Acute Myeloid Leukemia (AML

AML occurs due to genetic changes in the HPC that alter hematopoietic proliferation and differentiation. As a result, patients have large numbers of abnormal, immature myeloid cells in the bone marrow and peripheral blood. A pool of self-renewing malignant cells maintains the disease and is regarded as leukemic stem cells.

Leukemogenic mutations and cytogenetic changes can be inherited in familial AML cases or evolve from exposure to chemotherapy, ionizing radiation, chemical exposure, and infection with retroviruses. Specific recurrent cytogenetic abnormalities and mutations are part of the risk stratification of AML and predict the outcome of affected patients and the intensity of treatment.

AML account for 15% of all childhood leukemia. Between 1975 and 2010, the survival of patients with AML has increased from less than 20% to 68%.

Risk stratification The risk stratification of pediatric AML is based on cytogenetic and molecular features. Low-risk features are generally agreed upon; however, the consensus regarding high-risk molecular features is more complex.

Table 8.11: Risk stratification for AML (Know this!)

Favorable-risk (25%) Intermediate risk (60%) Unfavorable risk (15%)

• t(8;21); RUNX1-

RUNX1T1* • inv(16); CBFB-MYH11* • Mutated NPM1# • Mutated CEBPA • t(15;17); PML-RARA • Neither favorable nor unfavorable risk • t(6;9); DEK-NUP214 • NUP98-NSD1 • Specific MLL/KMT2A rearrangements## • CBFA2T3A-GLIS2 • Abnormalities of 3q • Monosomy 5 or del(5q) • Monosomy 7 • Complex karyotype$ • High allelic FLT3-ITD ratio+

*Core binding factor AML #No co-occurring FLT3-ITD mutations ##Not t(9;11)(p22;q23) which accounts for 50% of all MKT2A rearrangements $Three or more cytogenetic abnormalities +Greater than 0.4 to 0.5; refers to the number of ITD-mutated versus wildtype alleles

Response to induction chemotherapy The role of MRD at the end of induction has not been shown to have definitive prognostic significance in children with favorable or unfavorable risk. However, for children with intermediate-risk, who do not carry cytogenetic or molecular abnormalities with prognostic value, the most important prognostic factor is MRD at the end of induction therapy. MRD-negative patients have an EFS of 65%. MRD-positive patients have a risk of relapse of 60%.

Table 8.12: Clinical presentation of patients with AML Symptoms Complications

• Fever • Malaise • Musculoskeletal pains • Lymphadenopathy • Hepatosplenomegaly • Bleeding • DIC • Hyperleukocytosis and leukostasis • Neurologic deficits o Lethargy o Mental status changes o Cranial nerve palsies • Acute kidney injury DIC: Disseminated intravascular coagulopathy

Diagnostic work-up Aside from a thorough history and physical examination, the following tests are performed in children with suspected AML: • CBC • Peripheral blood smear • Coagulation studies • Electrolyte panel • Lumbar puncture with CSF analysis for cell count, protein, glucose, cytology • Unilateral bone marrow biopsy for pathologic review • Unilateral bone marrow aspirate for immunophenotyping, metaphase cytogenetics, FISH analysis, specific molecular testing

Table 8.13: Diagnostic findings in AML Test Finding

Morphology • Immature cells with large nuclei, usually with prominent nucleoli, a variable amount of pale blue cytoplasm • Auer rods • Use FAB classification Immunophenotype • Expression of CD11b, CD34, CD33, CD45, CD64, CD65, CD117, MPO, lysozyme Cytogenetics/FISH • t(8;21), inv(16), t(15;17), and 11q23 translocations Mutation analysis • PCR or NGS for NPM1, CEBPA, FLT3-ITD, KIT, WT1 CSF analysis • Cytologic confirmation of leukemia cells • Positive if >5/µL WBCs in the CSF with blasts in a non-bloody lumbar puncture FAB: French-American-British; CSF: Cerebrospinal fluid; MPO: Myeloperoxidase; NGS: Nextgeneration sequencing; PCR: Polymerase chain reaction; WBCs: White blood cells There are two systems according to which AML is classified. The FAB classification was the first system that classified AML based on morphologic and histochemical features. Taking the degree of differentiation into account, AML is assigned from M0 to M7 and requires 30% of blasts in the bone marrow. This classification system has been replaced mainly by the WHO system, which groups AML based on morphology and IHC detection of surface markers, lower the required percentage of blasts to be 20% in the bone marrow.

