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The Oncologist, Vol. 8, No. 2, 174–186, April 2003
© 2003 AlphaMed Press

ORIGINAL PAPER
Pediatric Oncology

Advances in the Diagnosis, Molecular Genetics, and Treatment of Pediatric Embryonal CNS Tumors

Tobey J. MacDonalda, Brian R. Rooda, Maria R. Santib, Gilbert Vezinac, Kimberly Bingamand, Philip H. Cogend, Roger J. Packere

a Departments of Hematology/Oncology, b Pathology, c Radiology, d Neurosurgery, and e Neurology, Children’s Hospital National Medical Center, Washington, DC, USA

Correspondence: Tobey J. MacDonald, M.D., Children’s Hospital National Medical Center, Department of Hematology/Oncology, 111 Michigan Avenue, NW, Washington, DC 20010, USA. Telephone: 202-884-2800; Fax: 202-884-5685; e-mail: tmacdona{at}cnmc.org


   LEARNING OBJECTIVES
Top
 Learning Objectives
Abstract
 Introduction
 Medulloblastomas and Primitive...
 Atypical Teratoid/Rhabdoid...
 Novel Therapeutic Strategies for...
 Summary
 References
 
After completing this course, the reader will be able to:

  1. Recognize the classification, clinical presentation, and diagnosis of embryonal CNS tumors.
  2. Explain the important molecular genetic alterations identified in embryonal CNS tumors.
  3. Describe the current management and novel treatment strategies for embryonal CNS tumors.

Access and take the CME test online and receive one hour of AMA PRA category 1 credit at CME.TheOncologist.com


   ABSTRACT
Top
 Learning Objectives
 Abstract
Introduction
 Medulloblastomas and Primitive...
 Atypical Teratoid/Rhabdoid...
 Novel Therapeutic Strategies for...
 Summary
 References
 
Embryonal central nervous system (CNS) tumors are the most common group of malignant brain tumors in children. The diagnosis and classification of tumors belonging to this family have been controversial; however, utilization of molecular genetics is helping to refine traditional histopathologic and clinical classification schemes. Currently, this group of tumors includes medulloblastomas, supratentorial primitive neuroectodermal tumors, atypical teratoid/rhabdoid tumors, ependymoblastomas, and medulloepitheliomas. While the survival of older children with nonmetastatic medulloblastomas has improved considerably within the past two decades, the outcomes for infants and for those with metastatic medulloblastomas or other high-risk embryonal CNS tumors remain poor. It is anticipated that the emerging field of molecular biology will greatly aid in the future stratification and therapy for pediatric patients with malignant embryonal tumors. In this review, recent advances in the diagnosis, molecular genetics, and treatment of the most common pediatric embryonal CNS tumors are discussed.

Key Words. Primitive neuroectodermal tumor • Medulloblastoma • Atypical teratoid/rhabdoid tumor • Diagnosis • Molecular genetics • Treatment


   INTRODUCTION
Top
 Learning Objectives
 Abstract
 Introduction
Medulloblastomas and Primitive...
 Atypical Teratoid/Rhabdoid...
 Novel Therapeutic Strategies for...
 Summary
 References
 
Embryonal central nervous system (CNS) tumors comprise the most common group of childhood malignant brain tumors (21%) [1]. The World Health Organization (WHO) classification of tumors recognizes the following entities within this group: medulloblastoma (MB), supratentorial primitive neuroectodermal tumor (PNET), atypical teratoid/ rhabdoid tumor (AT/RT), ependymoblastoma, and medulloepithelioma [2]. MBs, PNETs, and ependymoblastomas share a histologically similar, undifferentiated morphology, while medulloepitheliomas and AT/RTs have distinctly different histologies and appear to evolve by different genetic pathways. The incidence of CNS embryonal tumors is constant from infancy to 3 years of age (11.6 to 10.2 per million) and then steadily declines thereafter [1]. MBs, PNETs, and AT/RTs make up the majority of these tumors, the remaining being rare, and thus are the focus of this review. Controversy exists regarding the division between MBs and PNETs, but emerging molecular, biologic, and clinical evidence supports the separation of these tumors [3]. The incidence and classification of the more recently described entity, AT/RT, is also evolving due in large part to the expanded use of diagnostic molecular genetics. Historically, AT/RTs have been confused with MBs or PNETs.

Treatment of these tumors has traditionally relied on surgery and radiation therapy (RT). More recently, chemotherapy has been utilized to improve outcome and/or delay or reduce the dose of RT in an attempt to lessen its neurotoxic effects. While the survival of older children with nonmetastatic MBs has improved considerably within the past two decades, the outcomes for infants and for those with metastatic MBs or other high-risk embryonal CNS tumors remain poor. It is hoped that the field of molecular biology will aid in the development of novel therapeutics that target specific characteristics of individual tumors, while minimizing toxicity to normal organ systems. This review discusses important advances in the diagnosis, molecular genetics, and treatment of the most common pediatric embryonal CNS tumors.


