profound cytopenias lasting 2 to 3 weeks, during which patients require supportive care with transfusions, antimicrobial prophylaxis, and management of febrile neutropenia. Response assessment with bone marrow biopsy occurs after count recovery to document remission status and inform consolidation planning.

Consolidation therapy options depend on risk stratification determined by genetic features and response to induction, with the primary decision being whether to proceed with high-dose chemotherapy alone or allogeneic stem cell transplantation. Favorable-risk patients including those with core binding factor AML and NPM1-mutated disease without FLT3-ITD are typically consolidated with high-dose cytarabine alone, achieving long-term survival rates of 60 to 70 percent. Intermediate-risk and adverse-risk patients benefit from allogeneic stem cell transplantation in first complete remission, which provides the greatest chance of cure through combined chemotherapy conditioning and graft-versus-leukemia effect. Autologous transplantation using the patient's own stem cells is considered in selected patients who lack suitable allogeneic donors and are at higher risk of relapse with chemotherapy alone.

Targeted therapies have transformed AML treatment by providing options for specific molecular subsets and for patients unable to tolerate intensive chemotherapy. FLT3 inhibitors including midostaurin added to induction and gilteritinib for relapsed disease improve outcomes in FLT3-mutated AML. IDH1 and IDH2 inhibitors ivosidenib and enasidenib induce differentiation and responses in IDH-mutated disease. Venetoclax, a BCL2 inhibitor, combined with hypomethylating agents such as azacitidine provides an effective lower-intensity option for older or unfit patients with response rates of 60 to 70 percent. Gemtuzumab ozogamicin, an anti-CD33 antibody-drug conjugate, provides benefit in favorable-risk disease when added to chemotherapy. These targeted approaches have expanded treatment options across the age and fitness spectrum.

<image>Panel A: 7+3 induction regimen showing cytarabine infusion schedule and anthracycline administration days. Panel B: Risk stratification algorithm directing consolidation with high-dose cytarabine versus allogeneic transplantation. Panel C: Targeted therapy mechanism for FLT3 inhibitors, IDH inhibitors, and venetoclax. Panel D: Treatment algorithm for fit versus unfit patients showing intensive versus lower-intensity approaches.</image>

VII. Acute Lymphoblastic Leukemia - Overview

Acute lymphoblastic leukemia arises from malignant transformation of lymphoid progenitors committed to either B-cell or T-cell lineage, with classification based on immunophenotype and genetic features that determine prognosis and guide treatment. B-cell ALL comprises approximately 85 percent of cases and is characterized by expression of B-lineage markers including CD19, CD10 (CALLA), CD22, and CD79a, with cytoplasmic immunoglobulin in more mature stages. T-cell ALL represents approximately 15 percent of cases and demonstrates CD3 expression (cytoplasmic or surface), CD7, and variable CD4 and CD8, often arising from thymic T-cell precursors. The distinction between B-ALL and T-ALL is essential because prognosis, risk factors, and treatment intensity differ between lineages, though both follow similar multi-agent chemotherapy principles.

Genetic abnormalities in ALL define disease biology, prognosis, and increasingly direct targeted therapy, with several recurrent abnormalities having well-established prognostic significance. Favorable abnormalities in childhood ALL include hyperdiploidy with greater than 50 chromosomes and t(12;21) creating ETV6-RUNX1 fusion, both associated with excellent outcomes exceeding 90 percent cure rates. The Philadelphia chromosome resulting from t(9;22) creating BCR-ABL1 fusion historically conferred adverse prognosis but has been transformed by addition of tyrosine kinase inhibitors to chemotherapy. Adverse-risk features include KMT2A (MLL) rearrangements at 11q23, hypodiploidy, and iAMP21 amplification, all requiring more intensive therapy. BCR-ABL1-like or Philadelphia-like ALL demonstrates a gene expression profile similar to Ph-positive ALL without the characteristic translocation and carries intermediate to adverse prognosis.