Table 8.14: FAB classification FAB group Features

M0 No differentiation • No MPO • Positive for CD13, CD33, CD117 • No lymphoid differentiation.

M1 Minimal differentiation • Positive for MPO

M2 With differentiation • Auer rods • t(8;21) • Chloromas

M3 Promyelocytic, hypergranular type • Auer rods • t(15;17) • DIC

M3v Promyelocytic, microgranular type M4 Myelomonocytic

M4Eo With eosinophilia • Same clinical, cytogenetic, therapeutic implications as M3 • Fine granularity

• Myeloblasts (at least 20%) and monoblasts • Often peripheral blood monocytosis • 11q23 rearrangement • >5% dysplastic eosinophil precursors with basophilic granules • inv(16)

M5 Monocytic

M6 Erythroblastic • >5% monoblasts • M5a: Monoblastic • M5b: Monocytic • 11q23 rearrangement • Chloromas, CNS involvement, gingival hyperplasia • M6a: Erythroleukemia • M6b: Pure erythroid leukemia • M6c: Presence of myeloblasts and proerythroblasts.

M7 Megakaryoblastic

• t(1;22) • Often in Down syndrome • Myelofibrosis DIC: Disseminated intravascular coagulopathy; MPO: Myeloperoxidase

Figure 8.6: Auer rods

Auer rods are needle-like structures that are pathognomonic for AML. They emerge after an abnormal fusion of the primary (azurophilic) granules.

Table 8.15: 2016 WHO classification of AML Category Specifics

Recurrent genetic abnormalities

No minimum blast percentage needed

• inv(16); CBFB-MYH11 • t(8;21); RUNX1-RUNX1T1 • t(15;17); PML-RARA • t(9;11); MLLT3-KMT2A • t(6;9); DEK-NUP214 • inv(3) or t(3;3); GATA2 • t(1;22); RBM15-MKL1 • t(9;22); BCR-ABL1 • Mutated NPM1 • Biallelically mutated CEBPA

Therapy-related AML AML with MDS features • Monosomy 7 • del(5q) • Monosomy 5

Myeloid proliferations with Down syndrome • TAM • AMKL with Down syndrome

Myeloid sarcoma AML, NOS • No maturation (equivalent to M0) • With minimal differentiation (M1) • With maturation (M2) • Myelomonocytic (M4) • Monoblastic/monocytic leukemia (M5a/5b) • Pure erythroid leukemia (M6) • AMKL (M7) • Acute basophilic leukemia • Acute panmyelosis with myelofibrosis AMKL: Acute megakaryoblastic leukemia; AML: Acute myeloid leukemia; MDS: Myelodysplastic syndrome; NOS: Not otherwise specified; TAM: Transient abnormal myelopoiesis

Histologic examples of FAB subtypes:

Figure 8.7: FAB-M0 Figure 8.8: FAB-M1 Figure 8.9: FAB-M2

Figure 8.10: FAB-M3 Figure 8.11: FAB-M4 Figure 8.12: FAB-M5a

Figure 8.13: FAB-M5b Figure 8.14: FAB-M6 Figure 8.15: FAB-M7

Definition of CR • Peripheral blood counts recover to normal levels o WBC count >1,000/μL o ANC >1,000/μL o Platelet count >100,000/μL • Mildly hypocellular to normal cellular marrow with fewer than 5% blasts • No clinical signs or symptoms of the disease including at extramedullary sites

Induction therapy Chemotherapy for AML is administered as two courses of intensive induction chemotherapy, followed by consolidation with cytarabine or HSCT. Standard induction chemotherapy comprises an anthracycline and extended exposure to cytarabine. Some regimens use a third drug, such as etoposide or 6-TG.

Consolidation therapy Consolidation therapy incorporates high-dose cytarabine or allogeneic HSCT. Children with highrisk cytogenetics in first CR or with therapy-related AML are offered to undergo allogeneic HSCT, whereas patients with favorable features should receive consolidative chemotherapy. Matched sibling donors are preferred. In patients with FLT3-ITD translocations, the FLT3 inhibitor sorafenib improves survival in children when given for up to two years after HSCT.