   MEDULLOBLASTOMAS AND PRIMITIVE NEUROECTODERMAL TUMORS
Top
 Learning Objectives
 Abstract
 Introduction
 Medulloblastomas and Primitive...
Atypical Teratoid/Rhabdoid...
 Novel Therapeutic Strategies for...
 Summary
 References
 
Medulloblastomas account for 40% of all posterior fossa tumors and 15%–20% of all childhood brain tumors. The peak incidence occurs between 3 and 4 years of age, with a male predilection of 1.5- to two-fold [1]. PNETs constitute 2% of all childhood brain tumors and are most often located in the cerebrum, suprasellar, or pineal region of children in their first decade of life [2]. Metastatic disease at diagnosis occurs in 11%–43% of MB/PNET cases and is one of the most important clinical predictors of outcome [4]. Extraneural spread of MBs/PNETs is an uncommon event, with bone, bone marrow, lymph nodes, liver, and lung involvement occurring in decreasing order of frequency.

Clinical Presentation
Medulloblastomas arise from the cerebellum, typically growing into the fourth ventricle. Patients often present with hydrocephaly and raised intracranial pressure (ICP) symptoms, such as headache, lethargy, and morning vomiting. Infants in whom the cranial sutures have not fused can present with increasing head circumferences. Cerebellar invasion results in ataxia and dysmetria. Patients with PNETs present with symptoms dependent upon tumor location. Paresis and seizures can occur with tumors of the cerebral cortex, raised ICP symptoms occur with tumors that obstruct cerebrospinal fluid (CSF) flow, and endocrinopathies or visual deficits may result from suprasellar tumors.

Neuropathologic Diagnosis
The classic histologic appearance of an MB is that of densely packed cells with hyperchromatic nuclei, indiscernible cytoplasms, and numerous mitoses (Fig. 1AGo). Homer Wright rosettes and neuroblastic differentiation are observed in a minority of cases (Fig. 1BGo). These tumors may be strongly immunoreactive for vimentin and at least focally for synaptophysin [5]. Although classic MB is the most common form, the WHO classification of CNS tumors describes three additional subtypes of MB [2] (Table 1Go). These subtypes include large cell, occurring in approximately 4% of cases, desmoplastic (Fig. 1C and DGo), and the rare MB variant characterized by extensive nodularity and advanced neuronal differentiation, also known as "cerebellar neuroblastoma" [2, 6].



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  		Figure 1. Histologic features of medulloblastomas. Undifferentiated, classic medulloblastoma (A) is characterized by patternless sheets of small round hyperchromatic cells. Homer Wright rosettes (B), the histologic expression of neuroblastic differentiation, are seen in a minority of cases. In desmoplastic lesions, the tumor cells are compressed into slender columns (C) or are organized in nodular zones (arrow) (D). (Hematoxylin-eosin stain, 200x magnification).

 

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  		Table 1. Histopathologic subtypes of medulloblastomas by WHO classification of CNS tumors
 
PNETs are histologically similar to classic MBs [2, 7]. Nuclear polymorphism, brisk mitotic activity, and necrosis may be present. Rarely, Homer Wright or Flexner-Wintersteiner rosettes are seen. Fields of neuronal cells, glial cells, ependymal canals, and striated muscle or melanin-bearing cells may be identified, confirming divergent differentiation along neuronal, astrocytic, ependymal, muscular, or melanocytic lines, respectively [8].

Molecular Genetics and Neurobiology
Molecular biology has augmented traditional histopathologic and clinical classification schemes by providing further insight into the biological diversity of MBs/PNETs. This emerging field is expected to have a great impact on the diagnosis, classification, and prognosis of MBs/PNETs as well as aid in the rational development of innovative molecularly targeted therapies. A summary of the most common molecular genetic alterations recognized in MBs/PNETs is shown in Table 2Go.


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  		Table 2. Common molecular alterations detected in MB and PNET
 
Expression of the neurotrophin-3 receptor trkC was the first molecular alteration in MBs to be correlated with outcome [9]. Neurotrophin receptors regulate cell differentiation, growth, and apoptosis in the developing cerebellum. TrkC activation in MB cells induces apoptosis by initiating c-jun and c-fos early gene expression [10]. trkC expression has been found in up to 48% of MB cases [9, 11]. High trkC expression is the single most powerful independent predictor of favorable outcome, with 5-year survival rates as high as 89%, compared with 46% for those patients with low trkC expression levels [9, 11].

