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Neuroblastoma, a tumor of the sympathetic
nervous system, is the most common solid tumor in childhood. Interestingly, some
infants with metastatic disease experience complete tumor regression
without therapy, and other patients may have maturation of their
tumor into a benign ganglioneuroma. Nevertheless, the majority of
patients have metastatic disease at diagnosis that progresses despite
intensive multimodality therapy.1-3 Current risk classification
schemes use biological and clinical features at diagnosis to predict
tumor behavior and to stratify patients to an appropriate treatment.
Children with tumors that have lower risk features are spared unnecessary
therapies yet still achieve excellent outcomes. However, the survival
of patients with high-risk neuroblastoma is still unacceptably low.
Advances in understanding the molecular pathogenesis of this tumor,
including how alterations in specific biological pathways impact
tumor behavior, may lead to novel therapeutics to reduce toxicity
in patients with favorable disease and improve outcomes in those
with unfavorable disease.4,5
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Epidemiology,
Genetics, and Molecular Pathogenesis
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Neuroblastoma is the most common malignancy diagnosed in infants
and accounts for 8% to 10% of childhood cancers
overall. Unfortunately, it also accounts for 15% of childhood
cancer-related deaths. The prevalence is about 1 per 7000 live births,
and there are about 650 new cases per year in the United States,
with an incidence of 10.5 per million per year in white and 8.8
per million per year in black children less than 15 years of age.1-3 This
incidence appears fairly uniform throughout the world. The tumor
is slightly more common in males than in females, with a male-to-female
ratio of 1.2:1 in most large studies. The median age at diagnosis is
22 months, and less than 5% of patients are diagnosed after
10 years of age. The rare adolescent or young adult with neuroblastoma
poses a unique treatment challenge, as their tumors appear biologically distinct
with an indolent and often progressive course.
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The etiology of neuroblastoma is unknown in most cases, but it
appears unlikely that environmental exposures play a major role,
because no prenatal or postnatal drug, chemical, or radiation exposure
has been strongly or consistently associated with an increased risk.
Neuroblastoma has been reported in patients with neurofibromatosis
type 1, as well as central congenital hypoventilation syndrome (CCHS)
and Hirschsprung disease, suggesting that disordered neural crest
development may predispose to neuroblastoma. Mutations in the PHOX2B gene,
a key regulator of autonomic neural development, have been identified
in neuroblastomas associated with these latter disorders, and in
occasional sporadic tumors.6,7 Diverse congenital anomalies
have also been reported in association with neuroblastoma, but without
a causal genetic etiology. Finally, there may be a decreased prevalence
of neuroblastoma in patients with Down or Klinefelter syndromes,
and an increase in Turner syndrome, but the reasons for this are
unclear.
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As with many embryonal tumors, a small subset of neuroblastoma
patients has a familial predisposition that follows an autosomal
dominant pattern of inheritance. Analysis of such pedigrees predicted
that neuroblastoma fits the 2-mutation hypothesis proposed by Knudson
for the origin of childhood cancers. The median age at diagnosis
is younger (9 months in contrast with 22 months), and multiple primary tumors
are more prevalent in familial cases. Heritable mutations within
the ALK gene, an oncogenic tyrosine kinase deregulated
in multiple human cancers, is causative in the majority of familial
cases, with gene duplication or amplification believed to act as
the “second hit.”8 Acquired mutations
in ALK also play a role in a subset of sporadic
neuroblastomas, providing a potential therapeutic target.9
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Trk Receptors,
Differentiation, and Regression
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Neuroblastoma arises from sympathetic neuroblasts and often demonstrates
neuronal differentiation. Some tumors undergo spontaneous or therapy-induced
differentiation to ganglioneuroma, so the malignant behavior of
these cells may be maintained in part by a failed differentiation
program. Additionally, some neuroblastomas regress spontaneously,
particularly in infants. The factors responsible for regulating differentiation
or regression in these tumors are not well understood but may involve
neurotrophin signaling, as these pathways are paramount in modeling
the sympathetic nervous system. Three neurotrophin receptors (NTRK1, NTRK2, and NTRK3 encoding
TrkA, TrkB, and TrkC) were identified. Their primary ligands are
nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF),
and neurotrophin-3 (NT-3), respectively.
