Acute Promyelocytic Leukemia
APL is morphologically identified as AML-M3. In the late 1970s, it was discovered that leukemia cells could be forced to differentiate in vitro when exposed to retinoic acid. Based on this observation, a 5-year-old girl with refractory APL was treated with ATRA and remained disease-free. Subsequently, in 24 patients the CR rate improved to 96% with ATRA alone. In addition to high CR rates, ATRA has improved long-term outcomes. Of relevance, ATRA diminishes the initial bleeding tendency the disease is associated with and reduces early mortality.
ATRA is not the only targeted agent for APL. Ai-ling 1, a Chinese remedy historically used to treat a variety of illnesses, was tested in the 1970s in more than 1000 patients with different cancers and was found to induce remissions in approximately two-thirds of patients with APL. The effective component of this remedy was arsenic (III) trioxide (ATO). Importantly, ATO was able to induce remissions in patients who had failed ATRA and conventional chemotherapy. At high concentrations, ATO induces apoptosis of APL, while low doses promote maturation and differentiation.
Approaches to the diagnosis, supportive care measures, and treatment of childhood APL are similar to those in adults with APL, including the immediate initiation of ATRA when APL is suspected, followed by the continued use of ATRA and ATO, with or without the addition of conventional chemotherapy. Follow-up of patients after the end of treatment using Q-polymerase chain reaction (PCR) is essential, as treating molecular relapse yields better results than salvaging overt relapse. An overview of recent pediatric APL studies is given in Table 1.
Table1. Results of Recent Clinical Trials for Pediatric Acute Myeloid Leukemia
Transient Myeloproliferative Disease and Myeloid Leukemia of Down Syndrome
ML DS is associated with a hematologic disorder in infancy, termed TMD. In this disorder, a clonal population of megakaryoblasts accumulates in the peripheral blood, originating from the fetal liver where hematopoiesis takes place prenatally. In the majority of cases, remission is spontaneous within 3 months in the absence of treatment. In certain cases, treatment with cytarabine may be indicated, for example, to avoid liver failure in case of severe hyperbilirubinemia or in case of massive organomegaly. Both Children’s Oncology Group (COG) and BFM have described the natural history of children with TMD. The BFM group tried unsuccessfully to treat children with persistent TMD with low-dose cytarabine to prevent later ML DS. Results of the two studies are detailed in Table 1.
In approximately 20% of TMD cases, patients will later develop AMKL, typically between 1 and 4 years of age. AMKL is the most frequent type of AML in children with DS, and the incidence in these patients is 500-fold higher than in the general population.
Children with ML DS have a favorable prognosis, with OS rates greater than 90%, and should be treated in cooperative group trials that are designed to minimize toxicity while maintaining high cure rates by reducing cumulative chemotherapy exposure, as first demonstrated by Ravindranath and colleagues in the AML8498 study. An overview of more recent trials in ML DS is given in Table 1. The excellent outcome has been at least partly attributed to increased drug sensitivity of the leukemic blasts to chemotherapeutic agents, allowing for dose-reducing chemotherapy. This nicely makes up for the greater risk of toxicity (especially mucositis and infections) with chemotherapy in these children, especially for anthracyclines. Unfortunately, children with ML DS suffering from relapse are almost incurable, which is a poorly understood phenomenon.
Clinical and Laboratory Manifestations and Diagnosis of Acute Myeloid Leukemia in Children
Most children with AML present with symptoms of bone mar row failure, including pallor, fatigue, bleeding (spontaneous nose or gum bleeding), bruising (petechiae), fever, and/or infection. Hepatosplenomegaly, lymphadenopathy, and bone pain are common but usually less prominent than in children with ALL. Because of the acute nature of the process, weight loss and other signs of chronic disease are rare. In some cases, the predominant clinical signs result not from bone marrow disease, but from extramedullary myeloid tumors, referred to as chloromas or granulocytic sarcomas, which most often arise in the orbital or spinal regions. Other clinical manifestations include gingival hypertrophy, commonly seen in AML cases with a monocytic component, and skin or subcutaneous nodules. Cutaneous lesions may be single or multiple violaceous papules or nodules, are most common in infants, and may be mistaken for the blueberry muffin lesions that are associated with congenital infection. Skin nodules sometimes develop before bone marrow involvement and may regress spontaneously. Patients with highly elevated leukocyte counts (hyperleukocytosis) may also present with central nervous system (CNS) symptoms (bleeding, seizure, or stroke) or pulmonary symptoms related to hyperviscosity. Testicular infiltration may occur in a minority of patients.