High expression of the erbB-2 (c-erbB-2) oncogene product, HER2, a member of the epidermal growth factor receptor family, correlates with poor outcome in MB patients. HER2 expression has been found in 84% of MB cases, and in those patients with more than 50% positive tumor cells, the 10-year survival rate was 10%, compared with 48% for all others [12]. Low expression level of the MYCC (C-myc) oncogene is predictive of greater survival in MB patients [13]. MYCC expression has been detected in 42% of MB cases. A recent study showed that MYCC amplification occurs in only 5% of MB cases; however, all patients with this amplification died of aggressive disease within 7 months of diagnosis [14].

The nevoid basal cell carcinoma syndrome (NBCCS, Gorlin’s syndrome) is an autosomal dominant disease resulting from mutations of the PTCH gene on chromosome 9q22.3. This mutation leads to the development of MB in about 4% of affected patients. Similarly, NBCCS is responsible for 1%–2% of all MBs. Studies have shown PTCH mutations in about 10% of sporadic MB cases, particularly in desmoplastic MBs [15]. PTCH encodes a membrane receptor important for cell growth in the developing cerebellum. Experimental models have shown that loss of p53 accelerates the development of MBs in mice heterozygous for PTCH [16], indicating that PTCH acts as a tumor suppressor gene. Sonic hedgehog (SHH), a major ligand for the PTCH receptor, is considered a putative oncogene.

Loss of genetic material from the short arm of chromosome 17 (17p) is the most common cytogenetic abnormality in MBs, occurring in 35%–50% of cases [17]. Among the genes localized to the common breakpoint at 17p13.3, HIC-1 is the leading tumor suppressor gene candidate inactivated by 17p deletion. HIC-1 encodes for a zinc finger transcriptional repressor whose expression is upregulated by p53 and is silenced by hypermethylation. Hypermethylation of the HIC-1 gene is a frequent event in MB that predicts for a poor outcome [18]. Other frequent cytogenetic abnormalities include deletions of regions on chromosomes 10q and 11 as well as rearrangements of chromosomes 3, 14, 10, 6, 13, 18, and 22 [19, 20].

Despite similar histological appearances, many of the molecular genetic aberrations found in MBs are absent in PNETs. For example, loss of genetic material from chromosome 17p is not found in PNETs [21]. Patterns of aberrant methylation in the region of the 17p breakpoint cluster of MBs are also absent [22]. Recent microarray studies have revealed that MBs and PNETs could be separated based on their specific patterns of gene expression [3]. Furthermore, this work illustrated that the sporadic form of desmoplastic MB is molecularly similar to that of MB associated with NBCCS, yet distinct from classic MB, predominantly due to differential expression of the PTCH/SHH genes. Most importantly, the clinical outcomes of children with MBs were best predicted by the gene expression profile of the individual’s tumor.

Using similar methodology, another study compared gene expression profiles of metastatic (M+) and non-metastatic (M0) MBs. This analysis discovered that the platelet-derived growth factor receptor alpha (PDGFR-{alpha}) and the Ras/mitogen-activated protein (MAP) kinase pathway genes were significantly upregulated in M+ tumors [23]. This finding suggests that the PDGFR-{alpha} and Ras/MAP kinase signal transduction pathways may be rational therapeutic targets for M+ disease.

Neuroradiographic Findings
The imaging features of MBs/PNETs are fairly homogeneous throughout the CNS. On T1-weighted images, the solid components generally have low signals and strong contrast enhancements. On T2-weighted images, the solid component signals are intermediate between gray and white matter; on fast fluid-attenuated inversion recovery (FLAIR) images, the signals are isointense to gray matter. In contrast, most other CNS tumors tend to have T2-weighted and FLAIR signal that are greater than gray matter [24]. MBs typically arise in the cerebellar vermis and roof of the fourth ventricle, growing forward into the fourth ventricle, which is displaced anteriorly (Fig. 2AGo and 2BGo). Invasion of the dorsal brain stem or extension into the medial cerebellar hemisphere may occur. MBs are typically 3–5 cm in maximal diameter. In older children and adolescents, MBs have a tendency to present either in the lateral cerebellar hemisphere or near the cerebellopontine angle cistern [24]. Atypical imaging features include an extensive or complete lack of enhancement in up to 25% of lesions, cystic or large necrotic areas, and hemorrhage. On computerized tomography (CT) scans, MBs have a hyperdense appearance compared with the cerebellum, and calcifications are seen in approximately 10% of cases [25].



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  		Figure 2. Medulloblastoma with extensive subarachnoid metastatic dissemination. Axial T2-weighted (A) and postcontrast sagittal T1-weighted (B) images show a mass filling the fourth ventricle. The ventral brainstem is coated with enhancing tumor (B).

 
PNETs replicate the appearance of MBs (Fig. 3A-3CGo). However, these lesions are generally larger and more commonly display large cystic/necrotic areas. They are typically well defined rather than infiltrative, most often located in the frontoparietal region, and can arise either cortically or in the deep periventricular white matter. Calcifications and hemorrhage are more common, especially within the larger cystic or necrotic foci. These characteristics result in more heterogeneous magnetic resonance imaging (MRI) features, including areas of high T1-weighted signal (hemorrhage) and a mixed low and high T2-weighted signal (high cellularity and cystic, necrotic, and/or hemorrhagic changes) [26]. Peritumoral edema is common, though often minimal given the large size of these tumors [27].



