B-Precursor Cell ALL

The B-precursor cell phenotype is present in 80% to 85% of children with ALL. These leukemia cells are characterized by reactivity with monoclonal antibodies specific for B-cell–associated antigens (e.g., CD9, CD19, CD22, and CD79a) and are distinguished from mature B-cell ALL by the absence of surface immunoglobulin. In the vast majority (80% to 90%) of B-precursor cell ALL, the cells express CD10, also known as the common ALL antigen (CALLA).

Over 90% of cases of B-precursor cell ALL have evidence of immunoglobulin gene rearrangements, predominantly involving the immunoglobulin heavy chain (IgH). Of note, T-cell receptor (TCR) gene rearrangements also may be observed in B-precursor cell ALL and IgH gene rearrangements have been observed in T-cell ALL. These leukemia-specific clonal IgH and TCR gene rearrangements have been used to quantify submicroscopic levels of residual leukemia in patients using polymerase chain reaction (PCR)-based assays.

The presence of intracytoplasmic immunoglobulin (cIg) and various cell surface markers has been used to distinguish different subsets of B-precursor cell ALL based on their level of differentiation. These subsets include pro-B ALL (3% to 4% of pediatric patients), early pre-B ALL (60% to 70% of pediatric patients), and pre-B ALL (20% to 30% of pediatric patients):

• •
  

  Pro–B-cell ALL, believed to be derived from a very immature B-cell precursor, is characterized by CD10-negative immunophenotype and the absence of cIg. It is most frequently observed in infants with ALL, especially those with abnormalities involving chromosome 11q23 ( MLL [also known as KMT2A ] gene locus).

  
  
• •
  

  Early pre–B-cell ALL is the most common subtype of B-precursor cell ALL in children. It is believed to be derived from a more mature B-cell precursor than pro-B ALL. Early pre–B cells frequently express CD10 but still lack cIg. CD10 (CALLA)-positive leukemias without cytoplasmic immunoglobulin are also referred to as common ALL (c-ALL).

  
  
• •
  

  Pre–B-cell ALL is characterized by the presence of cIg. It is believed to derive from an intermediate B-precursor cell, more mature than those lacking cIg (early pre–B cells) but not as mature as those with surface immunoglobulin (mature B cells). Like early pre–B cells, pre–B lymphoblasts typically express CD10 and human leukocyte antigen (HLA)-DR. Approximately 25% of cases of pre–B-cell (cIg+) ALL have the chromosomal translocation t(1;19)(q23;p13), which fuses the TFPT (formerly E2A ) gene on chromosome 19 with the PBX1 gene on chromosome 1. This translocation is observed in only 1% of cases of early pre-B (or common) ALL.

Mature B-Cell ALL

Mature B-cell ALL accounts for 1% to 2% of childhood ALL cases. It is characterized by the presence of surface immunoglobulin, most often IgM, which is monoclonal for κ or λ light chains. The cells generally express other B-cell antigens, including CD19, CD20, and HLA-DR. Morphologically, the blasts tend to be classified as FAB L3, with intensely staining cytoplasmic basophilia similar to that of erythroblasts. Cases of mature B-cell ALL without L3 morphologic features have been reported but are rare. Almost all cases of mature B-cell ALL are associated with one of three nonrandom chromosomal translocations—t(8;14)(q24;q32) or, less commonly, t(2;8)(p12;q24) or t(8;22)(q24;q11). The chromosomal breakpoints involve the MYC oncogene on chromosome 8 and the genes for the IgH (chromosome 14), κ light chain (chromosome 2), or λ light chain (chromosome 22). Mature B-cell ALL is clinically indistinguishable from disseminated Burkitt lymphoma and is much more successfully treated with regimens used for that disease than when treated with conventional childhood ALL therapy.

T-Cell ALL

The T-cell immunophenotype is present in 10% to 15% of children with ALL. Compared with B-precursor cell ALL, T-cell ALL is more frequently associated with older age at diagnosis, higher presenting leukocyte counts, and bulky extramedullary disease (including lymphadenopathy, hepatosplenomegaly, overt central nervous system [CNS] leukemia, and an anterior mediastinal thymic mass). Historically, patients with T-cell ALL had an inferior outcome compared with those with B-precursor cell ALL, although this prognostic difference is not observed when more intensive regimens are used. However, the time period in which relapses occur differs between the two immunophenotypic subtypes, with earlier relapses observed more frequently in T-cell ALL.

T-cell ALL can be subclassified using monoclonal antibodies that recognize surface antigens expressed during discrete stages of normal T-cell development. Most of the phenotypically defined subsets of T-cell ALL have limited clinical relevance. However, gene expression profile studies have identified a biologically distinctive subset of T-cell ALL that, in retrospective analysis, was associated with a high risk for treatment failure. These patients were noted to have a distinctive “early T-cell precursor” phenotype consisting of weak expression of CD5 and coexpression of stem cell or myeloid markers. Similarly, another retrospective study using array comparative genomic hybridization (CGH) and quantitative DNA-PCR techniques identified the absence of V(D)J recombination at the TCRγ locus, a feature that is characteristic of early T-cell precursor cells, as a predictor of early treatment failure in patients with T-cell ALL. Thus, T-cell ALL patients with features of development arrest at the earliest stages of T-cell development appear to have an inferior outcome.

Myeloid Antigen Coexpression

In some instances, individual leukemic cells simultaneously express lymphoid and myeloid surface antigens. Up to 20% of cases of ALL (i.e., with lymphoid surface antigen predominance) also coexpress myeloid antigens. Myeloid antigen coexpression is associated with young age at diagnosis (<12 months), pro–B-cell (CD10−) immunophenotype, and certain chromosomal abnormalities, such as rearrangements of chromosome 11q23 (frequently observed in infants with ALL), the Philadelphia (Ph) chromosome [t(9;22)], and the ETV6-RUNX1 [t(12;21)] fusion. Older studies suggested that patients with biphenotypic ALL had a worse prognosis, but more recent reports have indicated that myeloid antigen coexpression lacks independent prognostic significance. Cases of ALL with myeloid antigen coexpression should be distinguished from leukemia of ambiguous lineage, an uncommon entity in which the predominant lineage (lymphoid or myeloid) cannot be determined by immunophenotype and histochemistry. The World Health Organization classification system requires that cells express myeloperoxidase or evidence of monocytic differentiation, along with either T-cell antigen CD3 (cytoplasmic or surface) or B-cell surface antigens (CD19, CD79a, CD22, and CD10).

ETV6-RUNX1 Fusion Genes in B-Precursor Cell ALL

In 20% to 25% of childhood ALL cases a cryptic t(12;21) translocation results in expression of a fusion ETV6-RUNX1 (also known as TEL-AML1 ) translocation. ETV6 and RUNX1 are transcription factors that are each required for normal hematopoiesis, and both are involved in other leukemia-related translocations. The t(12;21) translocation results in the fusion of the HLH domain of ETV6 to almost all of RUNX1 . The mechanisms through which ETV6-RUNX1 fusions drive leukemogenesis remain incompletely understood, but recent work has shown that expression of this fusion oncoprotein in hematopoietic stem cells markedly increases their numbers and promotes their transformation after chemical mutagenesis. Interestingly, ETV6-RUNX1 translocations can often be detected in prenatal blood specimens collected from children who went on to develop ETV6-RUNX1 –rearranged ALL many years later, indicating that it is likely the initiating mutation in ALL. However, this translocation is not sufficient for leukemogenesis because the incidence of detectable ETV6-RUNX1 fusions in normal neonatal blood is 100-fold greater than the incidence of leukemia with this translocation. Several studies have shown that the presence of an ETV6-RUNX1 translocation is associated with a favorable prognosis in childhood ALL. Indeed, ETV6-RUNX1 expression is associated with decreased expression of the ABCB1 (also known as MDR1 ) multidrug resistance gene and of genes involved in purine metabolism, which may explain the particular sensitivity of these cases to mercaptopurine and methotrexate.