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High TrkA expression in neuroblastomas is associated with younger
age, lower stage, and a favorable outcome.10 Explanted
primary neuroblasts with high TrkA expression differentiate when
exposed to NGF and undergo apoptosis when deprived of NGF. Thus,
NGF/TrkA signaling may inhibit cancer cells by inducing differentiation
or cell death depending on the particular microenvironment. In contrast,
more aggressive tumors, especially those with MYCN amplification,
express TrkB and its ligand, BDNF.11 This potential
autocrine or paracrine signaling pathway promotes cancer cell survival,
therapy resistance, and metastasis. Finally, the expression of TrkC is
found predominantly in lower-stage tumors, and, like TrkA, is generally
not expressed in MYCN amplified tumors. Overall,
the pattern of expression of the neurotrophin receptors may play
an important role in the behavior of neuroblastomas, including the
likelihood that a tumor may undergo spontaneous regression or differentiation. As
such, these receptors are of potential pharmacologic interest, and
inhibitors of these signaling pathways are under development.
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Genomic Classification
of Neuroblastomas
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Several distinct genomic changes have been identified as characteristic
of neuroblastoma subsets. Amplification of the MYCN proto-oncogene
is seen in 20% to 25% of tumors and strongly associates
with unfavorable disease outcome.12 Testing for
this genomic alteration is recommended for all neuroblastomas, as
these results are used for treatment stratification worldwide.4,5 In
addition, common regions of deletion or allelic loss involving 1p
or 11q (among other loci) and unbalanced gain of distal 17q have
been associated with advanced disease stage and poor outcome in
numerous studies.13-16CHD5 has
been identified as a tumor suppressor gene deleted from 1p36.31,
but the cancer-associated genes from the other regions have yet
to be identified.17Diploid or tetraploid DNA content
has also been recognized as a feature of unfavorable neuroblastomas.18 In
contrast, tumors with hyperdiploid or near triploid DNA content
with whole chromosome gains have a more favorable outcome.
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More recently, technologies capable of identifying genome-wide
copy-number alterations in tumor cells have been applied to neuroblastoma. These
data suggest that there appear to be three distinct genomic subsets
of neuroblastoma that are highly correlated with clinical outcome.4,5 The
first is characterized by mitotic dysfunction leading to a hyperdiploid
modal karyotype, with whole chromosome gains but few if any structural
chromosome rearrangements (Fig. 457-1). These
patients are generally infants or toddlers with localized disease
and a very good prognosis. The majority of tumors, however, are
characterized by a near-diploid or tetraploid karyotype with segmental chromosomal
aberrations, and unbalanced 17q gain is found in the majority. Within
this less favorable genomic type, two subsets can be distinguished.
One demonstrates segmental chromosomal changes, such as 11q or 3p
deletion. The other is defined by high-level amplification of the MYCN proto-oncogene,
frequently with 1p deletion. These latter tumors are highly aggressive
and often lethal. Patients with these unfavorable subtypes are generally
older than 1 year with more advanced stages of disease, which is
often progressive. Current evidence suggests that these are genomically
and biologically distinct, and that there is no progression from
one genomic subtype to another. The underlying causes of the genomic
instability that lead to these patterns of genomic change remain
unknown.
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About half of all neuroblastomas originate in the adrenal medulla;
30% occur in nonadrenal abdominal sites in the paravertebral ganglia,
pelvic ganglia, or the organ of Zuckerkandl; and 20% occur
in the paravertebral ganglia of the chest or neck.1 Most
primary tumors cause symptoms such as abdominal mass or pain. Paraspinal
tumors frequently invade the spinal canal through neural foramina
and may cause spinal cord compression. In the thoracic or upper
lumbar region, this may lead to paraplegia, whereas lower lumbar
invasion leads to a cauda equina syndrome with loss of bowel or
bladder function. These are true oncologic emergencies, and rapid
institution of therapy may be necessary to preserve neurologic function.
Midline tumors can displace or compress structures such as the trachea
or esophagus and lead to obstructive symptoms. Involvement of the
superior cervical ganglion can produce Horner syndrome as its initial
manifestation.