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  		Figure 3. Supratentorial PNET. Axial T2-weighted (A), postcontrast axial (B), and sagittal (C) T1-weighted images reveal a large mass in the left temporal lobe and the subfrontal region. Necrosis is seen on the contrast image (B, C) as serpiginous nonenhancing regions within the tumor. A metastatic nodule is present behind the vermis of the cerebellum.

 
At the time of diagnosis, meticulous imaging of the entire CNS is required for all MB/PNET patients, as these tumors have a propensity to spread throughout the subarachnoid spaces. In general, metastatic deposits are identified on gadolinium T1-weighted images as enhancing nodules or "carpet-like" coverings of the meningeal surfaces of the brain and spinal cord. However, nonenhancing metastatic disease can also be present, especially when the primary tumor does not enhance. The nonenhancing deposits are often only identified on T2-weighted images as areas of distortion of the subarachnoid spaces and can also be seen as areas of abnormal signal on FLAIR or diffusion images.

Diffusion-weighted imaging, which reflects Brownian diffusion of water molecules, reveals abnormal restriction of water movement in most MBs/PNETs. In contrast to most CNS tumors, MBs/PNETs are hyperintense on diffusion-weighted images. The restricted diffusion characteristics likely reflect the high cellularity and dense packing of MBs/PNETs [26]. The MR spectroscopy (MRS) signatures of MBs/PNETs reflect that of malignant tumors and are not as specific as the imaging features on conventional and diffusion images. In general, choline levels are markedly increased, N-acetyl aspartate (NAA) is either markedly decreased or absent, and lactate/lipid moieties can be identified. Choline is a cellular membrane marker; its increase reflects increased membrane turnover within the tumor. NAA is a neuronal marker; its diminution or absence confirms the lack of neuronal differentiation of MBs/PNETs. Lactate is a product of anaerobic glycolysis and indicates the presence of necrosis or nonaerobic cellular metabolism.

Therapeutic Considerations

Clinical Prognostic Factors
Treatment groups for MB are designated high risk and average risk based upon the criteria of age greater than or less than 3 years, residual tumor greater than or less than 1.5 cm2, and the presence or absence of metastatic disease on neuroimaging or CSF sampling [4, 24]. Age younger than 3 years is predictive of poor outcome. One explanation for this is that younger children more commonly present with metastatic disease [28]; however, they are also less likely to be treated with conventional doses of RT [29] and are more likely to have subtotal tumor resection [30]. Extent of resection correlated with better survival for patients without metastatic disease in the Children’s Cancer Group (CCG) study 921 [31]. Metastatic disease at diagnosis has been repeatedly correlated with poor survival, the exception being M1 disease, defined as only CSF cytology positive for MB cells [4, 32]. PNETs are considered high risk regardless of the patient’s age, extent of resection, or the presence or absence of metastatic disease at diagnosis, and as such, are treated in a similar fashion as high-risk MBs as outlined below.

Surgery
Most MBs are located in the midline of the fourth ventricle and/or cerebellar vermis, with associated important hydrocephalus. If the child presents in extremis from his or her hydrocephalus, an emergency ventriculostomy should be performed through a frontal burr hole, often at the bedside using conscious sedation. The CSF may then be sampled for tumor cells as well as drained to a level sufficient to relieve the acute symptoms. If the child is not obtunded and responds to intravenous corticosteriods alone, a burr hole can be placed in the occipital skull at the time of the tumor resection and an external ventricular drain placed [33]. In the rare instances where hydrocephalus is not initially present, a burr hole should usually be placed anyway at the time of tumor resection to allow bedside ventriculostomy should postoperative swelling result in CSF obstruction. The child is usually operated upon in the prone position: we favor the use of a craniectomy rather than replacing the bone flap, for these highly malignant tumors often produce considerable posterior fossa edema postoperatively. It may be necessary to remove the posterior arch of the first cervical vertebra to gain access below the cerebellar tonsils.