MYC Translocations in Mature B-Cell ALL

Chromosome translocations that place the coding region of the MYC protooncogene under the control of gene regulatory elements of one of the immunoglobulin receptor genes, such as the t(8;14), t(2;8) or t(8;22), lead to aberrant overexpression of the MYC oncogenic tran­scription factor. MYC overexpression is potently oncogenic in a wide range of experimental systems. Although MYC has long been believed to be a traditional transcription factor that regulates a specific set of target genes, recent work has revealed that MYC accumulates at the promoters of essentially all transcriptionally active genes and acts as a nonlinear amplifier of existing gene expression programs. Thus, rather than inducing expression of a specific gene expression program, these findings suggest that MYC instead acts to lock a cell’s gene expression program into its existing form, which presumably can be oncogenic when it occurs in a cell that is in a highly proliferative state that normally should be transient. MYC translocations are characteristically found in mature B-cell leukemia, which is best considered a disseminated form of Burkitt lymphoma that responds poorly to standard ALL therapy. Outcomes for these patients are much improved with treatment regimens specifically designed for high-grade B-cell non-Hodgkin lymphomas.

Activating NOTCH1 Mutations in T-Cell ALL

NOTCH1 encodes an unusual cell surface receptor that, on activation by ligand binding to its extracellular domain, undergoes two distinct proteolytic cleavage events, culminating in release of the intracellular domain of NOTCH1 (ICN) into the cytoplasm. The ICN protein subsequently translocates to the nucleus, where it functions as a transcription factor that generally activates expression of its targets. NOTCH1 is required at multiple stages of T-cell development, and its ectopic activation in murine T-cell progenitors is highly leukemogenic. Although NOTCH1 can be activated by chromosomal translocations in very rare cases of T-cell ALL, more than 50% of T-cell ALL cases harbor activating NOTCH1 mutations, and these occur in all of the molecular subtypes of T-cell ALL. Activating NOTCH1 mutations typically occur either as missense mutations in the heterodimerization domain, where they promote ligand-independent proteolytic activation of the ICN domain, or as truncating or frameshift mutations in the C-terminal PEST domain of ICN, which lead to ICN protein accumulation as a result of impaired proteasomal degradation. The pathogenic role of NOTCH1 in T-cell ALL is mediated largely by its role as a transcriptional activator of two key downstream targets, MYC and HES1 .

One of the key proteolytic cleavage events required for NOTCH1 activation is mediated by the γ-secretase complex. Treatment of NOTCH1-mutant T-cell ALL cell lines with γ-secretase inhibitors leads to modest reductions in cell proliferation in a subset of cell lines, but a number of NOTCH1-mutant T-cell ALL cell lines fail to respond to these inhibitors. The mechanisms of resistance to NOTCH1 inhibition have been an area of intense investigation, and recent work has revealed that inactivation of the FBXW7 tumor suppressor, an E3 ubiquitin ligase that targets both ICN and MYC for proteasomal degradation, is one mechanism of resistance to γ-secretase inhibitors. Additionally, resistance to inhibition of NOTCH1 or its downstream target MYC can be mediated by PTEN inactivation and resultant AKT activation in T-cell ALL, thus suggesting the potential therapeutic utility of AKT pathway inhibitors as a strategy to reverse resistance to γ-secretase inhibitor. Moreover, glucocorticoid resistance in T-cell ALL has been shown to be reversible by γ-secretase inhibitors, and it appears that glucocorticoid therapy can ameliorate the intestinal toxicity of γ-secretase inhibitors. Taken together, these findings provide a compelling rationale for clinical trials combining glucocorticoids with small molecule inhibitors of NOTCH1 and AKT.

Cytokine Receptor Signaling

Cytokine receptor signaling plays key roles in normal lymphocyte development, with inactivating mutations in key cytokine receptor subunits (e.g., interleukin-2 receptor γ [ IL2RG ] and interleukin-7 receptor [ IL7R ]) or their downstream effector kinases (e.g., Janus kinase-3 [ JAK3 ]), resulting in severe lymphopenia and the lack of a functional adaptive immune system characteristic of severe combined immunodeficiency. Conversely, aberrant activation of cytokine receptor signaling has recently been implicated in the molecular pathogenesis of ALL.

Cytokine receptor–like factor 2 (CRLF2) encodes a cytokine receptor subunit that heterodimerizes with IL7R to form the receptor for the cytokine TSLP (thymic stromal lymphopoietin), which is a proinflammatory cytokine secreted by endothelial cells that drives lymphocyte proliferation. CRLF2 rearrangements leading to aberrant overexpression have recently been described in B-precursor cell ALL, typically as a result of an intrachromosomal deletion leading to a P2RY8-CRLF2 fusion, or translocations of the IgH locus to CRLF2 . CRLF2 rearrangements are found in 5% to 10% of B-precursor cell ALL cases but in more than 50% of patients with Down syndrome–associated ALL, strongly suggesting that these collaborate with trisomy 21 in B-lymphoblast transformation. CRLF2 rearrangements are often associated with activating mutations of Janus kinase 2 (JAK2) , and CRLF2 overexpression and mutated JAK2 cooperate to induce cytokine-independent growth to Ba/F3 pro-B cells.

IL7R encodes a cytokine receptor subunit that can heterodimerize with the IL2Rγ (the common γc chain) to form IL7R, or with CRLF2 to form the TSLP receptor. Recent work has identified activating IL7Rα point mutations in 9% of cases of T-cell ALL, as well as in the “BCR-ABL–like” subset of B-precursor cell ALL (discussed in more detail later). Interestingly, most of these mutations result in the introduction of a novel cysteine residue into the extracellular domain of IL7Rα, and aberrant signaling results because disulfide bond formation between these mutant cysteines can allow mutant IL7Rα proteins to aberrantly dimerize, leading to activation of downstream signaling pathways independent of ligand or of the common γc chain. The potential therapeutic utility of targeting aberrant oncogenic signaling downstream of mutationally activated cytokine receptors is an area of active investigation.

PTEN and the PI3K-AKT Axis

The PI3K -AKT oncogenic pathway, which is negatively regulated by the PTEN tumor suppressor, mediates prosurvival and proliferative signaling downstream of activated growth factor receptors and is aberrantly activated in a broad range of human cancer subtypes. Recent work has revealed that nearly half of T-cell ALL cases harbor mutational activation of oncogenic signaling through this pathway, most often as a result of truncating mutations or deletions of the PTEN tumor suppressor, but activating mutations of PI3K and AKT genes also occur. PTEN deletions are a strong predictor of early treatment failure in T-ALL cases and PTEN inactivation has been shown to induce resistance to NOTCH1 or MYC inhibition in zebrafish and murine models of T-cell ALL. These findings highlight the potential application of small molecule inhibitors of the PI3K-AKT pathway for high-risk T-cell ALL.

BCR-ABL and BCR-ABL –like ALL

The discovery of the Ph chromosome, which arises from the t(9;22)(q34;q11) translocation resulting in expression of a BCR-ABL fusion oncoprotein, was a major milestone in molecular oncogenesis that provided compelling support for the hypothesis, first put forth by Boveri in 1914, that cancer arises due to a genetic lesion in a single cell, resulting in outgrowth of a clonal population. BCR-ABL translocations are pathognomonic of chronic myeloid leukemia and are also found in 4% to 5% of childhood and 25% of adult cases of B-precursor cell ALL. The translocation underlying formation of the Ph chromosome results in expression of a BCR-ABL fusion oncoprotein with constitutively active tyrosine kinase activity, which transforms cells at least in part due to activation of the RAS-MAPK pathway, PI3K, and JUN-kinase pathways. Although historically associated with an extremely poor prognosis, hematopoietic stem cell transplantation (HSCT) proved to be the first highly effective therapeutic modality for this high-risk subtype of ALL. Additionally, the development of imatinib mesylate, a small molecule inhibitor of oncogenic signaling by the BCR-ABL kinase, has opened novel therapeutic opportunities for the management of Ph chromosome–positive ALL. Although single-agent therapy with imatinib leads to rapid development of resistance in Ph chromosome–positive ALL, pilot trials of the combination of conventional chemotherapy with imatinib or other tyrosine kinase inhibitors have been promising.