Philadelphia chromosome-positive ALL occurs in approximately 25 percent of adult ALL but only 3 percent of childhood ALL, representing the most common cytogenetic abnormality in adult disease and historically the most adverse prognostic factor. The BCR-ABL1 fusion protein is a constitutively active tyrosine kinase that drives leukemic proliferation and survival, the same molecular driver found in chronic myeloid leukemia though the fusion protein size differs. Before tyrosine kinase inhibitors, Ph-positive ALL had dismal prognosis with long-term survival rates below 20 percent despite intensive chemotherapy and transplantation. The addition of TKIs including imatinib and second-generation agents dasatinib and ponatinib to chemotherapy has dramatically improved outcomes, with some regimens achieving cure rates approaching those of Ph-negative ALL.

Risk stratification in ALL integrates clinical features, genetic abnormalities, and response to therapy to guide treatment intensity and determine need for transplantation. Age at diagnosis is a key factor, with infants under 1 year and older children above 10 years having worse prognosis than children in the favorable 1 to 9 year age range. White blood cell count at diagnosis above 50,000 per microliter indicates higher-risk disease, as does presence of central nervous system involvement at diagnosis. Treatment response assessed by minimal residual disease (MRD) measurement has emerged as the most powerful prognostic factor, with MRD negativity after induction strongly predicting favorable outcomes regardless of other features. Patients with persistent MRD positivity require treatment intensification including consideration of transplantation or novel immunotherapies.

<image>Panel A: B-ALL versus T-ALL immunophenotype showing lineage-specific marker expression patterns. Panel B: Genetic abnormalities in ALL with prognosis including favorable, intermediate, and adverse risk categories. Panel C: Philadelphia chromosome positive ALL showing BCR-ABL1 translocation and mechanism of TKI action. Panel D: Risk stratification factors including age, WBC, cytogenetics, CNS involvement, and MRD status.</image>

VIII. ALL - Clinical Presentation

The clinical presentation of acute lymphoblastic leukemia reflects both bone marrow failure from leukemic infiltration and extramedullary involvement more prominent than in most AML cases. Symptoms of marrow failure including fatigue from anemia, bleeding from thrombocytopenia, and infections from neutropenia parallel those seen in AML and drive many patients to seek medical attention. Bone pain is particularly common in children with ALL and may be severe enough to cause limp, refusal to walk, or night-time awakening, reflecting leukemic expansion within the marrow cavity and periosteal involvement. The combination of bone pain with systemic symptoms in a child should raise suspicion for leukemia and prompt appropriate evaluation.

Lymphadenopathy and hepatosplenomegaly occur more frequently in ALL than in AML, reflecting the lymphoid origin and tropism of the malignant cells for lymphoid tissues. Generalized lymph node enlargement may be appreciated in cervical, axillary, and inguinal regions, with nodes typically firm but non-tender. Hepatomegaly and splenomegaly from leukemic infiltration are common and may cause abdominal discomfort or early satiety. Central nervous system involvement is more frequent in ALL than AML and may present with headache, cranial nerve palsies, visual changes, or signs of increased intracranial pressure. Testicular involvement represents a "sanctuary site" where standard chemotherapy achieves lower penetration, requiring specific attention in male patients.

T-cell ALL has distinctive clinical features related to its frequent origin from thymic precursors, often presenting as a mediastinal mass with associated complications. The anterior mediastinal mass composed of malignant T cells may cause superior vena cava syndrome with facial swelling, venous distension, and respiratory distress from airway compression. Adolescent and young adult males are most commonly affected by T-ALL, presenting with higher white blood cell counts compared to B-ALL. The mediastinal mass may be detected on chest radiograph obtained for respiratory symptoms or as an incidental finding. Recognition of T-ALL is important because some treatment protocols differ for T-lineage disease and the mediastinal mass requires monitoring during treatment.

Laboratory findings in ALL demonstrate variable white blood cell counts that may range from leukopenia with few circulating blasts to marked leukocytosis exceeding 100,000 per microliter. Peripheral smear examination reveals lymphoblasts that are typically smaller than myeloblasts, with scant cytoplasm, high nuclear-to-cytoplasmic ratio, fine chromatin, and absent cytoplasmic granules or Auer rods, distinguishing them from myeloid blasts. Anemia and thrombocytopenia are common, reflecting marrow infiltration and suppression of normal hematopoiesis. Elevated lactate dehydrogenase and uric acid levels reflect high tumor burden and cell turnover, with extreme elevations indicating higher risk for tumor lysis syndrome. Lumbar puncture is required for staging to evaluate cerebrospinal fluid for occult CNS involvement that would alter treatment approach.