Using the operating microscope, the cerebellar tonsils should be carefully separated following the dural opening, and the floor of the fourth ventricle can be identified and protected with a cottonoid pledgett. The majority of these tumors arise from this region, and their attachment may be identified. The bulk of the tumor can then be resected by splitting the vermis and retracting the cerebellar hemispheres. Useful surgical adjuncts include the Cavitron ultrasonic aspirator. Care must be taken to avoid undue dissection of the roof of the third ventricle, which results in ocular pareses, but the tumor must be fully resected from this location to remove the inferior third ventricular obstruction that is almost always present. Dissection at the junction of the cerebellar peduncles and brainstem may be the origin of the phenomenon of postoperative mutism [34]. In general, an attempt should be made to remove the entire tumor [35]. This may not be possible when there is encasement of the posterior inferior cerebellar artery or extensive involvement of the brainstem. However, it is sometimes possible that residual tumor detected on the postoperative MRI scan can be safely resected, and under these circumstances, a second operation should be attempted to achieve a complete resection in patients with nonmetastatic disease. If there is already leptomeningeal dissemination seen at the time of the resection, then no attempt should be made to route out every last cell of the primary mass. Common postoperative deficits in addition to mutism include ataxia, hemiparesis, and sixth nerve palsy, which generally resolve over time [36]. Approximately 60%–75% of children in whom a total or near-total resection of the mass is achieved will not require permanent CSF diversion. The remainder of these children should undergo placement of a ventricular shunt generally at day 5–7 postoperatively, when the CSF has cleared from blood and debris, and it is clear that a permanent implant will be required.

Medulloblastomas that present in the cerebellopontine angle, once classified as reticulum cell sarcomas (primarily now known as the desmoplastic variant), should be approached through a laterally placed incision and craniectomy. These tumors are generally completely resectable, as they do not involve the fourth ventricle, and often present with hydrocephalus. This is also a common location for AT/RTs, although this latter type tends to envelop the cranial nerves, arteries, and brainstem, making their resection more problematic. Supratentorial PNETs should be approached through a craniotomy placed in relation to their site of origin. These tumors are most often extremely large and vascular. An attempt should be made to resect the entire primary mass, unless there is widespread leptomeningeal disease. The use of intraoperative neuronavigation (frameless stereotactic guidance) can be quite helpful in the resection of these tumors.

Radiation and Chemotherapy
The cornerstone of MB/PNET treatment has been RT of the primary tumor site. However, given the propensity of MBs/PNETs to spread, the addition of craniospinal radiotherapy (csRT) for prophylactic treatment of metastasis has been necessary to maximize survival [37]. Unfortunately, the neurocognitive and endocrine effects resulting from irradiation of the developing neuraxis have presented a high price for this protection. In an attempt to lessen RT-induced neurotoxicity, clinical trials utilizing adjuvant chemotherapy have been explored.

Medulloblastomas respond to a range of alkylator and platinum-based drugs. A CCG study of patients with average-risk MBs reduced the csRT dose from the standard 3,600 cGy to 2,340 cGy (total boost 5,580 cGy) and added adjuvant chemotherapy consisting of vincristine, cisplatin, and lomustine (CCNU). Progression-free survival was 86% at 3 years and 79% at 5 years [38]. These rates compared favorably with historical controls. A CCG trial using an identical RT dose followed by a randomization between the chemotherapy described above and one substituting cyclophosphamide for the CCNU was recently completed. These data are awaited to confirm the promising results for reduced-dose csRT in this group of patients.

Despite this reduction in csRT, neurocognitive deficits were still noted. Patients who underwent longitudinal intelligence testing demonstrated an estimated rate of change from baseline of -4.3 Full Scale Intelligence Quotient points per year, -4.2 Verbal IQ points per year, and -4.0 Nonverbal IQ points per year (p < 0.001 for all three outcomes). Females, children aged less than 7 years, and those with higher baseline IQs were at greatest risk [39].

Doses of 3,600 cGy csRT with total tumor boost to 5,400 cGy have been used to treat high-risk MBs and PNETs in neurodevelopmentally appropriate patients. However, when used as the sole postoperative treatment, results were dismal. Yet objective responses to chemotherapy were observed in up to 50% of patients. Postoperative chemoradiotherapy for non-pineal PNETs have produced 5-year survivals in approximately one-third of patients, with children less than 2 years faring more poorly [30]. Although infants with pineal PNETs did poorly, older patients with this type of tumor in this location appeared to have a better prognosis [30].

In very young children, for whom the long-term neurocognitive sequelae of RT are unacceptable, high-dose chemotherapy (HDCT) and autologous stem cell (ACS) support have been used in an attempt to delay or obviate the need for RT. In a study of 23 relapsed MB patients who received HDCT consisting of carboplatin, thiotepa, and etoposide with autologous stem cell (ASC) rescue, 3-year event-free survival (EFS) and overall survival (OS) rates were 34% and 46%, respectively [40]. Trials of HDCT and ASC as front-line therapy are ongoing in patients less than 3 years of age with MBs/PNETs and as therapy following csRT for older children with high-risk MBs or PNETs.