Although the poor prognosis of BCR-ABL ALL was traditionally believed to be due to expression of the BCR-ABL oncoprotein, recent work has shown that this subtype of ALL is also characterized by inactivation of the DNA-binding protein Ikaros, which is encoded by the IKZF1 gene. IKZF1 deletions have been identified in up to 15% of BCR-ABL–negative pediatric ALL cases and have been shown to be an independent predictor of adverse outcome. These cases have been classified as “BCR-ABL–like” ALL on the basis of similarities in gene expression profiles with Ph chromosome–positive ALL. Recent work has revealed that most of these “BCR-ABL–like” cases harboring Ikaros alterations, but lacking BCR-ABL translocations, commonly harbor mutational activation of tyrosine kinase signaling as a result of translocations or activating point mutations of receptor or cytoplasmic kinases, such as PDGFRB, CRLF2, EPOR, IL7R, FLT3, JAK2, and ABL1. Importantly, preclinical studies have demonstrated the potential therapeutic utility of targeting the aberrantly activated kinase involved for at least some of these cases.

Hypodiploid ALL

The presence of hypodiploidy (<44 to 45 chromosomes) portends a poor prognosis in B-precursor cell ALL, with event-free survival (EFS) rates less than 40%. Alterations in whole-chromosome number (aneuploidy) are very common in human cancer and can be oncogenic. However, the mechanisms through which hypodiploidy contributes to leukemogenesis in B-precursor cell ALL remain largely unknown.

A recent genomic study has shed key insights into the molecular pathogenesis of hypodiploid ALL. Both copies of chromosome 21 were always retained in each of the 124 cases of hypodiploid ALL cases analyzed, which, given that trisomy 21 is one of the most common genetic alterations in B-precursor cell ALL, highlights the pathogenic role of additional copies of genes on chromosome 21 in B-precursor cell ALL.

Rather than representing a single subtype of ALL, hypodiploid ALL instead harbors distinct biologic subsets, including low hypodiploidy (32-39 chromosomes) and near haploidy (24-31 chromosomes), which differ strikingly on a molecular basis.

• •
  

  Near-haploid ALL (24 to 31 chromosomes): Seventy percent of these cases are characterized by genetic lesions leading to activation of growth factor signaling pathways, including NRAS, KRAS, NF1, MAPK, FLT3, and PTPN11. Notably, deletions of tumor suppressors such as NF1 and PTPN11 were typically homozygous as a result of hypodiploidy, and loss of heterozygosity of tumor suppressors provides one mechanism through which hypodiploidy can contribute to oncogenesis. These data support the potential therapeutic utility of targeting these kinases, or their key downstream effectors, although such interventions await further preclinical investigation.

  
  
• •
  

  Low-hypodiploid ALL (32 to 39 chromosomes): In marked contrast to near-haploid ALL, almost all cases of low-hypodiploid ALL harbored mutations of the TP53 tumor suppressor, whereas mutations of growth factor signaling pathways were rare in this subset. Mutations of TP53 were present in remission bone marrow specimens in almost half of pediatric cases, strongly suggesting a germline origin consistent with the Li-Fraumeni syndrome; indeed, germline transmission was confirmed in one kindred. Given that the induction of aneuploidy triggers TP53- dependent cell cycle arrest, these data support a model in which TP53 inactivation occurs early in the pathogenesis of low-hypodiploid ALL, thus allowing the subsequent acquisition of hypodiploidy to be tolerated by these cells. However, it remains entirely unclear why TP53 mutations should be so rare in other ALL subtypes with striking degrees of aneuploidy, including near-haploid and hyperdiploid ALL. Low-hypodiploid cases were also associated with a high frequency of inactivating mutations of the RB1 tumor suppressor, as well as a very high frequency of mutations involving Ikaros family members, particularly IKZF2 (encoding Helios), which was mutated in more than 50% of cases.

A subset of cases with an apparently hyperdiploid chromosome complement was found to in fact represent “masked” hypodiploidy in which the hypodiploid genome has undergone reduplication, based on loss of heterozygosity apparent on the single-nucleotide polymorphism arrays performed. Although these cases might be classified clinically as hyperdiploid ALL (which has a favorable prognosis), masked hypodiploid ALL cases are indistinguishable from the respective hypodiploid ALL subtype (low-hypodiploid or near-haploid) based on gene expression profiling and pattern of oncogenic lesions, thus suggesting the need for clinical tests to distinguish this subtype from cases of “typical” hyperdiploidy, which has a favorable prognosis. Given that chromosomes 4 and 17 are lost in almost all cases of hypodiploid ALL, the presence of these trisomies may help to distinguish cases of true hyperdiploidy from masked hypodiploidy.

Early T-Cell Progenitor T-Cell ALL

Recent work has revealed that differentiation arrest at the earliest stages of T-cell development, defined either by absence of biallelic TCRγ deletion (ABD; indicating failure to complete V(D)J recombination) or by an early T-cell progenitor gene expression profile pattern, identifies an overlapping subset of T-cell ALL cases at very high risk for treatment failure. These cases are associated with aberrant oncogenic signaling through the RAS-MAPK, PI3K-AKT, and JAK-STAT pathways, typically owing to associated mutations in RAS , FLT3 , IL7R , BRAF , and JAK kinases, as well as deletions of the PTEN tumor suppressor. These cases are defined as T-cell ALL based on expression of cytoplasmic CD3 and harbor mutations in defining T-cell ALL oncogenes such as NOTCH1 . However, these cases also harbor additional mutations that are typically seen in AML, myelodysplastic, and myeloproliferative syndromes, such as RUNX1 , ETV6 , EED , EZH2 , SUZ12 , DNMT3A , IDH1 , and IDH2 . Moreover, gene expression analyses have shown that early T-precursor T-cell ALL harbors similarities to normal hematopoietic stem cells and AML leukemic stem cells. The identification of effective targeted therapies is a subject of active investigation.

CNS Manifestations

Up to 20% of children will have blast cells visible on a cytocentrifuged cerebrospinal fluid (CSF) specimen at diagnosis. CSF lymphoblasts can usually be identified by the use of cytocentrifugation and Wright-Giemsa staining. Such morphologic evaluation is necessary to distinguish the pleocytosis of leukemic meningitis from that induced as a result of intrathecal chemotherapeutic agents (arachnoiditis) or from CNS infections.

The vast majority of children with detectable CSF lymphoblasts at initial diagnosis do not have any symptoms related to CNS involvement. Symptomatic patients may present with diffuse or focal neurologic signs and symptoms, including manifestations of increased intracranial pressure (vomiting, headache, papilledema, and lethargy), seizures, and nuchal rigidity. Cranial nerve palsies can occur, with the facial nerve being the most frequently involved.

Intracranial hemorrhage in patients with high presenting leukocyte counts is uncommon in ALL but can lead to severe and devastating neurologic consequences. Clinically significant spinal cord involvement has been reported in ALL, manifesting as a localized epidural leukemic infiltrate compressing the cord.

Anterior Mediastinal Mass

Leukemic infiltration of the thymus appears as an anterior mediastinal mass on a chest radiograph. It is observed in about 10% of newly diagnosed patients and is almost always associated with T-cell immunophenotype (see Table 50-1 ). Leukemic infiltration of the mediastinal structures may cause life-threatening tracheobronchial or cardiovascular compression. Pleural effusion may also be associated with thymic enlargement, which can exacerbate respiratory distress. Prompt initiation of systemic chemotherapy (e.g., corticosteroids) is necessary to handle such emergencies and, rarely, emergent radiation may be indicated.

Genitourinary Tract Manifestations

Bone and Joint Manifestations

Bone pain, joint pain, and limp are common presenting symptoms in childhood ALL. Bone pain may be the result of direct leukemic infiltration of the periosteum, periosteal elevation of underlying cortical disease, bone infarction, or expansion of the marrow cavity by the leukemic cells. Pain and swelling of the joint are less frequent but may be presenting manifestations of disease and can initially cause confusion in the diagnosis. Migratory joint pain accompanied by swelling and tenderness can be misdiagnosed as juvenile rheumatoid arthritis (JRA) or rheumatic fever.

Up to 25% of children with ALL have characteristic radiographic changes, such as osteopenia and fracture at diagnosis, including vertebral compression fractures. Some patients have radiographic changes in the absence of bone pain, whereas others have bone pain unaccompanied by radiographic changes. Radiographic changes are most often seen in the long bones, especially around the areas of rapid growth (e.g., the knees, wrists, and ankles), and include subperiosteal new bone formation, transverse metaphyseal radiolucent bands, osteolytic lesions involving the medullary cavity and cortex, diffuse demineralization, and transverse metaphyseal lines of increased density (growth arrest lines). The latter probably represent regions of growth arrest during active phases of the disease rather than direct infiltration by leukemic cells.