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More than half of all neuroblastomas are metastatic at the time
of diagnosis. Frequent sites of metastasis are regional or distant
lymph nodes, cortical bone, bone marrow, and liver. In infants,
there is a characteristic pattern of metastases which shows massive
infiltration into the liver, skin nodules, and variable bone marrow
involvement. This pattern of disease is referred to as stage 4S
disease using the International Neuroblastoma Staging System, and
it has a very favorable outcome.19 However, in
older patients (Å 18 months old), metastasis frequently involves
significant bone marrow and cortical bone dissemination, commonly
including the skull and orbits. Rarely, disease may spread to lung
and brain parenchyma, usually as a manifestation of relapsing or
end-stage disease. The outlook for patients such as these is poor,
even with intensive, multimodality therapy.
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Paraneoplastic
Syndromes
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Several paraneoplastic syndromes have been rarely associated
with neuroblastoma, representing only 1% to 3% of
patients.1
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1. Vasoactive intestinal peptide (VIP) syndrome. These
patients have intractable secretory diarrhea and abdominal distention,
sometimes associated with hypokalemia and dehydration, which is
a manifestation of tumor secretion of VIP. The biological functions
of VIP are relaxation of smooth muscle, stimulation of intestinal
water and electrolyte secretion, and stimulation of release of other
polypeptide hormones. The VIP syndrome usually is associated with ganglioneuroblastoma
or ganglioneuroma, and these symptoms usually resolve after eradication
of the tumor.
2. Opsomyoclonus-myoclonus-ataxia (OMA) syndrome.
This syndrome consists of myoclonic jerking and random, uncontrolled
eye movement, sometimes associated with cerebellar ataxia. These
patients usually have biologically favorable tumors and an excellent tumor
outcome with surgery alone. Unfortunately, many have persistent
severe neurologic abnormalities. OMA syndrome may be caused by antineuronal
autoantibodies. The symptoms vary in severity, worsening with intercurrent
illnesses, and may respond to immune modulation. Chemotherapy treatment
may also improve the neurologic outcomes by more rapidly eliminating
antigenic tumor cells, as well as providing immune suppression.
3. Excessive catecholamine secretion syndrome. Rarely,
neuroblastoma patients may present with tachycardia, hypertension,
palpitations, profuse sweating, and flushing. This syndrome is more
common in patients with pheochromocytomas; neuroblastoma patients
with hypertension most often have a renovascular etiology, rather
than catecholamine secretion.
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Differential
Diagnosis
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Because of its myriad clinical presentations, neuroblastoma may
be confused with other neoplastic and nonneoplastic conditions.
This may be a problem particularly in the 5% to 10% of
patients with tumors that do not produce excess catecholamines,
as well as the 1% who do not have an obvious primary tumor.
The VIP syndrome can be confused with inflammatory bowel disease,
and those with opsoclonus-myoclonus and ataxia syndrome can resemble
a primary neurologic disorder. Histologically, neuroblastoma tissue
from primary or metastatic sites may be quite undifferentiated and
may be confused with other embryonal pediatric cancers such as rhabdomyosarcoma,
Ewing sarcoma, lymphoma, or even leukemia (especially megakaryoblastic
leukemia). Molecular, biochemical, and immunological characterization
can usually distinguish among these various possibilities.
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Diagnostic Evaluation
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To confirm a diagnosis of neuroblastoma, characteristic histological
or ultrastructural features are sought using light microscopy, immunohistochemistry,
and rarely electron microscopy. Stratification to an appropriate
therapy often requires that tumor genomic features and histopathologic
grade be known, so sufficient tumor material should be obtained
in these cases. Some patients with neuroblastoma and metastatic
disease are diagnosed based on the presence of tumor cells within
the bone marrow accompanied by increased urinary catecholamine metabolites.
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The majority of neuroblastomas (greater than 90%) produce
sufficient catecholamines to result in increased urinary metabolites.1 This
provides an ancillary diagnostic test, as well as a means to follow
disease activity, including surveillance for relapse. The two enzymes
primarily responsible for the catabolism of catecholamines are catechol-O-methyl
transferase and monamine oxidase. DOPA and dopamine are converted
primarily to homovanillic acid (HVA), whereas norepinephrine and
epinephrine are converted primarily to vanillylmandelic acid (VMA).
Both urinary VMA and HVA should be normalized for age and for urinary
creatinine.
