Leukemia Pathology — ALL, AML, CLL, CML

Acquisition of Transforming Mutations

• Two-hit model: Leukemia typically arises from sequential acquisition of mutations affecting oncogenes (RAS, MYC, KIT) and tumor suppressor genes (TP53, CDKN2A, IKZF1)
• Chromosomal translocations generating fusion oncoproteins (e.g., t(9;22) BCR-ABL in CML, t(15;17) PML-RARA in APL) provide proliferative and/or anti-apoptotic signals
• Mutations impair epigenetic regulation (DNMT3A, TET2 in AML) and transcription factor function (PAX5, EBF1 in ALL)

Loss of Differentiation and Apoptotic Block

• Maturation arrest occurs at the level of hematopoietic differentiation; ALL blasts are typically lymphoid precursors (L1-L3 morphology), while AML blasts represent myeloid progenitors (M0-M7)
• Anti-apoptotic mechanisms include upregulation of BCL2 family members (especially BCL2 itself), NF-κB activation, and TP53 inactivation
• Leukemic blasts show decreased expression of differentiation-associated antigens (CD34+, HLA-DR+) and increased proliferation (high Ki-67 index)

Clonal Expansion and Bone Marrow Replacement

• Single mutant hematopoietic stem cell undergoes unlimited self-renewal, progressively replacing normal hematopoiesis
• Leukemic cells produce hematopoietic cytokines (IL-6, G-CSF, TNF-α) that may further promote proliferation and systemic inflammation
• Myelophthisis (replacement of marrow space) results in cytopenias: anemia (from red cell suppression), thrombocytopenia (from platelet suppression), and neutropenia (in acute leukemias)
• In CML, myeloid hyperplasia with preserved maturation (in chronic phase) contrasts with blast transformation showing maturation arrest

Disease Heterogeneity by Type

Genetic/Hereditary Factors

• Inherited predisposition syndromes: Down syndrome (↑ risk of ALL, AML), Bloom syndrome, Fanconi anemia, Li-Fraumeni syndrome (TP53 germline mutation → ALL, AML)
• Familial clustering: Increased leukemia risk in monozygotic twins and first-degree relatives suggests inherited germline mutations in epigenetic modifiers (DNMT3A, TET2)

Acquired/Environmental Factors

• Prior chemotherapy and radiation: Alkylating agents and topoisomerase II inhibitors increase risk of therapy-related AML/ALL (t-AML, t-ALL); latency typically 5-7 years for alkylators, 1-3 years for topoisomerase inhibitors
• Benzene exposure: Occupational/environmental exposure linked to AML (and rarely other leukemias)
• Smoking: Modest increased risk for AML; unclear mechanism

Underlying Hematologic Disorders

• Myelodysplastic syndrome (MDS): ~30% progress to AML; cytopenia and dysplastic changes precede transformation
• Polycythemia vera, essential thrombocythemia, primary myelofibrosis: JAK2, CALR, MPL mutations predispose to AML
• Chronic hemolytic anemias (spherocytosis, sickle cell disease): Transfusional iron overload and chronic hypoxia increase AML risk

Viral Factors

• HTLV-1: Causes adult T-cell leukemia/lymphoma (ATLL) with characteristic "flower cell" morphology and hypercalcemia

Age and Prior Disease

• Age: ALL peaks in childhood (2-5 years) but has second peak in elderly; AML, CLL, CML increase with age
• Prior autoimmune disease (systemic lupus erythematosus, rheumatoid arthritis): Associated with increased ALL risk
• Immunosuppression (HIV, post-transplant): Increased NHL and secondary AML/ALL

Common to All Leukemias (Onset and Symptoms)

Acute Leukemias (ALL and AML) — Presentation Features

• Rapid onset (days to weeks) of symptoms: high-grade fever, severe cytopenias, bleeding manifestations
• High white blood cell (WBC) counts often presenting (>50,000–100,000/μL); can reach >500,000 in blast crisis CML or ALL with high disease burden
• Leukostasis syndrome: Complications from extreme leukocytosis (especially in AML with large, deformable blasts and ALL); microvascular occlusion in lungs (dyspnea, hypoxemia) and CNS (confusion, stroke, seizures)
• Disseminated intravascular coagulation (DIC): Particularly in acute promyelocytic leukemia (APL, AML-M3) with t(15;17) PML-RARA; leukemic blasts release tissue factor (TF), triggering coagulation cascade; presents as serious bleeding, thrombocytopenia, ↓fibrinogen, ↑D-dimer, ↑PT/INR
• Tumor lysis syndrome (TLS): Risk especially in ALL (high proliferation rate, high disease burden); hyperuricemia, hyperkalemia, hyperphosphatemia, hypocalcemia from cell lysis; presents with acute renal failure, arrhythmias, seizures
• Meningeal (CNS) involvement: More common in ALL (~5% at diagnosis); presents with headache, cranial nerve palsies, confusion; requires intrathecal chemotherapy
• Testicular infiltration (male ALL): Chloroma (granulocytic sarcoma) formation; presents as painless testicular enlargement