   ATYPICAL TERATOID/RHABDOID TUMORS
Top
 Learning Objectives
 Abstract
 Introduction
 Medulloblastomas and Primitive...
 Atypical Teratoid/Rhabdoid...
Novel Therapeutic Strategies for...
 Summary
 References
 
Atypical teratoid/rhabdoid tumors, first described by Rorke et al. in 1987, are considered by some as a subtype of PNET [41–44]. With the wider utilization of immunohistochemistry and new molecular genetic probes, AT/RTs have been increasingly diagnosed, especially in infants and very young children [42, 44]. AT/RTs also have been diagnosed in older children and young adults [45–48]. The exact incidence of this tumor is unknown, but it has been suggested that approximately 10%–15% of children less than 3 years of age thought to have MBs or other forms of PNETs, actually had AT/RTs [45–48]. Others have reported that the ratio of AT/RTs to other more common PNETs is as low as 1:4 among children less than 3 years of age [49].

Clinical Presentation
AT/RTs present in a similar fashion to other PNETs and can arise throughout the nervous system. Approximately one-half of patients will have tumors originating in the posterior fossa, with a possible predilection for the cerebellopontine angle [42, 47]. Supratentorial AT/RTs tend to be extremely large at the time of diagnosis and may have cystic/necrotic components [42–48]. The tumors can be intra- or extra-axial and often invade adjacent structures. The incidence of leptomeningeal dissemination at the time of diagnosis has not been firmly established. Early review suggested that as much as 30%–40% of patients had leptomeningeal dissemination, although in more recent studies the incidence of dissemination was noted to be closer to 15% [42–48].

Neuropathologic Diagnosis
AT/RTs are malignant embryonal tumors composed of rhabdoid cells usually with additional, variable components of primitive neuroectodermal, mesenchymal, and epithelial cells [42, 44, 50]. The typical rhabdoid cell is medium sized, round to oval, with distinct borders, an eccentric nucleus, and commonly a prominent nucleolus (Fig. 4Go). The cytoplasm has a fine granular character or may contain a poorly defined pink "body" resembling an inclusion. Variable elements from small cells with tapering cytoplasmic tails to large bizarre cells may be identified. The primitive neuroectodermal component may consist of sheets of small round blue cells or may display Homer Wright or Flexner-Wintersteiner rosettes. The mesenchymal component appears as loose arrangements of small spindle cells or tightly arranged in a fascicular pattern resembling sarcoma. Epithelial differentiation is uncommon, and if present, is confined to few gland-like spaces. Mitoses are abundant, and field necrosis is common. The immunophenotype is broad, as the large rhabdoid cells display a range of immunoreactivity with clusters of cells almost always positive for epithelial membrane antigen and vimentin. Also frequent is reactivity for glial fibrillary acidic protein and cytokeratin, and less frequent is reactivity for smooth muscle actin and neurofilament protein. The rhabdoid cells are negative for desmin and any of the markers for germ cell tumors [42, 44].



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  		Figure 4. Histologic features of an AT/RT. The cells have large nuclei with prominent nucleoli (arrowhead), and some cells possess abundant eosinophilic cytoplasm (arrow) (A). Vimentin reactivity (B) is universal, and positive staining for epithelial membrane antigen (C) is common in groups of cells (arrow). (Hematoxylin-eosin, vimentin, and epithelial membrane antigen stains, 400x magnification).

 
Molecular Genetics and Neurobiology
Molecular genetic analysis has aided greatly in the diagnosis and understanding of AT/RTs. The vast majority of AT/RTs demonstrate monosomy 22 or deletions of chromosome band 22q11 [51, 52]. Other CNS tumors may demonstrate chromosome 22 abnormalities, and this abnormality alone is not sufficient for diagnosis. MBs and other PNETs may show a deletion of chromosome 22, but can be distinguished from AT/RTs by the presence of associated chromosome abnormalities. Eighty-five percent or more of AT/RTs show alterations of the hSNF5/INI1 gene [52–54]. The direct function of this gene in tumor development is unknown, but homozygous inactivation of the hSNF5/INI1 gene likely results in altered transcriptional regulation of downstream targets by the chromatin remodeling complex (SWI/SNF). The mutations in this gene are predominantly point mutations that result in the coding of a novel stop codon, which predicts premature truncation of the protein [53–55].

Neuroradiographic Findings
The CT findings of AT/RTs are relatively characteristic, but not diagnostic. These tumors are usually hyperdense and enhance intensely [42]. Calcifications may occur but are not common, while cysts are more common in the supratentorial lesions. On MRI imaging (Fig. 5Go), the T1 signal of the solid portion of the tumor is typically isointense; there are frequent T1 hyperintense foci (secondary to intratumoral hemorrhage) and hypointense foci (secondary to cystic/necrotic change). AT/RTs commonly display intense contrast enhancement. The T2 appearance is heterogeneic. The MRS appearance of an AT/RT is similar to that of a PNET, with marked elevation of choline and low or absent NAA and creatine; lipids and lactate peaks can often be identified.



