Leukemia Limfoblastik Akut (ALL)

01Pathophysiology

ALL arises from the clonal expansion of lymphoid progenitor cells that have acquired genetic mutations disrupting normal differentiation. These immature cells, called lymphoblasts, accumulate in the bone marrow and progressively replace normal hematopoietic tissue. This is the central mechanism that connects all of the patient's symptoms to a single unifying process: bone marrow failure.

Because the marrow is overtaken by nonfunctional blasts, the production of mature blood cell lines is suppressed. Reduced red blood cell production causes anemia, which manifests as fatigue, pallor, and exertional dyspnea. Thrombocytopenia from inadequate megakaryocyte development leads to petechiae, easy bruising, mucosal bleeding, and epistaxis. Neutropenia impairs the immune response, making the child vulnerable to recurrent or severe infections, often presenting as unexplained fevers.

The proliferating blasts also expand beyond the marrow. Infiltration of the periosteum and bone cortex causes the characteristic bone pain, which in young children may present as refusal to bear weight or a new-onset limp. This is a classic vignette trigger. Blast infiltration of the liver, spleen, and lymph nodes produces hepatosplenomegaly and diffuse lymphadenopathy. In T-cell ALL, blasts frequently accumulate in the thymus, producing a large anterior mediastinal mass that may cause superior vena cava syndrome or respiratory distress.

Extramedullary spread to the central nervous system (CNS) and testes represents sanctuary sites where chemotherapy penetration is limited. CNS involvement may cause headaches, cranial nerve palsies (especially CN VI and VII), vomiting, and papilledema. Testicular involvement presents as painless testicular enlargement. Both of these sites are critically important in treatment planning because they require directed prophylactic or therapeutic intervention.

At the molecular level, several recurrent genetic alterations drive leukemogenesis. The t(12;21) translocation producing the ETV6-RUNX1 (TEL-AML1) fusion is the most common genetic abnormality in childhood B-ALL and confers a favorable prognosis. In contrast, the t(9;22) Philadelphia chromosome producing the BCR-ABL1 fusion is associated with a poor prognosis but is targetable with tyrosine kinase inhibitors. Hyperdiploidy (>50 chromosomes) in blast cells is another favorable marker, while hypodiploidy and MLL (KMT2A) gene rearrangements (especially in infants) carry an unfavorable prognosis. Understanding these is essential because exam questions frequently test the association between a cytogenetic finding and the expected clinical outcome.

02Classification and Clinical Manifestation

Immunophenotypic Classification

Risk Stratification (Pediatric B-ALL)

Clinical Manifestation by Organ System

03Diagnostic Workup

The workup begins with a CBC and peripheral blood smear when a child presents with unexplained cytopenias, bone pain, or persistent fever. The classic finding is pancytopenia (low hemoglobin, low platelets, variable WBC), though it is important to recognize that the white blood cell count can be low, normal, or extremely elevated. A common exam trap is assuming that leukemia always means leukocytosis; in reality, about one-quarter of patients present with leukopenia. The peripheral smear is examined for circulating lymphoblasts, which appear as large immature cells with high nuclear-to-cytoplasmic ratios, open (fine) chromatin, and inconspicuous nucleoli.

If blasts are identified or strongly suspected, the next step is a bone marrow aspiration and biopsy, which is the gold standard for diagnosis. The WHO defines ALL as the presence of ≥20% lymphoblasts in the bone marrow. In most cases, the marrow is packed with blasts far exceeding this threshold. This is the test that confirms the diagnosis.

Flow cytometry is then performed on the marrow aspirate to determine the immunophenotype. This is the test that distinguishes B-ALL from T-ALL and excludes acute myeloid leukemia (AML). The hallmark markers include TdT (terminal deoxynucleotidyl transferase), which is positive in virtually all precursor lymphoblastic leukemias but negative in mature B-cell neoplasms and myeloid leukemias. CD10 (CALLA, common acute lymphoblastic leukemia antigen) positivity is characteristic of B-ALL and is associated with a favorable outcome.