Gastrointestinal Manifestations

Hepatosplenomegaly is a common feature at the time of diagnosis. Severe hepatic dysfunction, manifested as hyperbilirubinemia, presumably from leukemic infiltration, has been observed at diagnosis in some children with ALL and can complicate initial therapy, because many induction agents are hepatically metabolized or hepatotoxic. A short course of corticosteroid monotherapy before the initiation of multiagent induction chemotherapy may improve hyperbilirubinemia.

Ocular Manifestations

Ophthalmic manifestations of leukemia can be observed in more than a third of all newly diagnosed patients. Leukemia can involve almost all ocular structures. Retinal hemorrhages, the most frequent ocular abnormality, are presumably caused by thrombocytopenia or anemia. However, local infiltration of the capillary vessel walls with subsequent rupture and hemorrhage also may occur, especially in patients with very high leukocyte counts. Ocular motor palsies and papilledema are indicative of meningeal leukemia. Occasionally the optic nerve may be directly involved by leukemic infiltration. In such situations visual acuity may be markedly affected and patients may present with monocular blindness. Prompt radiation therapy may be necessary to salvage useful vision. Leukemic infiltration of the anterior chamber with hypopyon and iritis has also been reported and can be the first manifestation of relapse. Symptoms include conjunctival injection, photophobia, pain, blurring, and decreased vision.

Pulmonary Manifestations

Pulmonary leukostasis, which can lead to respiratory failure, can be seen at diagnosis, especially in patients with high presenting leukocyte counts. This complication is more common in AML than in ALL. Radiographic distinction among infection, leukemic infiltration, and hemorrhage may be difficult.

Dermatologic Manifestations

Skin infiltration is uncommon in childhood leukemia, with the exception of congenital leukemia (both AML and ALL). Leukemia cutis typically manifests as red or violaceous papules, nodules, or plaques.

Idiopathic Thrombocytopenic Purpura

ITP is the most common cause of the acute onset of petechiae and purpura in children. Patients with ITP usually present with isolated thrombocytopenia, often with large platelets seen on a blood smear, contrasting sharply to the more generalized blood cell abnormalities and small platelets typically observed in patients with ALL. Physical findings in ITP are usually limited to bruising or bleeding associated with thrombocytopenia. An occasional patient develops modest splenomegaly, probably caused by a recent viral infection. In most cases, the leukocyte level and differential count are unremarkable and there is no anemia unless bleeding has been substantial or an unrelated anemia is present. Bone marrow examination is rarely necessary in uncomplicated cases of pediatric ITP (i.e., children with otherwise normal blood cell counts, consistent peripheral blood smear, and absence of adenopathy, hepatosplenomegaly, or other concerning findings on physical examination). However, if the diagnosis of ITP is in question or any of these concerning findings are present, clinicians should consider marrow evaluation to rule out leukemia before starting corticosteroids to avoid partially treating occult ALL. The bone marrow aspirate is usually diagnostic because the marrow elements in ITP appear normal or show increased megakaryocytes.

Aplastic Anemia, Myelodysplasia, and Myeloproliferative Disorders

Like patients with leukemia, those with aplastic anemia, myelodysplasia, and myeloproliferative disease may present with pancytopenia and have fevers or infections associated with granulocytopenia. Lymphadenopathy and hepatosplenomegaly are unusual in aplastic anemia. Also, the radiographic skeletal changes sometimes seen in leukemia do not occur in aplastic anemia. To differentiate acute leukemia from these other disorders, marrow biopsy is mandatory and usually revealing.

Rarely, patients with ALL present with a transient pancytopenia preceding the leukemic phase. The pancytopenic period may persist for a few days or weeks and be accompanied by high fevers. Spontaneous recovery with a period of normal blood cell counts often occurs before the onset of ALL. In those cases, ALL typically becomes manifest several months after the period of aplasia. However, ALL may also develop after a period of aplasia, without any recovery of blood cell counts. Distinguishing pre-ALL pancytopenia from aplastic anemia may be difficult. Repeated bone marrow aspirates or biopsies ultimately establish the correct diagnosis by demonstrating areas of marrow that are infiltrated with lymphoblasts. The pathogenesis of the preleukemic pancytopenia prodrome is unclear; in one case report, leukemia-associated genetic lesions were identified in the aplastic marrow when it was retrospectively analyzed.

Juvenile Rheumatoid Arthritis and Connective Tissue Disease

Because ALL often presents primarily as joint or extremity complaints (especially limp, arthritis, or arthralgia), it may be confused with JRA or other autoimmune disorders. Children with JRA can manifest fever, pallor, splenomegaly, and anemia, and patients with ALL can present with a positive test for antinuclear antibody, highlighting how difficult it may be to distinguish these conditions from each other. Because of this difficulty, bone marrow aspiration should be considered before initiating corticosteroid therapy in children with presumed JRA.

Infectious Mononucleosis and Other Viral Infections

Childhood infectious mononucleosis and other viral illnesses can masquerade as leukemia. Patients may have generalized lymphadenopathy, splenomegaly, rash, fevers, and peripheral blood lymphocytosis. The atypical lymphocytes observed with acute Epstein-Barr virus (EBV) and other viral infections can sometimes be confused with peripheral leukemic blasts because they are larger than normal lymphocytes. Usually, viral infections can be differentiated from leukemia without bone marrow aspiration, but the procedure is sometimes necessary for accurate diagnosis. The detection of acute EBV infection in the blood by serology or PCR testing may be helpful in establishing the correct diagnosis.

Metastatic Solid Tumors

ALL and neuroblastoma may have similar presenting signs and symptoms, including fever, bone pain, and pancytopenia. Children with neuroblastoma frequently have malignant involvement of liver, lymph nodes, bone, or bone marrow; the marrow involvement may be extensive. In the bone marrow, neuroblasts tend to cluster and form pseudorosettes, in contrast to the diffuse involvement generally observed in ALL. Other solid tumors also rarely present as extensive marrow involvement, including rhabdomyosarcoma and Ewing sarcoma, and can initially be confused with leukemia. If questions remain after the examination of marrow morphology, other studies, such as immunophenotype and cytogenetics, may be helpful in distinguishing leukemia from a metastatic solid tumor.

Age

The age of patients with ALL significantly correlates with clinical outcome. In childhood ALL, infants and adolescents have a worse prognosis than patients in the intermediate age group (aged 1 to 10 years). ALL in infancy (<1 year at diagnosis) is associated with high presenting leukocyte counts, increased frequency of CNS leukemia at presentation, and a high incidence of rearrangements of the MLL gene on chromosome 11q23 (see Table 50-2 ). Even when treated with intensified regimens, infants with MLL gene rearrangements have a dismal prognosis, with long-term EFS rates ranging from 10% to 40%.

Adolescents (10 to 21 years of age) with ALL also have a less favorable outcome than children aged 1 to 10 years, although it is not as poor as that in infants. Adolescents with ALL more frequently present with “high risk” features at diagnosis, including T-cell immunophenotype, higher presenting leukocyte counts, and a lower incidence of cytogenetic abnormalities associated with a favorable prognosis, including high hyperdiploidy and the ETV6-RUNX1 gene fusion (see Table 50-2 ). Multiple retrospective studies have shown that adolescents fare better with pediatric ALL regimens than on treatments designed for adults with ALL.

Leukocyte Count

The initial peripheral blood leukocyte count is a significant predictor of treatment outcome, with worsening outcomes as the leukocyte count increases. Since 1996, based on the recommendation of the Cancer Therapy Evaluation Program of the National Cancer Institute (NCI), many investigators have considered a leukocyte count of 50,000 cells/mm ³ as the level separating patients with a higher risk for relapse from those with a more favorable prognosis.

Immunophenotype

Historically, immunophenotype was considered an important prognostic factor, with inferior outcomes observed in patients with mature B-cell and T-cell disease. However, if they are treated with more intensive regimens, children with T-cell phenotype fare as well as those with B-precursor cell disease. As noted earlier, mature B-cell ALL is more effectively treated with the same therapy used for advanced-stage Burkitt lymphoma.