<image>Panel A: Clinical manifestations of ALL showing bone pain in children, lymphadenopathy, and hepatosplenomegaly. Panel B: Mediastinal mass in T-ALL with chest radiograph appearance and potential for SVC syndrome. Panel C: Lymphoblast morphology demonstrating small cells with scant cytoplasm and high N:C ratio compared to myeloblasts. Panel D: Flow cytometry for B-ALL showing CD19, CD10, CD22, and TdT positivity.</image>

IX. ALL - Treatment

The treatment of acute lymphoblastic leukemia follows a multi-phase approach developed primarily in pediatric oncology and now adapted for adult protocols, consisting of induction, consolidation, CNS-directed therapy, and extended maintenance. This prolonged treatment duration spanning 2 to 3 years distinguishes ALL therapy from AML, where maintenance has not proven beneficial, reflecting different disease biology and patterns of relapse. The multi-agent chemotherapy approach uses drugs with different mechanisms of action to prevent resistance development while balancing toxicity through sequential administration. Treatment intensity is risk-adapted based on presenting features and response to therapy, with higher-risk patients receiving more intensive regimens and consideration of allogeneic transplantation.

Induction therapy aims to achieve complete remission and typically combines corticosteroids (prednisone or dexamethasone), vincristine, an anthracycline such as daunorubicin, and asparaginase as the core components. Corticosteroids have potent lympholytic activity specific for lymphoid malignancies and are essential components achieving rapid initial cytoreduction. Asparaginase depletes asparagine, an amino acid that lymphoblasts cannot synthesize, making it uniquely effective against lymphoid malignancies with minimal activity in myeloid disease. Philadelphia chromosome-positive ALL requires addition of a tyrosine kinase inhibitor to chemotherapy, with some protocols achieving excellent results with TKI combined with less intensive chemotherapy. Complete remission rates exceed 95 percent in children and 80 to 90 percent in adults with modern regimens, though remission must be consolidated and maintained to prevent relapse.

Central nervous system prophylaxis is mandatory in ALL treatment because the CNS serves as a sanctuary site where standard systemic chemotherapy does not achieve adequate drug levels, risking CNS relapse even when systemic disease is controlled. Intrathecal chemotherapy with methotrexate, often combined with cytarabine and hydrocortisone (triple intrathecal therapy), is administered repeatedly throughout treatment to prevent CNS relapse. High-dose systemic methotrexate, which crosses the blood-brain barrier, provides additional CNS coverage and is incorporated into many protocols. Cranial radiation, while effective for CNS prophylaxis, has been largely eliminated from frontline therapy due to significant late effects including neurocognitive impairment and secondary malignancies, reserved primarily for patients with overt CNS disease at diagnosis.

Maintenance therapy with daily oral 6-mercaptopurine and weekly oral methotrexate continues for 2 to 3 years total treatment duration and is essential for preventing late relapse in ALL. This prolonged low-intensity therapy is unique to ALL treatment among acute leukemias and reflects the different biology allowing effective disease control with continuous suppressive therapy. TPMT (thiopurine methyltransferase) enzyme activity testing identifies patients with reduced drug metabolism who require dose reduction to avoid severe myelosuppression from 6-mercaptopurine. Novel therapies including blinatumomab (a bispecific T-cell engager targeting CD19), inotuzumab ozogamicin (an anti-CD22 antibody-drug conjugate), and CAR-T cell therapy targeting CD19 have transformed treatment of relapsed/refractory disease and are increasingly incorporated into frontline approaches for high-risk patients.

<image>Panel A: Treatment phases showing induction, consolidation, CNS prophylaxis, and maintenance over 2-3 year timeline. Panel B: Induction regimen components with mechanisms of steroids, vincristine, anthracycline, and asparaginase. Panel C: CNS prophylaxis approaches including intrathecal chemotherapy and high-dose methotrexate. Panel D: Novel immunotherapy mechanisms showing blinatumomab BiTE, inotuzumab ADC, and CAR-T cells targeting CD19.</image>

X. Complications and Prognosis

Treatment complications in acute leukemia arise from both the disease itself and the intensive therapy required for cure, requiring comprehensive supportive care throughout the treatment course. Tumor lysis syndrome results from rapid release of intracellular contents during leukemic cell death, causing hyperuricemia with risk of uric acid nephropathy, hyperkalemia with cardiac arrhythmia risk, hyperphosphatemia causing secondary hypocalcemia with tetany and seizure risk, and acute kidney injury from crystal precipitation. Prevention with aggressive hydration, allopurinol prophylaxis to block uric acid production, or rasburicase for rapid uric acid breakdown in high-risk patients is essential. Monitoring and management of electrolyte derangements and renal function during induction is critical, with dialysis support required for severe tumor lysis syndrome.