Cytogenetic analysis and molecular studies (FISH, PCR) are essential for risk stratification and treatment planning. The presence of the Philadelphia chromosome, t(9;22), is particularly important because it changes management by adding a tyrosine kinase inhibitor (imatinib or dasatinib) to the chemotherapy regimen. Hyperdiploidy and t(12;21) are favorable findings, while hypodiploidy and MLL rearrangements indicate a need for intensified therapy.

A lumbar puncture is performed at diagnosis, both for staging and to initiate intrathecal chemotherapy. The CNS status is classified as:

• CNS-1: No blasts in CSF

• CNS-2: Blasts present but <5 WBC/μL

• CNS-3: Blasts present with ≥5 WBC/μL, or cranial nerve palsy, or hypothalamic involvement

A chest X-ray should be obtained early, particularly in adolescent males, to evaluate for an anterior mediastinal mass suggestive of T-ALL. This finding has immediate clinical implications because a large mediastinal mass can cause airway compression or SVC syndrome, which may be a medical emergency requiring careful anesthetic planning.

Baseline metabolic studies are drawn to assess for tumor lysis syndrome (TLS), which can occur spontaneously before treatment begins (especially with very high WBC counts) or upon initiation of chemotherapy. The classic TLS laboratory profile includes elevated uric acid, elevated potassium, elevated phosphorus, decreased calcium, elevated LDH, and rising creatinine.

04Management and Treatment

Pre-treatment stabilization is the first priority. Before any chemotherapy is initiated, the clinician must assess for and prevent tumor lysis syndrome. This involves aggressive intravenous hydration (typically 2 to 3 times maintenance fluid rate) and administration of rasburicase (0.2 mg/kg IV, single dose) in patients with a high tumor burden or elevated uric acid. Rasburicase is a recombinant urate oxidase that rapidly converts uric acid to allantoin. It is preferred over allopurinol in the acute setting because it reduces existing uric acid levels rather than merely preventing new formation. Rasburicase is contraindicated in patients with G6PD deficiency because it generates hydrogen peroxide, which can cause severe hemolytic anemia in these individuals. This is a frequently tested contraindication.

Induction therapy aims to achieve complete remission, defined as <5% blasts in the bone marrow with recovery of normal blood counts. The standard regimen includes:

• Vincristine: 1.5 mg/m² IV weekly (capped at 2 mg per dose) for 4 weeks

• Dexamethasone: 6 mg/m²/day orally for 28 days (preferred over prednisone in many pediatric protocols due to better CNS penetration) or prednisone 40 to 60 mg/m²/day

• PEG-asparaginase: 2,500 IU/m² IM or IV, typically one dose during induction

• Daunorubicin (anthracycline): added for high-risk patients, 25 mg/m² IV weekly

Complete remission is achieved in approximately 95 to 98% of children with standard-risk ALL after induction. However, remission alone is not cure. Minimal residual disease (MRD) assessment by flow cytometry or PCR at the end of induction is a powerful predictor of outcome. MRD-positive patients are escalated to higher-risk treatment arms.

Consolidation and intensification phases use rotating multi-agent blocks to deepen the remission and eliminate resistant clones. High-dose methotrexate (typically 5 g/m² IV with leucovorin rescue) is a backbone agent in this phase. Cyclophosphamide, cytarabine, and additional asparaginase are included in various combinations depending on the protocol.

CNS-directed therapy is administered throughout treatment. The standard approach involves intrathecal methotrexate given at scheduled intervals during all treatment phases. This replaced routine cranial irradiation, which was previously standard but caused unacceptable long-term neurocognitive toxicity and secondary malignancies. Cranial radiation is now reserved only for patients with CNS-3 disease at diagnosis or those with CNS relapse.