Some investigators have reported prognostic differences within subsets of patients with B-precursor cell ALL. Initial studies have suggested that patients with pre–B-cell ALL (cIg+) have a worse outcome than those with early pre–B-cell ALL (cIg−). Subsequent reports indicated that the adverse outcome of patients with pre–B-cell ALL was caused by the subset with the t(1;19) translocation. However, with greater treatment intensity, even patients with this cytogenetic abnormality do not appear to have an adverse prognosis.

An early T-precursor phenotype is observed in 10% to 15% of cases of childhood T-cell ALL. This subset was initially indentified by gene expression profile analyses and is characterized by a distinctive immunophenotype (CD1a and CD8 negativity, with weak expression of CD5 and coexpression of stem cell or myeloid markers). Retrospective analyses suggested that patients with early T-precursor cell ALL have a poorer prognosis. In another retrospective study, patients with T-cell ALL with early T-precursor cell features were identified by ABD, indicating failure to complete V(D)J recombination at this locus. Like early T-precursor cell ALL patients defined by immunophenotype, T-cell ALL patients with ABD were also found to have an inferior prognosis.

In 15% to 30% of patients with newly diagnosed ALL, flow cytometry reveals coexpression of at least one myeloid antigen on the cell surface of the lymphoblasts. Myeloid antigen coexpression has been associated with several genetic abnormalities, including the ETV6-RUNX1 fusion [t(12;21)], MLL gene (11q23) rearrangements, and the Ph chromosome ( BCR-ABL rearrangement), but is almost never observed in high hyperdiploid ALL (51 to 65 chromosomes). Myeloid antigen coexpression was previously believed to be associated with an inferior outcome, but several more recent reports have indicated that it is not an independent prognostic factor.

Chromosomal Abnormalities

Several recurrent chromosomal abnormalities are important prognostic factors in childhood ALL. Two abnormalities, high hyperdiploidy (51 to 65 chromosomes or a DNA index greater than or equal to 1.16) and the cryptic t(12;21) ( ETV6-RUNX1 fusion), have been associated with a favorable prognosis. High hyperdiploidy is observed in 25% to 30% of cases of childhood ALL and is more common in younger (noninfant) children with B-precursor cell phenotype and low leukocyte counts. The most favorable outcomes in high hyperdiploid ALL patients have been associated with the presence of trisomies of chromosomes 4, 10, and 17.

The ETV6-RUNX1 fusion, which is rarely detected by karyotypic analysis, has been identified in approximately 20% of children with ALL using other more sensitive techniques, such as fluorescence in situ hybridization and the reverse-transcriptase PCR assay. Like high hyperdiploidy, the ETV6-RUNX1 fusion is more common in younger (noninfant) patients with low leukocyte counts and is observed almost exclusively in patients with B-precursor cell phenotype. Up to 80% of children with B-precursor cell ALL diagnosed between the ages of 2 and 7 years have either high hyperdiploidy or the ETV6-RUNX1 fusion, although almost never both; these two chromosomal abnormalities are almost always mutually exclusive. Several studies have indicated that children with ETV6-RUNX1– positive ALL have favorable EFS rates. When relapses are observed in children with ETV6-RUNX1– positive ALL, they tend to occur relatively late and may be more responsive to postrelapse salvage therapy.

Chromosomal abnormalities associated with an adverse prognosis include hypodiploidy (fewer than 44 or 45 chromosomes), rearrangements of the MLL gene on chromosome 11q23, and the Ph chromosome [t(9;22)]. MLL gene rearrangements are more frequent in infants than in older children with ALL. The incidence of Ph chromosome–positive ALL increases with age at diagnosis and is more common in adolescents and young adults. Patients with Ph chromosome–positive ALL appear to fare better if treated with allogeneic hematopoietic stem cell transplantation (HSCT) in first remission ; recent work has suggested that favorable outcomes may also be achieved with the combination of tyrosine kinase inhibitors and intensive chemotherapy without transplant.

Intrachromosomal amplification of the AML1 gene on chromosome 21 (iAMP21), detected in 1% to 2% of children with ALL, may also be associated with an inferior outcome. Patients with this abnormality appear to fare better with more intensified “high risk” treatment than with less intensive regimens intended for patients with “standard risk” disease. The t(1;19) translocation, observed in approximately 5% of cases of childhood ALL, had been previously associated with inferior EFS rates, but more recently conducted clinical trials have not found that this translocation is associated with an adverse prognosis. Some investigators have reported that patients with the t(1;19) translocation are at higher risk for CNS relapses, but this finding has not been confirmed by others. Chromosomal abnormalities in childhood ALL are reviewed in depth in Chapter 44 .

Overt CNS Disease at Diagnosis

Lymphoblasts in the CSF can be detected in 15% to 20% of children who present with ALL. Some children, such as those diagnosed within the first 12 months of life and those with T-cell ALL, have a higher incidence of CNS leukemia at diagnosis.

Most investigators consider the presence of overt CNS disease at diagnosis to be an adverse prognostic indicator, and these patients are usually treated with more aggressive therapy. CNS status at presentation is usually classified as CNS-1 (no blast cells), CNS-2 (fewer than five leukocytes/µL with blast cells), and CNS-3 (five or more leukocytes/µL with blast cells or cranial nerve palsy). Historically, CNS leukemia was defined as CNS-3 status (observed in approximately 5% of patients at diagnosis). CNS-3 status at diagnosis is associated with a lower probability of EFS. Several investigators have reported that patients with CNS-2 status also have a higher risk for relapse, although it does not appear to be as high as in patients with CNS-3 status at diagnosis. Intensification of CNS-directed therapy (e.g., increasing the frequency of intrathecal chemotherapeutic treatments) may abrogate the adverse prognostic significance of CNS-2 status. Thus patients with CNS-2 status at diagnosis usually receive extra doses of intrathecal chemotherapy without other changes in treatment whereas those with CNS-3 status are typically classified as higher risk and receive more intensive systemic and CNS-directed therapies.

Traumatic lumbar punctures with lymphoblasts on cytospin have also been associated with an adverse prognosis. Like patients with CNS-2 status, those with traumatic lumbar punctures with lymphoblasts at diagnosis may also benefit from additional doses of intrathecal chemotherapy.

Early Response to Induction Chemotherapy

The rapidity with which a patient responds to initial chemotherapy is a significant predictor of long-term outcome. Early response to therapy has been evaluated using morphologic measures (residual microscopic leukemia) and more sensitive techniques, such as PCR assay and flow cytometry.

Other Prognostic Factors

Risk-Adapted Therapy

After the addition of CNS-directed therapy improved cure rates to approximately 50% in the 1970s, investigators compared presenting features in patients who were cured and those who relapsed to establish clinically relevant prognostic factors. Subsequent clinical trials used these prognostic factors to stratify therapy. More intensive therapy was administered to those patients considered to be at the highest risk for relapse. In contrast, some of the more morbid components of therapy were modified or eliminated for those children considered to have the best prognosis. The goal of risk-adapted therapy is to treat away adverse presenting features so that high-risk and low-risk patients have similar cure rates. For example, on Dana-Farber Cancer Institute (DFCI) ALL Consortium protocols, children with T-cell ALL (historically, a subgroup with an inferior prognosis) are treated as high-risk patients (receiving higher cumulative dosages of anthracycline and corticosteroid than lower-risk patients); with that therapy, their outcomes are as favorable as those with B-precursor cell disease.

For many years, pediatric cooperative groups and institutions applied prognostic factors differently when defining risk categories for clinical trials. A more uniform approach to risk classification was proposed and agreed on at an NCI-sponsored workshop held in 1993. For patients with B-precursor cell ALL, the standard-risk category was defined as age between 12 months and younger than 10 years and initial leukocyte count lower than 50,000/mm ³ . The remaining patients were considered to have high-risk ALL. Other characteristics used by the various cooperative groups to classify patients as high risk include T-cell phenotype, CNS-3 status at diagnosis, and slow early response to initial therapy, including a high peripheral blood absolute blast count at the end of a 7-day prednisone prophase. These various prognostic factors are discussed earlier.