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  		Figure 5. Atypical teratoid/rhabdoid tumor in the right cerebellopontine angle. Axial T2-weighted (A), postcontrast axial (B), and sagittal (C) T1-weighted images demonstrate a heterogeneous mass (A) with central enhancement and dural extension (B, C).

 
Therapeutic Considerations
To date, the therapy for AT/RTs has been suboptimal. Information about response to therapy and outcome has been primarily gathered from retrospective reviews of a handful of patients [42–48]. An AT/RT registry has added some useful information [47]. The role of surgery for AT/RTs is unsettled [56]. Although initial reports suggested that, because of the age of the patients, the large extent of the tumors, and their tendency to be more laterally placed in the cerebellopontine angle, total or near-total resection was quite uncommon. In the AT/RT registry, six of the eight patients who survived for greater than 18 months had undergone "total" resection.

Given the young age of the patients, chemotherapy has been the primary modality of treatment after radiation therapy [56]. Even after aggressive surgery and chemotherapy, overall survival rates for children, especially those less than 2 years of age, have been extremely poor, with less than 20% of patients surviving less than 12 months from diagnosis. A variety of different chemotherapeutic agents have been utilized, but no one agent or combination of agents has been shown to be most effective. The majority of children have been treated with chemotherapeutic regimens developed for infantile brain tumors that have included drugs such as cyclophosphamide, cisplatin, etoposide, and vincristine. The use of myeloablative doses of chemotherapy, supported either by autologous bone marrow transplant or peripheral stem cell rescue, has not been shown to increase survival.

Because of the histological appearance of these tumors, another approach has been to utilize sarcoma chemotherapy regimens [56]. In general, these regimens have shown a slightly higher overall response rate; however, the majority of patients treated with such regimens have been somewhat older. In general, the results of chemotherapeutic studies suggest that a variety of chemotherapeutic regimens may result in tumor stabilization and, for fewer patients, objective tumor shrinkages. The benefit of chemotherapy has not been durable for most patients. Because of the age of patients, radiotherapy has been less widely employed in children with AT/RTs [42–48, 56]. Most of the children reported to the AT/RT tumor registry that survived for greater than 18 months received at least local RT [47, 56]. However, conclusions are difficult to draw, since many of those patients were older at the time of diagnosis.

In summary, therapeutic approaches have been suboptimal, with the majority of patients developing progressive disease within 12 months of diagnosis and dying soon after. As the prognosis of children with AT/RTs seems to differ from those with MBs/PNETs, investigators have suggested that AT/RTs be removed from present infant brain tumor protocols and entered on protocols designed specifically for AT/RTs [56]. There is sentiment to use high-dose chemotherapy for a shorter period of time and institute at least local radiotherapy earlier for patients with localized disease at the time of diagnosis. The optimal induction therapy is not clear from available data and there is no treatment that has shown significant efficacy for children with disseminated disease at the time of diagnosis.


   NOVEL THERAPEUTIC STRATEGIES FOR EMBRYONAL CNS TUMORS
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 Learning Objectives
 Abstract
 Introduction
 Medulloblastomas and Primitive...
 Atypical Teratoid/Rhabdoid...
 Novel Therapeutic Strategies for...
Summary
 References
 
The development of therapies with acceptable toxicities that can adequately penetrate the CNS yet remain relatively unsusceptible to the emergence of tumor resistance is critical to improving the outcome of pediatric embryonal CNS tumors. Treatment strategies can be broadly separated into two categories: methods that increase the total dose of drug/radiation delivered to the focal sites of CNS disease and novel therapeutics that exploit the specific biological characteristics of the tumor. Clinical strategies that are currently active are summarized in Table 3Go.


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  		Table 3. Active clinical trials utilizing novel therapeutic strategies for embryonal CNS tumors
 
High-dose systemic chemotherapy, with ASC or peripheral blood stem cell (PBSC) support, is being evaluated in children with CNS tumors. The aim of HDCT is to increase the tumor’s exposure to cytotoxic agents by overcoming the limited permeability of the blood-brain barrier (BBB). Classic alkylating agents, which generally have nonoverlapping hematological toxicities, show little cross-resistance, and maintain steep and linear dose-response curves, have been predominantly investigated by this approach. Because of its lipid solubility, thiotepa has been commonly used. Initial results with thiotepa and busulfan in 20 children with relapsed malignant tumors showed five partial responses (4/8 MB/PNET) for an overall response rate of 26% [57]. A more recent CCG study using carboplatin, thiotepa, and etoposide followed by ASC support for 23 patients with recurrent MBs reported a 3-year EFS rate of 34% and an OS rate of 46% [40]. A subsequent study evaluated this regimen in 62 patients with newly diagnosed malignant brain tumors. The EFS and OS rates at 3 years were 25% and 40%, respectively [58]. The most impressive responses were again noted in MB/PNET patients. Despite these promising responses, the toxicity associated with these regimens has been excessively high (5%–15% death rate). In an effort to reduce toxicity, more recent investigations have used multiple cycles of somewhat lower doses of chemotherapy followed by PBSC support. This has decreased transplant-related complications; however, the data relating to efficacy from ongoing trials are still premature.