Maintenance therapy is the longest phase, extending the total treatment duration to 2 to 3 years from the time of diagnosis. It consists of:

• 6-mercaptopurine (6-MP): 75 mg/m²/day orally, taken once daily at bedtime on an empty stomach. Importantly, 6-MP should not be taken with milk or dairy products because xanthine oxidase in milk can reduce drug bioavailability. Patients with TPMT (thiopurine methyltransferase) deficiency are at risk for severe myelosuppression and require dose reduction or alternative therapy. TPMT and NUDT15 genotyping should be performed before starting 6-MP.

• Methotrexate: 20 mg/m² orally once weekly

Some protocols add monthly vincristine and dexamethasone pulses during maintenance.

For Philadelphia chromosome-positive ALL, the addition of a tyrosine kinase inhibitor (TKI) has dramatically improved outcomes. Imatinib or dasatinib is incorporated into each phase of treatment alongside the standard chemotherapy backbone. This combination has shifted Ph+ ALL from one of the worst prognostic categories to one with substantially improved survival, reducing the need for hematopoietic stem cell transplant in first remission.

Relapsed ALL is managed based on the site and timing of relapse. Early bone marrow relapse (within 18 months of diagnosis) carries the worst prognosis and often requires allogeneic hematopoietic stem cell transplantation after achieving a second remission. Novel agents for relapsed or refractory ALL include blinatumomab (a bispecific T-cell engager targeting CD19), inotuzumab ozogamicin (an anti-CD22 antibody-drug conjugate), and CAR T-cell therapy (tisagenlecleucel, targeting CD19). These represent high-yield emerging topics on modern exams.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common trap with ALL is misdiagnosing the initial presentation as a rheumatologic or orthopedic condition. A child who presents with bone pain, a limp, or refusal to walk is frequently worked up for JIA, osteomyelitis, or a fracture. The teaching point embedded in many exam questions is that any child with bone pain and unexplained cytopenias needs a CBC and peripheral smear before a rheumatologic diagnosis is considered. The clue in the vignette is usually a concurrent finding of pallor, petechiae, or an abnormally low platelet count that would not be expected in a purely musculoskeletal condition. If a student sees "child with joint pain and low platelets," the answer is almost always leukemia, not JIA.

Another commonly tested concept is the distinction between ALL and AML on a peripheral smear. Auer rods are pathognomonic for AML. TdT positivity points to a lymphoid neoplasm. MPO positivity indicates myeloid lineage. Exam writers exploit the superficial similarity between the two to test whether students can apply these markers correctly.

Tumor lysis syndrome is a high-frequency testing topic. Students must know the classic metabolic profile (high potassium, high phosphorus, high uric acid, low calcium) and that rasburicase is contraindicated in G6PD deficiency. A question may describe a child about to start chemotherapy with high uric acid and then ask for the next step, offering both allopurinol and rasburicase as options. The correct answer depends on whether G6PD deficiency is mentioned in the stem.

The role of TPMT testing before initiating 6-mercaptopurine is another pearl. A homozygous TPMT-deficient patient given standard-dose 6-MP will develop life-threatening myelosuppression. This pharmacogenomic concept is increasingly testable.

Finally, students should recognize the presentation of T-ALL as an adolescent male with an anterior mediastinal mass, respiratory distress, and a very high WBC count. This is distinct from the typical B-ALL presentation and is designed to test whether the student can link the mediastinal mass to thymic involvement and the T-cell immunophenotype. The differential for an anterior mediastinal mass in an adolescent (the "4 T's": thymoma, teratoma, terrible lymphoma, thyroid) is a classic board question, and T-ALL belongs in that differential.

The core competency being tested across ALL questions is the ability to recognize bone marrow failure as the unifying mechanism, to correctly sequence the diagnostic workup from CBC to bone marrow biopsy to flow cytometry, and to integrate cytogenetic findings into risk-stratified treatment decisions.