Contemporary regimens have incorporated cytogenetic abnormalities and MRD levels into risk group stratification, and in some cases, only these factors are considered (and not more standard factors, such as age and leukocyte count) when assigning risk groups. Patients with certain chromosomal abnormalities (e.g., MLL gene rearrangements and low hypodiploidy) are treated as high risk, regardless of other presenting features. Similarly, end-induction MRD is used by almost all groups as a factor determining the intensity of postinduction treatment, with patients found to have higher levels allocated to more intensive therapies. On some protocols, MRD levels at a later time point (week 12) are also considered when risk-stratifying patients, because these have been shown to be prognostically important, especially in patients with T-cell ALL.

MRD measurements, in conjunction with cytogenetics, have also been used to identify subsets of patients with an extremely low risk for relapse who may be successfully treated with less intensive therapies. The Children’s Oncology Group reported a very favorable prognosis for patients with the B-precursor cell phenotype, NCI standard risk age/leukocyte count, CNS-1 status, and favorable cytogenetic abnormalities (either high hyperdiploidy with favorable trisomies or the ETV6-RUNX1 fusion) who had low peripheral blood MRD levels at day 8 and low marrow MRD at end of induction.

Ultimately, treatment is the most important prognostic factor, and those factors used to risk-stratify patients on one regimen may not be useful for patients treated on a different regimen.

Remission Induction

After the diagnosis of ALL has been established and any urgent medical issues, such as metabolic imbalances, have been addressed, antileukemic chemotherapy should be initiated without delay. The initial phase of treatment, remission induction, is designed to reduce the leukemic cell burden to a clinically and hematologically undetectable level. Hematologic remission is defined as attainment of a normocellular bone marrow with 5% or fewer blasts and peripheral blood without lymphoblasts, with return of “normal” peripheral blood counts, that is, a granulocyte count exceeding 500 to 1000/mm ³ and a platelet count exceeding 100,000/mm ³ . Complete remission is defined as achievement of these criteria along with the absence of any signs and symptoms of extramedullary leukemia. A complete remission must be induced before the next component of therapy is begun. The duration of most induction regimens is 4 to 6 weeks.

Clinical trials in the 1960s demonstrated that combinations of two agents were consistently superior to single agents for inducing complete remission in children with ALL. Vincristine and prednisone produced complete remission in approximately 90% of pediatric patients with ALL. Addition of a third drug increased both the number of patients who achieved complete remission and the long-term relapse-free survival. Current induction regimens, therefore, consist of at least three agents, most often l -asparaginase or an anthracycline, in addition to vincristine and prednisone.

Many groups have further intensified induction regimens to include four or more agents. In theory, these more intensive induction regimens may prevent the emergence of drug-resistant leukemic clones by an initial leukemic cell lysis of greater rapidity and magnitude. The benefit in terms of long-term EFS of using four drugs during induction therapy (vincristine, prednisone, l -asparaginase, and an anthracycline) is widely accepted in higher-risk patients but less clear in lower-risk patients. Intensive induction regimens may enhance long-term EFS but can result in increased short-term morbidity, especially from infections during periods of granulocytopenia. However, even with four-drug induction regimens, few toxic deaths are reported (i.e., 1% to 2% of patients).

The initiation of CNS treatment is an integral component of induction therapy. After the diagnosis of ALL has been established by bone marrow examination, a lumbar puncture is performed for diagnostic and therapeutic purposes. It is generally recommended that intrathecal chemotherapy be given with the first diagnostic lumbar puncture to reduce the theoretical risk for inadvertently seeding the meninges with peripheral blood lymphoblasts. In addition to the diagnostic lumbar puncture, intrathecal chemotherapy is usually administered at least one more time during the first month of treatment. Patients with lymphoblasts present on their initial CSF specimen (CNS-2 or CNS-3 status, or traumatic lumbar puncture with blasts) typically receive more doses of intrathecal chemotherapy during the induction phase than those without any detectable CSF lymphoblasts (CNS-1 status).

Failure to achieve remission after 1 month of therapy (induction failure) is uncommon, occurring in fewer than 5% of patients. Induction failure is observed more commonly in patients with high presenting leukocyte counts or T-cell phenotype. Successful treatment of refractory ALL has been reported with the use of drugs such as cytarabine, epipodophyllotoxins (e.g., etoposide, teniposide), and anthracyclines (e.g., idarubicin). Nelarabine has been shown to induce remissions in patients with refractory T-cell ALL. Ultimately, 80% to 90% of children with initially refractory disease are able to achieve a complete remission. Despite this relatively high complete remission rate, overall survival for patients with a history of initial induction failure is poor (long-term overall survival rates of 20% to 30%). Allogeneic HSCT in first remission may improve the outcome for patients with initial induction failure. In a large retrospective series, a trend for superior outcome with allogeneic HSCT compared with chemotherapy alone was observed in patients with initial induction failure and either the T-cell phenotype (any age) or B-precursor cell phenotype and age younger than 6 years. In that study, B-precursor cell ALL patients who were aged 1 to 5 years at diagnosis and did not have any adverse cytogenetic abnormalities ( MLL translocation, BCR-ABL ) had a relatively favorable prognosis, without any advantage in outcome with the utilization of HSCT compared with chemotherapy alone.

Consolidation (Intensification) Therapy

The goals of consolidation therapy are to reduce the disease burden further and to adjust the intensity of treatment based on the risk for subsequent relapse. A wide variety of agents and schedules have been used during postinduction consolidation in pediatric trials. In general, there has been an attempt in lower-risk patients to limit exposure to agents associated with significant acute and late toxicities while reserving the use of more intensive therapies for patients at higher risk for relapse.

A commonly used postinduction consolidation regimen was first introduced by the German BFM study group. This consolidation scheme includes (1) a 4-week chemotherapy course (sometimes referred to as “consolidation” or “induction IB”) immediately after the initial induction phase consisting of cyclophosphamide, low-dose cytarabine, and a thiopurine (mercaptopurine or thioguanine), followed by (2) an interim maintenance phase, which includes multiple doses of either high-dose or escalating doses of methotrexate with or without leucovorin rescue, and then (3) a reinduction course (or delayed intensification), which typically includes the same agents used during the initial induction/consolidation cycles. Augmentation of this regimen, including the use of two delayed intensification cycles instead of one, has led to improved outcomes for higher-risk patients, including those with a slow early morphologic response to initial induction therapy. The DFCI ALL Consortium uses an alternative postinduction consolidation regimen that includes 20 to 30 weeks of l -asparaginase and, for higher-risk patients, additional doses of doxorubicin.

Continuation Therapy

Almost all current treatment regimens for ALL include a phase of continuation or maintenance therapy, in which patients are treated with less-intensive chemotherapy to complete at least 2 years of therapy. Most continuation regimens consist of weekly low-dose methotrexate and daily oral 6-mercaptopurine. Some groups have added regular pulses of vincristine and corticosteroid to this regimen, although their benefit remains controversial.

Several pharmacologic studies have documented highly variable bioavailability after oral administration of 6-mercaptopurine and methotrexate during the continuation phase. This may have important prognostic implications, because lower intracellular levels of thioguanine nucleotides and methotrexate polyglutamates, the major metabolites of 6-mercaptopurine and methotrexate, have been correlated with a higher risk for relapse. Variable bioavailability may be the result of various factors, including concurrent food, which may interfere with drug absorption. Bioavailability of oral mercaptopurine appears to be improved if it is given in the evening, without food or milk. Polymorphisms within genes involved in drug metabolism also affect rates of drug activation and clearance. For example, patients with deficiency in the activity of TPMT, an enzyme that inactivates mercaptopurine, have higher rates of toxicity during continuation therapy (but also may have better survival) than patients with normal TPMT activity. Most physicians attempt to compensate for the interindividual variations of bioavailability and metabolism by tailoring the dose of both drugs to the leukocyte count. Administration of maximally tolerated doses of methotrexate and 6-mercaptopurine during continuation therapy may improve outcome, as suggested by studies demonstrating that patients with lower leukocyte or neutrophil counts during continuation chemotherapy have lower relapse rates. Lack of patient compliance with oral medications, as well as physician compliance with protocol dosages, may adversely affect the efficacy of continuation chemotherapy.