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A standard set of recommended tests to define the clinical stage
or extent of disease has been established. Computerized tomography
(CT) scan is the current preferred imaging modality for primary
tumors, although concerns regarding radiation exposure in young
children may lead to more frequent use of magnetic resonance imaging
(MRI) scan. MRI is superior in evaluating paraspinal mass lesions
for intraforaminal extension and spinal cord compression. 123I-Meta-iodobenzylguanidine
scintigraphy (MIBG scan) is used for evaluation of the primary tumor
and metastatic disease sites. MIBG is selectively concentrated by
catecholaminergic cells, including greater than 90% of
neuroblastomas, providing highly sensitive and specific detection
of metastases. If the primary tumor does not take up MIBG (as in
5% to 10% of neuroblastomas), then a 99mTc-diphosphonate
scintigraphy (bone scan) is recommended to assess for metastatic
cortical bone sites. The use of positron emission tomography (PET)
with fluorine-18 fluoro-deoxyglucose (FDG) is being evaluated alone
and in combination with CT (PET-CT) for its utility in staging and
monitoring disease status.
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Bone marrow disease is assessed via bilateral bone marrow aspirates
and biopsies with standard histological analysis. Molecular or immunocytochemical
assays that detect neuroblastoma cells in marrow or blood samples
at diagnosis and following treatment, have also been developed, to
assess minimal residual disease. Although there is no doubt that
these techniques increase the sensitivity for detecting neuroblastoma
cells by 1 or 2 orders of magnitude, it is not yet clear if this
level of sensitivity provides information that is useful clinically.
They are currently not used for assessing bone marrow involvement,
which instead relies on visual detection of neuroblast clumps by
light microscopy.
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Neuroblastomas arise from primitive pluripotential sympathetic
nerve cells (sympathogonia) that are derived from the neural crest.
These cells can differentiate into the normal tissues of the sympathetic
nervous system, such as the spinal sympathetic ganglia, the supporting
Schwannian cells, and adrenal chromaffin cells. Neuroblastic tumor
histopathologies include neuroblastoma, ganglioneuroblastoma, and
ganglioneuroma, reflecting increased neural differentiation. The
typical neuroblastoma is composed of small cells containing dense,
hyperchromatic nuclei and scant cytoplasm. The presence of neuritic
processes, or neuropil, is a pathognomonic feature of all but the
most primitive neuroblastoma. The fully differentiated, benign counterpart
of neuroblastoma is ganglioneuroma, which is composed of mature
ganglion cells, surrounded by a matrix of Schwannian cells and neuropil.
Ganglioneuroblastomas have intermediate maturation and may be focal
or diffuse or have nodules of malignant neuroblastoma within an
otherwise mature ganglioneuroma (nodular ganglioneuroblastoma). Shimada
and colleagues devised a classification schema relating histopathologic
tumor features to clinical outcome. Tumors are classified as favorable
or unfavorable depending upon the degree of neuroblast differentiation,
Schwannian stroma content, mitosis-karyorrhexis index (MKI), and age
at diagnosis. A modification of this system, the International Neuroblastoma
Pathology Classification System (INPC), is currently used,20 but
future systems will define histological features independent of
patient age.
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To provide a common staging system for comparison
of clinical trial results worldwide, the International Neuroblastoma
Staging System (INSS) was developed.18 In addition to using
image-defined tumor features (such as degree of locoregional or
distant spread), this system depends on surgical factors, such as
completeness of initial resection, that may differ from one institution
to another and lead to differences in stage assignment. A presurgical
International Neuroblastoma Risk Group Staging System (INRGSS) has
been proposed in which only the radiographic characteristics of
the tumor, as well as bone marrow morphology, are used to more uniformly
define extent of disease at diagnosis.21 This staging
system will distinguish locoregional tumors that do not involve
local structures from locally invasive tumors. Metastatic tumors
will remain similar to current INSS stage 4 and 4S patterns of disease,
respectively.
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Prognostic Considerations
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Variables shown to have prognostic significance in neuroblastoma
are collectively used to stratify patients to an appropriate treatment.