Administration of intrathecal (IT) chemotherapy or coadministration of systemic chemotherapy with biologic agents that disrupt BBB permeability are alternative methods to increase CNS drug penetration and control leptomeningeal disease. The former method had been limited by the lack of available active agents that can be given by IT administration. The availability of topotecan and mafosfamide, a preactivated derivative of cyclophosphamide, has led to renewed interest in regional therapy. A European trial with IT mafosfamide (20 mg) and systemic chemotherapy for disseminated pediatric brain tumors demonstrated complete responses in eight of nine evaluable patients and, at a median follow-up of 21 months, 11 of 16 patients remained in complete or partial remission [59]. For the latter method, bradykinin agonists, such as lobradimil, which cause vasodilatation and leakiness of the BBB, have been utilized. This agent has been used in conjunction with systemic carboplatin for refractory CNS tumors.

Poorly oxygenated cells comprise a significant portion of the total tumor mass and are nearly three times less sensitive than well-oxygenated cells to the effects of RT. Investigations have thus focused on particles that are less dependent on oxygen for their effect, such as neutrons, or agents that enhance the effect of radiation-induced free radicals, such as platinum agents and halogenated pyrimidines. Topotecan and paclitaxel, members of the camptothecin and taxane classes of chemotherapeutic agents, respectively, are under investigation for their effects as radiosensitizers. Pediatric trials are also investigating gadolinium-texaphyrin, a porphyrin compound that produces long-lived free radicals, conjugated to gadolinium [60]. This conjugate forms a tumor-selective radiosensitizer that can be visualized by MRI.

The delivery and transfer of foreign genes into tumor cells, a process known as gene therapy, has broad implications for the treatment of neoplastic diseases. The postmitotic environment of the CNS may provide an advantage over other tissues in that it allows for the specific uptake of foreign genetic material into the genome of the rapidly dividing tumor. To date, one study has been completed and reported in pediatric CNS tumors. In this phase I study, 12 patients with recurrent malignant supratentorial tumors were multiply injected in the rim of the resection cavity with murine vector-producing cells shedding the retroviral vector containing the herpes simplex virus-1 thymidine kinase gene, and then treated with cytotoxic ganciclovir [61]. The procedure was well tolerated and future trials are planned.

The advent of STI571 (imatinib mesylate), an inhibitor of the bcr-abl fusion protein found in Philadelphia-chromosome-positive leukemias, ushered in a new paradigm for cancer treatment based upon the identification of molecular targets [62]. Following this model, investigation is under way to find molecular targets in MBs/PNETs. A number of promising compounds are just entering phase I clinical trials in pediatric patients, including tyrosine kinase inhibitors that impede growth factor signaling and farnesyl transferase inhibitors that block Ras activation.

It is unclear whether chemotherapy alone can induce durable responses in a significant proportion of patients. Three-dimensional (3-D) conformal RT is a technique that attempts to minimize neurotoxicity by integrating many beams, precisely directing RT to the desired site while leaving untargeted areas minimally exposed. The achievement of this goal depends upon precise localization of the tumor and normal critical structures by integrating CT or MRI with reproducible positioning of the patient. Intensity-modulated radiation therapy (IMRT) is a new conformal technique that makes use of 3-D-based treatment planning and nonuniform radiation beams. The beams are of greatest intensity within the tumor, sparing nearby critical structures. The high-dose treatment volume can then be made to conform to an irregular target. When compared with conventional RT, IMRT delivered 68% of the dose to the auditory apparatus (mean dose, 36.7 versus 54.2 Gy), while the overall incidence of ototoxicity was lower in the IMRT group [63].


   SUMMARY
Top
 Learning Objectives
 Abstract
 Introduction
 Medulloblastomas and Primitive...
 Atypical Teratoid/Rhabdoid...
 Novel Therapeutic Strategies for...
 Summary
References
 
The current treatment of pediatric embryonal CNS tumors continues to be very challenging and too frequently results in significant long-term sequelae in survivors. This is especially true for very young children, the most common age group diagnosed with these tumors, in which the effects of chemoradiotherapy on the developing neuraxis are greatest. Innovative delivery and decreased neurotoxicity of chemoradiotherapy are major directives for future clinical trials. It is also anticipated that the expanded use of molecular genetics will help to better stratify patients, tailor individual therapy, and aid in the development of targeted therapeutics.


   REFERENCES
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 Learning Objectives
 Abstract
 Introduction
 Medulloblastomas and Primitive...
 Atypical Teratoid/Rhabdoid...
 Novel Therapeutic Strategies for...
 Summary
 References
 

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Received October 21, 2002; accepted for publication January 14, 2003.




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