6-Thioguanine is an alternative thiopurine that, unlike 6-mercaptopurine, does not rely on TPMT for its metabolism, resulting in higher intracellular levels of thioguanine nucleotides. 6-Thioguanine also has been shown to be more potent than 6-mercaptopurine in inducing cell death of lymphoblasts in vitro. The results of one randomized clinical trial comparing the two thioguanines indicated that 6-thioguanine was associated with a superior EFS (but not overall survival) in boys, primarily owing to a reduction in the frequency of CNS relapses. However, other randomized trials comparing the two thiopurines did not confirm an EFS advantage for 6-thioguranine. Additionally, the use of 6-thioguanine during the maintenance phase has been associated with significant hepatotoxicity, including veno-occlusive disease and cirrhosis, as well as higher remission death rates, primarily caused by infection. Thus 6-mercaptopurine remains the thiopurine of choice during the continuation phase.

Several investigators have studied the relative efficacy and toxicity of two corticosteroids used during postinduction therapy: prednisone and dexamethasone. Interest in substituting dexamethasone for prednisone stems from studies suggesting that dexamethasone has more potent in vitro antileukemic activity, higher free plasma levels, and enhanced CSF penetration. In several clinical trials conducted in the 1990s, the nonrandomized substitution of dexamethasone for prednisone appeared to affect outcome favorably. This finding was confirmed in several randomized clinical trials that indicated that dexamethasone was associated with a superior 5-year EFS. However, dexamethasone may also be associated with increased corticosteroid-related toxicities, including higher rates of fracture, osteonecrosis, and infectious complications, especially in older children and adolescents. Thus the optimal corticosteroid preparation and dosing have yet to be determined and may differ based on presenting features (e.g., age) and risk for relapse.

Attempts to intensify continuation therapy by administering rotating pairs of non–cross-resistant drugs, including many agents not traditionally incorporated during the continuation phase, such as cyclophosphamide, epipodophyllotoxins, and cytarabine, have led to mixed results. In studies conducted by St. Jude Children’s Research Hospital, this strategy resulted in improved outcomes for higher-risk but not lower-risk patients but also was associated with high rates of toxicity, including secondary AML. Based on this risk, these agents (especially epipodophyllotoxins) are, in general, no longer administered to most patients during the continuation phase.

CNS-Directed Therapies

In the 1960s, as systemic chemotherapy became more effective in prolonging the duration of hematologic remission in patients with ALL, the incidence of CNS leukemia as an initial site of relapse became progressively more common. It was hypothesized that leukemia cells, even if subclinical, were present in the CNS in all patients and that these cells were protected by the blood-brain barrier from systemically administered chemotherapy. Thus, the concept of the CNS as a “pharmacologic sanctuary” (i.e., an anatomic space that is poorly penetrated by systemically administered chemotherapeutic agents) emerged. The introduction of radiation therapy to treat subclinical CNS leukemia in the 1970s was a pivotal step in boosting long-term disease-free survival rates in childhood ALL to 50%. Although most pediatric patients are now treated without prophylactic cranial radiation, the importance of effectively treating subclinical CNS leukemia is undisputed for childhood ALL.

All successful treatment regimens for ALL include therapy directed at treating CNS leukemia. Options for CNS-directed therapies include cranial irradiation, intrathecal chemotherapy, and CNS penetrant systemic chemotherapy.

Radiation therapy was the first modality successfully used to prevent CNS relapses. The use of radiation for CNS treatment was based on experiments demonstrating that L1210 murine leukemia could be cured when cranial irradiation was added to systemic treatment with cyclophosphamide. In the 1960s and 1970s, studies performed at St. Jude Children’s Research Hospital documented the effectiveness of CNS radiation as preventive therapy in children with ALL. In one study, patients were randomized to receive 2400 cGy of craniospinal radiation or no radiation. The difference in CNS relapse rates was striking: only 2 of 45 irradiated patients (4%) had their initial complete remissions ended by meningeal relapse, compared with 33 of 49 nonirradiated patients (67%). Moreover, 31 of the patients who received prophylactic radiation remained alive after 18 to 20 years, compared with only 10 patients in the nonirradiated group. Subsequent studies demonstrated that 2400-cGy cranial irradiation with intrathecal methotrexate was as effective in preventing CNS relapse as 2400-cGy craniospinal irradiation without intrathecal chemotherapy. Because craniospinal radiation was associated with increased toxicity, including excessive myelosuppression and spinal growth retardation, radiation directed only to the cranium administered with intrathecal chemotherapy became the standard form of CNS treatment in the 1970s.

Although 2400-cGy cranial irradiation (with intrathecal chemotherapy) effectively prevents CNS relapses in children with ALL, it is also associated with subsequent learning disabilities, growth and neuroendocrinologic abnormalities, and an increased risk for second malignant neoplasms. Thus many investigators have studied alternative CNS treatments, including lower doses of cranial radiation (1200 to 1800 cGy), systemically administered CNS-penetrant chemotherapy (i.e., agents known to achieve therapeutic levels in CSF), or intrathecal chemotherapy. Current regimens for childhood ALL include some or all of these CNS-directed therapies.

The proportion of patients receiving cranial radiation has decreased significantly over the past several decades; on current regimens, most pediatric patients are treated without radiation. When it is administered, cranial radiation is generally restricted to patients with a higher risk for relapse involving the CNS, especially those with T-cell disease, high presenting leukocyte counts, or CNS-3 status at diagnosis. Doses lower than 2400 cGy have been shown to be effective, and therefore are typically used when cranial radiation is administered. For instance, CNS and marrow relapse rates were not significantly different when the dose of cranial radiation was lowered from 2400 to 1800 cGy in consecutive Children’s Cancer Study Group trials. The BFM and DFCI groups have successfully used 1200-cGy irradiation in the context of an intensive systemic regimen that included high-dose methotrexate (known to penetrate the CNS effectively). Irradiated patients also typically receive intermittent intrathecal chemotherapy in addition to cranial irradiation as CNS preventive therapy.

Systemic therapy plays an important role in the prevention of CNS leukemia. Penetration of the CSF by drugs has been clearly demonstrated with the use of glucocorticoids and high doses of methotrexate and cytarabine. It has also been shown that systemically administered asparaginase, whose efficacy is a function of asparagine depletion, effectively lowers CSF asparagine levels.

Intrathecal chemotherapy, used in conjunction with intensive systemic therapy and/or cranial irradiation, is an important component of CNS treatment. Many investigators have replaced cranial irradiation with frequent dosing of intrathecal chemotherapy, especially in lower-risk patients. The success of these efforts appears to depend on the frequency with which intrathecal chemotherapy is delivered, as well as the intensity and CNS penetration of the systemic chemotherapy administered. Investigators from the Children’s Cancer Group demonstrated that administering intrathecal methotrexate throughout all phases of therapy was as effective in preventing CNS relapses in lower-risk patients as 1800-cGy cranial irradiation combined with only 6 months of intrathecal methotrexate, but only in the context of an intensified systemic regimen. When a less-intensive systemic regimen was used, more CNS relapses were observed in nonirradiated patients. Similarly, in a DFCI ALL Consortium study, excessive CNS relapses were observed in standard-risk males when cranial irradiation was eliminated without increasing the frequency of intrathecal chemotherapy or the intensity of systemic chemotherapy. However, in a subsequent study conducted by the same group, standard-risk patients who received more frequent doses of intrathecal chemotherapy had as low a CNS relapse rate as irradiated patients. Some controversy exists regarding the relative efficacy of intrathecal methotrexate alone and “triple intrathecal chemotherapy” (intrathecal methotrexate, cytarabine, and hydrocortisone). In one randomized study in standard-risk patients, triple intrathecal chemotherapy was associated with a lower rate of isolated CNS relapse but there was no difference in overall EFS. Randomized studies have also demonstrated that the use of dexamethasone instead of prednisone, which is believed to be less CNS penetrant, is associated with a lower rate of isolated CNS relapses in nonirradiated patients. Thus those regimens that have successfully eliminated cranial radiation include extended and/or frequent dosing of intrathecal chemotherapy in conjunction with intensified, more CNS penetrant systemic therapy, including dexamethasone and/or high- or intermediate-dose methotrexate.