The most important clinical variables are stage of disease (see
above) and the age of the patient at diagnosis. Localized tumors
(stage 1 and 2) are more common in younger children and are associated
with favorable outcomes. Young children with stage 4S disease, a
unique pattern of dissemination, also have favorable outcomes despite
the presence of metastases. In contrast, older children more commonly
have tumors that are regionally infiltrative (stage 3) or disseminated
(stage 4) and have a poor overall survival. Although younger age
and lower stage of tumor are associated with good outcomes in general,
a subset of these tumors manifests a very aggressive clinical course. These
tumors frequently have unfavorable biological features, underscoring
the importance of using both clinical and biological information (see
below) for risk assessment. Younger patient age is independently
predictive of more favorable outcome in neuroblastoma. Age is likely
a surrogate for developmental changes occurring in the neuroblasts
that become cancerous or in the cancer microenvironment, and age
may be considered as a continuous variable. For obvious practical
reasons, however, age has traditionally been analyzed as a binary
variable with < 365 days defined as favorable. An analysis of
more than 3000 patients with neuroblastoma suggests that this value
may be too low, and 547 days (~18 months) may be more appropriate.22
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Tumor histopathology (discussed above), serum markers, and select
genomic features also have independent prognostic value. Serum levels
of ferritin, neuron-specific enolase, and LDH may correlate with
disease burden and outcome, but they have largely been supplanted
by more tumor-specific biological variables. The DNA index (reflecting
tumor cell DNA content, or ploidy) is a strong prognostic marker,
particularly for younger patients with disseminated disease. Hyperdiploid
(often near-triploid) DNA content correlates with favorable disease
behavior, whereas near-diploid (and near-tetraploid) tumors tend
to be more aggressive, but this correlation loses its significance
after 18 to 24 months of age. The specific genomic aberration most
consistently associated with poor outcome in neuroblastoma is amplification
of the MYCN proto-oncogene, which occurs in ~20% of
tumors and is strongly correlated with a poor outcome, even in patients
with favorable age and stage. Deletion of 1p is found in ~35% of
neuroblastomas and correlates with MYCN amplification
and advanced disease stage. 1p loss predicts for an increased risk
of relapse in patients with localized tumors. Allelic loss of 11q
is also present in ~30% of tumors. Unlike 1p loss, this
aberration is rarely seen in tumors with MYCN amplification,
yet remains highly associated with other high-risk features suggesting
it may be a biomarker of aggressive disease independent of MYCN pathways.
A gain of 1 to 3 additional 17q copies, often through unbalanced
translocation, may correlate with an aggressive phenotype. The presumptive
cancer genes deregulated by most of these genomic changes remain
unknown, and their prognostic significance relative to other genomic
and biological markers are being further evaluated.
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Treatment and Outcome
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The current Children’s Oncology Group (COG) risk stratification
system incorporates patient age and INSS stage at diagnosis, as
well as tumor histopathology, DNA index, and MYCN gene
status to assign patients to 1 of 3 risk groups (low-, intermediate-,
or high-risk) and to stratify treatment intensity accordingly. The
goal of risk-adapted therapy is to maintain outstanding outcomes
with minimal toxicity for children with favorable disease features
while ensuring that children with unfavorable high-risk tumors are
identified and receive intensive multimodality treatments, offering
them the best chance of cure.
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Neuroblastoma treatment may include surgery, chemotherapy, radiotherapy,
and immunotherapy in high-risk cases, or observation alone in carefully
selected low-risk cases. A comprehensive summary is beyond the scope
of this review. However, a simplified approach requires that 2 critical
issues be addressed. First, a determination must be made as to whether
residual tumor remains following the initial surgical procedure.
If so, integration of biologic and clinical data is crucial to predict
the behavior of this residual tumor. Neuroblastoma has a remarkable propensity
to regress or differentiate. Therefore, in some cases, the presence
of macroscopic residual tumor, even at metastatic sites, is not
an indication for adjuvant therapy. Conversely, some localized tumors
can be near-totally resected, yet the presence of unfavorable biological features
supports the need for adjuvant therapy.
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Surgery plays a pivotal role in the management of neuroblastoma.
The goals of a primary surgical procedure are to provide tissue
to establish the diagnosis and perform biomarker testing, and to
excise the tumor if feasible without undue surgical risk. The majority
of localized neuroblastomas have favorable biological features and
can be successfully treated with surgery alone. In patients with
unresectable tumors who require chemotherapy, subsequent delayed
or second-look surgery may remove residual disease and facilitate
histological assessment of treatment response.