Chronic Leukemias (CLL and CML) — Presentation Features

• Asymptomatic discovery in ~30-50% of CLL (found on routine blood work); indolent course with median survival 10-12 years if untreated
• Gradual symptom onset in CML: chronic phase (median 3-5 years before blast transformation) with mild splenomegaly, mild anemia; accelerated phase and blast crisis (AML-like or ALL-like) are medical emergencies
• CLL-associated autoimmunity: Autoimmune hemolytic anemia (AIHA) with positive direct Coombs test (~10-30%), immune thrombocytopenia (ITP), pure red cell aplasia (PRCA)
• Richter transformation: Conversion of indolent CLL to aggressive diffuse large B-cell lymphoma (DLBCL); occurs in ~5% and is rapidly fatal without intervention

Extramedullary Manifestations

• Chloroma (granulocytic sarcoma): Solid tumor of leukemic blasts; can occur in skin, bone, soft tissues, especially in AML and CML blast crisis
• Hyperuricemia and gout: Especially in ALL and high-burden AML; secondary to high cell turnover
• Hypercalcemia (rare except in ATLL): From osteoclast-activating cytokines (IL-1, TNF, PTHrP)

Peripheral Blood Smear Morphology

Bone Marrow Examination

• Hypercellular marrow (>90% cellularity) with >20% blasts = AML (diagnostic criterion); >80% blasts often seen
• ALL shows >80% lymphoblasts, often >90%; marrow space nearly entirely replaced
• CLL: Lymphocytic infiltration (often 50-90% lymphocytes); may mimic "packed" marrow but cells are mature-appearing
• CML chronic phase: Granulocytic proliferation with left shift, myeloid:erythroid (M:E) ratio markedly elevated (often 10:1 to 50:1 vs. normal 3:1)
• Fibrosis: Reticulin staining shows increased reticulin in CML and some AML; relevant for prognosis

Immunophenotyping (Flow Cytometry) — Gold Standard for Classification

Cytochemistry (Increasingly Replaced by Flow Cytometry)

• Myeloperoxidase (MPO) stain: Positive in AML (>3% of blasts), confirming myeloid lineage; negative in ALL and CLL
• PAS (periodic acid-Schiff) stain: Positive in ALL (coarse granules in cytoplasm); also positive in erythroleukemia
• Sudan black stain: Positive in AML (lipid inclusions); negative in ALL

Cytogenetics and Molecular Testing

ALL Prognostic Cytogenetics

AML Prognostic Cytogenetics

Immediate stabilisation (before definitive therapy)

• Tumor lysis prophylaxis: aggressive IV hydration plus a xanthine oxidase inhibitor (allopurinol) for lower-risk disease, or recombinant urate oxidase (rasburicase) for high burden/high LDH; rasburicase is contraindicated in G6PD deficiency (hemolysis, methemoglobinemia). NCCN supportive-care guidance frames this risk stratification.
• Febrile neutropenia: empiric antipseudomonal beta-lactam monotherapy (cefepime or piperacillin-tazobactam) within an hour of fever, per IDSA — cultures should not delay antibiotics.
• Leukostasis: cytoreduction with hydroxyurea and/or leukapheresis; avoid red cell transfusion before cytoreduction because it raises whole-blood viscosity. In APL, leukapheresis is avoided as it worsens coagulopathy.
• APL coagulopathy: start all-*trans* retinoic acid (ATRA) on morphologic suspicion, before cytogenetic confirmation, and support fibrinogen with cryoprecipitate and platelets — NCCN treats this as an oncologic emergency.

First-line therapy by subtype

• AML (fit): cytarabine plus an anthracycline (daunorubicin) — 7+3 induction; add a FLT3 inhibitor (midostaurin) for FLT3-mutated disease. Unfit/older patients: a BCL-2 inhibitor (venetoclax) plus a hypomethylating agent (azacitidine).
• APL: ATRA plus arsenic trioxide — chemotherapy-sparing and highly curative.
• ALL: multi-agent induction (glucocorticoid, vincristine, anthracycline, asparaginase) with mandatory CNS prophylaxis (intrathecal methotrexate), then consolidation and prolonged maintenance with 6-mercaptopurine and methotrexate; add a BCR-ABL1 TKI (imatinib or dasatinib) if Philadelphia-positive.
• CML: a BCR-ABL1 TKI (imatinib, or second-generation dasatinib/nilotinib), monitored by quantitative BCR-ABL1 transcripts on the International Scale.
• CLL: watchful waiting for asymptomatic early-stage disease — treat only for cytopenias, bulky symptomatic nodes, B symptoms, or rapid lymphocyte doubling; first-line is a covalent BTK inhibitor (acalabrutinib) or venetoclax plus obinutuzumab.

Escalation and definitive therapy

• Relapsed B-ALL: CD19-directed bispecific antibody (blinatumomab), inotuzumab ozogamicin, or CAR-T (tisagenlecleucel).
• Allogeneic HSCT is the definitive curative option for intermediate/adverse-risk AML, relapsed ALL, and CML blast crisis.
• T315I mutation confers resistance to earlier TKIs — use ponatinib or asciminib.