Several clinical trials have been conducted on which cranial radiation was omitted for all patients, including high-risk subsets. Most of these trials include multiple doses of either high-dose methotrexate or high-dose cytarabine and increased frequency of intrathecal chemotherapy. While the overall EFS on these trials was similar to that of contemporaneously conducted trials on which some patients received prophylactic radiation, some nonirradiated patient subsets had a significantly high risk for CNS relapse, including patients with T-cell phenotype or the presence of blasts in CSF at diagnosis. Additionally, some patients with high-risk features on these trials were treated with allogeneic SCT in first complete remission (which included total-body irradiation).

The goal of eliminating cranial radiation in pediatric patients is to minimize long-term sequelae (see later). Although several studies have indicated that intrathecal chemotherapy and intensive systemic therapy can be as effective as cranial irradiation in preventing CNS relapses, the relative late neurocognitive toxicities of these CNS treatment strategies remains unsettled. Moreover, higher rates of acute neurotoxicity, including seizures, have been observed with regimens that included multiple courses of high-dose methotrexate and frequent doses of intrathecal chemotherapy.

Cessation of Treatment

The optimal duration of therapy remains unknown. Most investigators continue to treat patients for 2 to 3 years, based on results of older studies, in which patients received therapy that was less intensive than in current regimens. Randomized studies conducted by the Children’s Cancer Group in the 1970s indicated that there was no significant difference in outcome when comparing 5 versus 3 years of therapy. In the Childhood Leukaemia Trial UKALL VIII, a randomized study of 2 versus 3 years of continuation therapy conducted in the 1980s, the researchers observed no significant survival advantage with longer therapy. Patients receiving the shorter duration of therapy had a higher relapse rate, but this was counterbalanced by a higher remission death rate in those receiving 3 years of treatment. Some early studies suggested that the optimal duration of therapy may be different for boys and girls, with boys benefiting from a more prolonged continuation phase, although this finding may be less relevant with current regimens.

Even with more intensive regimens, attempts to shorten therapy duration from 2 years have not been successful. The BFM group randomized patients to receive 18 or 24 months of treatment and observed a higher relapse rate with patients who received the shorter course of treatment. Similarly, high relapse rates were observed in a nonrandomized study conducted by the Tokyo Children’s Cancer Study Group, in which patients received intensified therapy for only 12 months, suggesting that truncated therapy, even if intensive, is inadequate for most children with ALL.

Philadelphia Chromosome–Positive ALL Patients

The Ph chromosome, t(9;22)(q34;q11), which leads to fusion of the BCR and ABL genes, is detectable in approximately 5% of children and 20% of adults with ALL. In pediatric ALL, it is more frequent in adolescents than in younger children. When treated on regimens with conventional chemotherapeutic agents, these patients have a dismal prognosis, with long-term EFS rates ranging from 0% to 25%. Allogeneic SCT in first remission has been shown to lead to more favorable outcomes and, prior to the availability of tyrosine kinase inhibitors, was considered the treatment of choice for children with Ph chromosome–positive ALL. In one study, which reviewed the outcome of 610 children with Ph chromosome–positive ALL treated by 10 study groups in the pre–tyrosine kinase inhibitor era between 1995 and 2005, transplantation in first remission from an HLA-matched related or unrelated donor was associated with a higher likelihood of disease-free survival compared with chemotherapy alone.

Imatinib mesylate is a selective inhibitor of the BCR-ABL tyrosine kinase. Phase I and II studies of single-agent imatinib in children and adults with relapsed or refractory Ph chromosome–positive ALL have demonstrated relatively high response rates, although these responses tended to be of short duration. Results from pediatric trials have suggested that the administration of imatinib in combination with multiagent chemotherapy is feasible, without excessive toxicity. In a pilot trial conducted by the Children’s Oncology Group, patients treated with the combination of chemotherapy and imatinib (but without transplant) appeared to do at least as well as other patients on the same trial who underwent SCT in first remission.

Infants

Patients diagnosed during the first year of life represent 2% to 4% of cases of childhood ALL. The outcome of infants with ALL is significantly worse than that for older children, with reported long-term EFS rates of 20% to 40%. The major cause of treatment failure is relapse, which tends to occur early, often in the first 1 to 2 years after diagnosis. In vitro chemosensitivity assays have indicated that lymphoblasts from infants with ALL are relatively resistant to agents commonly used in childhood ALL regimens, such as corticosteroids and l -asparaginase. Infants also appear to be more vulnerable to the toxic effects of therapy, especially severe infections.

The poor outcome of infants appears to be related to the biologic distinctiveness of the disease observed in this age group. Up to 80% of infants with ALL present with molecularly detectable rearrangements of the MLL gene on chromosome 11q23. The presence of a MLL gene rearrangement is an independent predictor of poor outcome in infant ALL, with the worst outcomes observed in MLL -rearranged patients who present at a very young age (<6 months at diagnosis) and with high presenting leukocyte counts.

Because of their poor prognosis, infants with ALL are usually treated on separate protocols with intensified therapy. In vitro drug sensitivity testing and small, nonrandomized clinical trials have suggested that high-dose cytarabine, an agent more often used in AML, might benefit infants with ALL. In the Interfant-99 trial, the largest clinical trial conducted for infants with ALL, a cytarabine-intensive regimen resulted in a 4-year EFS of 47%.

The role of allogeneic SCT in first complete remission in infants with ALL remains controversial. On the Interfant-99 trial, a subset analysis suggested that SCT in first remission may be beneficial in infants with high-risk features ( MLL gene rearrangement, age younger than 6 months at diagnosis, and presenting leukocyte count at or above 300,000/µL); an EFS advantage for transplant was not demonstrated in other MLL -rearranged infants.

Adolescents

Older children and adolescents (aged 10 to 18 years at diagnosis) have a less favorable outcome than children aged 1 to 10 years at diagnosis, and more aggressive treatments are generally administered to these patients. Adolescents present more frequently with adverse prognostic features, such as a high leukocyte count, T-cell immunophenotype, and the BCR-ABL translocation, and a lower frequency of favorable features, such as high hyperdiploidy and the ETV6-RUNX1 fusion gene. Adolescents also appear to be at higher risk than younger children for developing treatment-related complications, including osteonecrosis, pancreatitis, and thrombosis. Several investigators have compared outcomes of older adolescents with ALL (aged 15 to 20 years) treated in pediatric or adult clinical trials, and all have found that these patients fare better with a pediatric regimen. The superior outcome of older adolescents treated with pediatric ALL regimens may be related, in part, to the components of therapy; pediatric regimens tend to include higher cumulative dosages of vincristine, corticosteroid, and l -asparaginase than treatments designed for adults with ALL.

Children with Down Syndrome

Children with Down syndrome have increased risk for developing both ALL and AML. Approximately one half to two thirds of cases of acute leukemia in children with Down syndrome are ALL. Although the vast majority of cases of AML in children with Down syndrome occur before the age of 4 years, ALL in children with Down syndrome has an age distribution similar to that of ALL in non–Down syndrome children. Patients with ALL and Down syndrome almost always present with B-precursor cell phenotype and have a lower incidence of favorable and unfavorable cytogenetic abnormalities. They appear to be at higher risk for developing treatment-related complications, especially those related to methotrexate, such as mucositis and myelosuppression. Outcomes for children with Down syndrome and ALL have generally been reported to be less favorable than those of non–Down syndrome patients, which may be related to a higher incidence of treatment-related mortality because of infection and a lower frequency of favorable biologic features, such as high hyperdiploidy and the ETV6-RUNX1 fusion.

High expression of CRLF2 occurs in 50% to 60% of cases of ALL in children with Down syndrome, compared with 5% to 10% of non–Down syndrome B-precursor cell ALL patients. A higher frequency of activating mutations in JAK2 has also been observed in Down syndrome ALL (~20% of cases) ; almost all patients with JAK2 mutations also have CRLF2 overexpression. It does not appear that CRLF2 genomic aberrations or JAK2 mutations have independent prognostic relevance in Down syndrome ALL. However, in one study, deletions of the IKZF1 gene, observed in 35% of Down syndrome ALL cases (and frequently associated with CRLF2 overexpression), was found to be a strong independent predictor of inferior EFS.