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Chemotherapy is the mainstay of therapy for neuroblastomas with
unfavorable biological features. Phase I and II clinical trials
conducted in patients with neuroblastoma have identified a number
of effective drugs. Cyclophosphamide, ifosphamide, cisplatin, carboplatin, doxorubicin,
and etoposide (VP-16) yield complete and partial response rates
of 25% to 50% and have become the cornerstone
of multiagent regimens. Newer effective agents include topotecan,
irinotecan, and temozolamide. Drug combinations have been developed
that take advantage of drug synergism, mechanisms of cytotoxicity, and
differences in side effects. Treatment of children with advanced
stage neuroblastoma using dose-intensive chemotherapy combinations,
including myeloablative consolidation therapy with autologous stem
cell rescue, has resulted in improved outcomes, but disease relapse
and toxicities remain a significant problem. Newer agents under
evaluation include drugs targeting key biological pathways supporting
tumor progression and include angiogenesis inhibitors, retinoids,
proapoptotic agents, immunotherapy (such as antibodies targeting
the neural GD2 cell-surface marker), and kinase inhibitors.
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Neuroblastoma is a radiosensitive tumor, and radiotherapy is
effective in achieving local disease control or symptom palliation.
However, long-term control of neuroblastoma is seldom achieved with
radiation therapy alone because of the propensity of this tumor
to metastasize. Historically, radiation has been used in the multimodality
management of residual neuroblastoma and bulky unresectable tumors.
The availability of effective chemotherapy regimens, concern over
late toxicities of radiation exposure to developing tissues, and
more accurate risk stratification have led to a marked reduction
in radiotherapy use. Still, it is an important adjuvant therapy
in high-risk disease to control against tumor relapse at sites of
bulk disease. There is growing experience with targeted radiotherapeutic
approaches such as 131I-metaiodobenzylguanidine
(MIBG) to deliver a radiotherapy dose selectively to tumor cells.
This agent has demonstrated potent activity against relapsed or
refractory neuroblastoma.
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Myeloablative
Therapy with Hematopoietic Stem Cell Rescue
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Attempts have been made to improve on the modest gains of intensive,
combined-modality therapy by increasing the intensity of therapy.
More intensive therapy can be administered if accompanied by hematopoietic
stem cell transplant (SCT). Allogeneic SCT appears to offer little
if any advantage and greater toxicity compared to autologous BMT.
Currently, most centers use peripheral blood stem cells as the source
of progenitor cells for marrow rescue following myeloablative conditioning.
Although stem cell product contamination with neuroblasts is possible,
studies have not demonstrated any benefit to efforts to remove these
using monoclonal antibodies against tumor cells to purge the final
stem cell product if gross contamination is not apparent. Thus,
recurrence largely stems from neuroblasts not eradicated by high-dose
cytotoxic therapy rather than through the reinfusion of viable cancer
cells in the stem cell product.
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The use of dose-intensified chemotherapy combinations, with or
without autologous stem cell rescue, has improved immediate disease
control in neuroblastoma. Unfortunately, this has not translated
into durable remissions in the majority of children with high-risk
tumors. Biological therapy to treat persistent minimal residual
disease has been added following SCT. The use of 13-cis-retinoic
acid, which induces neuroblast differentiation, in the posttransplant
setting was tested in a randomized Phase III trial and showed improved
event-free survival with acceptable toxicity.23 Thus,
retinoid-based biotherapy in the posttransplant setting is now widely
used. Other compounds, including novel retinoids such as Fenretinide,
or immunotherapies, such as anti-GD2 antibody, have activity
against minimal residual disease in high-risk neuroblastoma patients
and warrant further study.
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Future Considerations
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Advances have been made in understanding the biology underlying
neuroblastoma initiation and progression. However, there remain
a variety of areas through which improvements in outcomes may be
realized. These include (1) the ongoing identification of the principal
genomic mutations driving tumor propagation (both germline and somatically
acquired) that will inform our biological understanding and provide
novel therapeutic targets, such as ALK; (2) continued
refinements to biological classification (such as implementation
of array-based technologies for biomarker determination) and risk-stratification
schemas; and (3) the development of rational biologically based
therapy targeted to the genes, proteins, and pathways responsible
for initiation or maintenance of the malignant state.