Among children with AML, the median leukocyte count at diagnosis is approximately 20 × 109/L, with a median hemoglobin con centration of approximately 9 g/dL and a median platelet count of 60 × 109/L. Although a minority of children with AML have no morphologically detectable circulating blasts, careful examination of the peripheral blood smear and/or flow cytometry will reveal leukemic cells in most cases. Sometimes a diagnosis will only become evident with a bone marrow aspirate/trephine or biopsy of a chloroma. Prolonged prothrombin, thrombin, and partial thromboplastin times, as well as decreased fibrinogen levels, are seen in the majority of patients with APL and about 5% of other AML cases. Clinically, these patients present with significant bleeding tendency typical for clotting disorders such as large bruises rather than just petechiae.
Abnormalities of serum chemistries may include hyperuricemia, although this is less common and less severe than hyperuricemia associated with ALL. The presence of hypokalemia, a rare finding in patients with ALL, suggests a diagnosis of monoblastic leukemia. Tumor lysis may also be evident from low calcium and high phosphate levels as well as increased creatinine levels. High LDH levels may be indicative of tumor burden and rates of cellular turn-over.
Among patients who present with circulating blasts, the differential diagnosis is generally limited to various types of leukemia, including AML, ALL, and juvenile myelomonocytic leukemia (JMML). For patients who do not have blasts in their blood at the time of presentation, the differential diagnosis is determined by signs and symptoms and may include aplastic anemia, autoimmune or inflammatory dis ease, infection, and solid malignancies.
In most cases, the final diagnosis of AML is made by bone mar row examination. A bone marrow aspiration should be performed for morphologic examination, immunophenotyping, genetic, and molecular studies, whereas a bone marrow biopsy is used to assess cellularity and dysplastic features, or when an aspirate fails.5 A diagnosis of AML is confirmed when 20% or more of nucleated bone marrow cells are blasts of myeloid origin, or when the blasts contain AML specific genetic lesions, regardless of blast percentage. In children, the blast percentage may not be the most reliable marker to differentiate pediatric AML from myelodysplastic syndrome (MDS), and other clinical factors may need to be taken into account. Nowadays, a morphological diagnosis of AML is confirmed using multiparameter flow cytometry, including markers that diagnose precursor stage such as CD34, CD117, CD133, and HLA-DR, as well as lineage-specific markers, for example, CD41, CD42, and CD61 for megakaryocytic differentiation. The mandatory minimal panel required to fulfill WHO and EGIL criteria for AML includes CD34, CD117, CD11b, CD11c, CD13, CD14, CD15, CD33, CD64, CD65, iMPO, i-lyso zyme, CD41, and CD61; and for MPAL: CD19, iCD79a, iCD22, CD10, and iCD3. Aberrant markers may be present and may help to define patient-specific leukemia-associated immune-phenotype (LAIP) that can be used for flow cytometric minimal residual dis ease (MRD) detection during treatment, including for example, CD2, CD7, CD19, or CD56. In case a LAIP cannot be established, MRD can still be followed using the “different from normal” approach, which uses aberrant differentiation/maturation profiles compared to normal, as described in the European Leukemia Net (ELN) MRD Working party guidelines. Moreover, flow cytometry may help to assess response after the first and second course of chemotherapy, as a reliable blast percentage after chemotherapy in the context of regenerating bone marrow may be difficult to assess. In difficult cases, FISH or Q-PCR may also be helpful in response determination (see box on Diagnosis of Acute Myeloid Leukemia).
When 20% to 30% of blasts are present and advanced MDS is considered in the differential diagnosis, repeated bone-marrow punctures may be needed to assess disease progression over time, as well as clinical signs such as organomegaly or CNS involvement that favor a diagnosis of AML over advanced MDS. Identifying underlying MDS is relevant as children with MDS need to be transplanted as definitive therapy.
When a diagnosis of AML is confirmed, most investigators classify each case according to the WHO 2016 criteria (see Table 2). This requires at least some genetic studies, and groups make use of various techniques including G-banding (conventional karyotyping), which can detect structural and numerical cytogenetic abnormalities in 70% to 80% of children with AML; FISH to confirm karyotype findings and identify cryptic rearrangements undetectable by G-banding; and molecular genetic approaches such as PCR, target-panel sequencing, genome-wide RNA sequencing, exome sequencing, or whole-genome sequencing. Diagnostic testing may be dependent on resources and the requirements to use these markers for risk-group differentiation. Quantitative PCR of certain markers such as fusion genes may also be used for tracking residual disease during therapy.
Table2. The 2016 World Health Organization Classification of Acute Myeloid Leukemia and Related Neoplasms
Since AML may involve the CNS, examination of the cerebrospinal fluid (CSF) should also be performed as part of the initial workup of all patients. Although CNS leukemia is traditionally defined as the presence of at ≥5 leukocytes/μL of CSF with leukemic blast cells present, the significance of lower levels of CNS involvement differs between studies. The diagnosis of CNS involvement may also be based on the presence of cranial nerve palsies or radiologic evidence of leukemic infiltration. Some groups defer examination of the CSF until day 8 of therapy, especially in cases with hyperleukocytosis.