Contraindicated

• Vincristine intrathecally — uniformly fatal ascending myeloencephalopathy.
• Chemoimmunotherapy in del(17p)/TP53-mutated CLL, and live vaccines during immunosuppression; TKIs are teratogenic and avoided in pregnancy.

Disease-related emergencies

• Tumor lysis syndrome (emergency): massive blast turnover releases intracellular contents — hyperkalemia, hyperphosphatemia, hyperuricemia, and secondary hypocalcemia from calcium-phosphate precipitation; signalled by rising creatinine, peaked T waves, tetany, or seizures. Highest risk in ALL and high-count AML.
• Leukostasis (emergency): rigid blasts plug pulmonary and cerebral microvasculature; signalled by hypoxemia with a clear chest film early, confusion, or retinal hemorrhages at WBC counts often exceeding 100,000/μL. AML blasts are larger and less deformable, so leukostasis occurs at lower counts than in CLL.
• DIC in APL (emergency): promyelocyte granules release tissue factor and annexin II-driven fibrinolysis; signalled by low fibrinogen, high D-dimer, prolonged PT, and intracranial or pulmonary hemorrhage.
• Febrile neutropenia (emergency): absent granulocyte defense; a single fever in a neutropenic patient mandates immediate empiric antibiotics per IDSA. Prolonged neutropenia predisposes to invasive Aspergillus and candidemia.

Treatment-related complications

• Differentiation syndrome (emergency): ATRA/arsenic-induced cytokine release from maturing promyelocytes causes fever, dyspnea, weight gain, pulmonary infiltrates, and hypotension — treat with dexamethasone.
• Anthracycline cardiotoxicity: cumulative dose-dependent free-radical myocyte injury producing dilated cardiomyopathy; signalled by falling LVEF on surveillance echo.
• Cyclophosphamide/ifosfamide hemorrhagic cystitis: acrolein metabolite injures urothelium — prevented with mesna and hydration.
• Asparaginase toxicity: depleted protein synthesis causes pancreatitis, hypofibrinogenemia, and paradoxical thrombosis (cerebral sinus thrombosis).
• Methotrexate toxicity: mucositis, myelosuppression, and leukoencephalopathy after intrathecal dosing; leucovorin rescue.
• CAR-T/blinatumomab: cytokine release syndrome with fever and shock — IL-6 receptor antagonist (tocilizumab); plus neurotoxicity (ICANS).
• Venetoclax ramp-up TLS; ibrutinib atrial fibrillation, hypertension, and bleeding; ponatinib arterial occlusive events.
• Allogeneic HSCT: graft-versus-host disease and, if products are not irradiated, transfusion-associated GVHD.

Late and disease-evolution complications

• Therapy-related AML/MDS from alkylators (longer latency, del5q/del7q) and topoisomerase II inhibitors (shorter latency, KMT2A rearrangement).
• CML blast crisis (≥20% blasts) and Richter transformation of CLL to DLBCL — abrupt clinical deterioration with a rising LDH.
• CLL hypogammaglobulinemia: recurrent encapsulated-organism sinopulmonary infection is the leading cause of death.

• Auer rods are myeloperoxidase-rich crystalline inclusions and are specific for myeloid lineage; bundles of them (faggot cells) inside abnormal promyelocytes mean APL. The single best next step is starting ATRA on suspicion, before t(15;17)/PML-RARA confirmation returns — waiting risks fatal intracranial hemorrhage from DIC.
• The Philadelphia chromosome is t(9;22) but not one disease: the p210 BCR-ABL1 fusion typifies CML, while p190 appears in Philadelphia-positive B-ALL. In CML it is near-universal and defines the disease; in adult ALL it is an adverse-risk marker that mandates adding a TKI.
• Smudge cells plus a mature-appearing lymphocytosis in an older adult is CLL, confirmed by the paradoxical CD5+ CD19+ CD23+ immunophenotype — a T-cell antigen aberrantly expressed on a B cell. CLL and small lymphocytic lymphoma are the same disease; nodal-predominant disease does not make it a different entity.
• TdT positivity marks lymphoblasts (ALL) and is absent in AML; conversely MPO positivity marks AML. This one stain distinguishes the two acute leukemias when morphology is ambiguous.
• ≥20% blasts in marrow or blood defines acute leukemia — and in a known CML patient it defines blast crisis, which converts an indolent chronic-phase disease into an allogeneic-transplant emergency. Basophilia is the peripheral clue that CML, not a leukemoid reaction, is present.
• Down syndrome is the association examiners test: increased risk of both ALL and AML (particularly megakaryoblastic AML in young children), often preceded by transient abnormal myelopoiesis in the neonate.
• ALL requires CNS-directed prophylaxis with intrathecal chemotherapy regardless of whether CNS disease is documented, because standard systemic agents penetrate the CSF sanctuary poorly. Vincristine is never given intrathecally.
• Common distractors: rasburicase is contraindicated in G6PD deficiency (use allopurinol); do not transfuse red cells into a hyperleukocytic patient before cytoreduction; and asymptomatic early-stage CLL is observed, not treated — chemotherapy offers no survival benefit at that stage per iwCLL criteria.