Infectious complications represent the major cause of treatment-related morbidity and mortality, reflecting the profound immunosuppression from disease and therapy-induced neutropenia. Prophylactic antibiotics including fluoroquinolones during neutropenia, antifungal agents for prolonged neutropenia, and antiviral medications to prevent herpes simplex and varicella zoster reactivation reduce infectious complications. Prompt recognition and treatment of febrile neutropenia with broad-spectrum antibiotics is essential, with empiric antifungal therapy added for persistent fever. Mucositis from chemotherapy-induced mucosal injury causes painful oral ulceration, difficulty swallowing, and risk of bacterial translocation, requiring supportive care with pain management, nutritional support, and meticulous oral hygiene.

Prognosis in acute leukemia varies substantially based on disease biology, patient factors, and treatment response. In AML, favorable-risk cytogenetics including core binding factor AML and NPM1-mutated disease achieve 60 to 70 percent cure rates with chemotherapy, while adverse-risk disease including complex karyotype and TP53 mutation has cure rates of 15 to 25 percent even with transplantation. Age is a major prognostic factor, with outcomes declining significantly in patients over 60 years who have higher-risk disease biology, more comorbidities, and poorer tolerance of intensive therapy. In ALL, pediatric outcomes are excellent with greater than 90 percent cure rates, while adult ALL achieves 30 to 50 percent long-term survival with standard therapy, though novel therapies are improving these outcomes.

Minimal residual disease assessment has emerged as the most powerful prognostic factor in both AML and ALL, guiding treatment decisions and identifying patients at high relapse risk who may benefit from treatment intensification. MRD is detected using flow cytometry or molecular methods with sensitivity of detecting 1 leukemic cell among 10,000 to 1,000,000 normal cells, far exceeding morphologic assessment. MRD negativity after induction or consolidation strongly predicts favorable outcomes with lower relapse rates regardless of presenting features. MRD positivity identifies patients with residual disease below morphologic detection who are at high risk for relapse and may benefit from treatment intensification, transplantation, or novel immunotherapies capable of eliminating MRD-positive disease. The incorporation of MRD into treatment algorithms represents a major advance in risk-adapted therapy.

<image>Panel A: Tumor lysis syndrome showing metabolic derangements, clinical consequences, and prevention with rasburicase mechanism. Panel B: Prognosis in AML by genetic risk category with survival curves. Panel C: Comparison of pediatric versus adult ALL outcomes. Panel D: Minimal residual disease detection methods and prognostic significance for treatment decisions.</image>

Summary

• Acute leukemia is defined by 20 percent or more blasts in bone marrow or blood, classified as AML or ALL based on lineage
• AML occurs predominantly in adults with Auer rods as a pathognomonic finding confirming myeloid lineage
• Genetic risk stratification is critical in AML, determining prognosis and consolidation approach
• APL with t(15;17) causes DIC but is highly curable with ATRA plus arsenic trioxide, requiring recognition of differentiation syndrome
• Standard AML induction with 7+3 achieves remission in 60-80 percent, followed by consolidation with high-dose cytarabine or transplantation based on risk
• ALL is most common in children, with B-ALL comprising 85 percent and T-ALL 15 percent of cases
• Philadelphia chromosome-positive ALL requires TKI therapy in addition to chemotherapy
• ALL treatment includes multi-agent induction, CNS prophylaxis, and maintenance therapy extending 2-3 years
• Novel immunotherapies including blinatumomab, inotuzumab, and CAR-T cells have transformed relapsed ALL treatment
• Tumor lysis syndrome prevention with hydration and allopurinol or rasburicase is essential in high-burden disease
• Minimal residual disease is the most powerful prognostic factor, guiding treatment intensity decisions

Key Terms

This content is subject to the MIT License. © 2024–2026 Hibbert School of Medicine.

Read this lecture as Markdown