**5. Classification**

**4.2. Macroscopic pathology**

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**4.3. Microscopic pathology**

**4.4. Electron microscopic examination**

**4.5. Immunohistochemistry**

**4.6. Genetic aspects**

cells can be recognized on electron microscopic analysis [9].

positive for vimentin and synaptophysin staining [32].

Macroscopically these midline globular, soft tumors are fairly well demarcated, and are apparently encapsulated. The marked vascularity gives them a dark, dirty brownish-red hue and occasionally they have extensive areas of necrosis. Consistency may vary from suckable to rubbery or to even firm sometimes. Calcifications are seen occasionally while hemorrhage in medulloblastoma is an extremely rare occurrence. Often marked edema of a wide zone of the neighboring tissue is also seen. They overlie the fourth ventricle as they develop in the midline in the roof of the fourth ventricle to occupy it, but most of the floors of the fourth ventricle in the great majority of cases are typically free. They usually block the aqueduct of Sylvius, with significant dilatation. These have a tendency to invade the meninges. Commonly they grow downward through the foramen magnum from the primary location between the tonsils of the cerebellum [1–3, 9]. CSF dissemination into the subarachnoid spaces often lead to local or widespread leptomeningeal metastases in the spinal axis or over the surface of the cerebral hemispheres or cerebellum and this also may cause characteristic pearly gray sheets of tumor in the meninges. Furthermore,

widespread implantation of tumor can be found throughout the ventricles [9, 31].

Generally medulloblastomas are categorized as small blue-cell tumors, based on presence of their deeply basophilic nuclei [25]. Histologically, these tumors comprise of densely packed rounded or pear-shaped or sometimes spindle-shaped cells having large ovoid nuclei containing plentiful network of chromatin with scanty cytoplasm and for the most part consisting of numerous embryonic glia fibrillae [1, 3, 8]. These are highly cellular tumors forming uniform sheets of cells, interposed with occasional thin-walled blood vessels. Low magnification reveals that the cells appear as a loose structureless mass, but the nuclei sometimes may form pseudo-rosettes or may show a palisade arrangement. Extensive areas of necrosis, numerous hemorrhagic foci and great vascularity often come across too [1, 3, 9]. Thin-walled blood vessels with delicate connective tissue confined to their walls can be demonstrated by special staining with Perdrau's stain [9].

The electron microscopic picture is a mosaic of cells, processes, and fibers. The tumor cells are arranged in a tightly packed mass as also seen in light microscopy. Both the glial and neuronal

Immunohistochemistry can help in diagnosis of medulloblastoma and can provide information augmenting the plan for further management. Mitosis is seen in up to 80% of tumors, as assessed by positive staining with the Ki-67/MIB1 antibody. Medulloblastomas are frequently

With the advent of genetic profiling and molecular analysis, evolving evidences point to the fact that the different precursor cell populations that form the cerebellum are vulnerable to From the very beginning of history of this special type of tumor, these has been tried to be classified in different ways. With the advent and development of molecular biology and incorporation of that with genetic profiling has taken the classification beyond the level of mere histological classification. The molecular subgroupings are helpful in prediction of course of the disease and outcome as well as choosing therapeutic options. Recently the treatment plan is devised in accordance with the classification that integrates both histological and the molecular subgroupings to have the best possible outcome on the basis of a personalized treatment for individual patients.

#### **5.1. Histological classification**

Histopathological classification of medulloblastoma has evolved with time. Rubinstein and Northfield [34] initially identified three variants of medulloblastoma: pigmented papillary medulloblastoma, medullomyoblastoma, and desmoplastic medulloblastoma. With newer technologies, newer histological types were being introduced. To alleviate the confusions, the World Health Organization (WHO) in 1976 developed a common classification of brain tumors in an effort to combine the various systems in use till then and the WHO publication "Histological Typing of Tumours of the Central Nervous System" in 1979 outlined and illustrated the new classification system [30, 35, 36]. Since then modifications, additions and characterization of classification of medulloblastoma has evolved a lot throughout the newer classifications of medulloblastoma of the WHO editions of classifications gradually in 1993 [35], 2000 [37, 38] and 2007 [30]. The present histological classification that is in practice has the following types:


**4. Medulloblastoma with extensive nodularity (MBEN)**: Similar to nodular type but has an expanded lobular architecture with more prominent reticulin-free zones that are more elongated and rich in neutrophil-like tissue. Reticulin staining highlights internodular desmoplasia. Advanced neurocytic differentiation in islands with strong nuclear NeuN expression is also seen.

Two other histologic types of medulloblastomas are also recognized but are not considered to be distinct variants. They are the Myogenic differentiation type and the Melanotic medulloblastoma. All the histopathological types have significant predictability of prognoses in accordance with the clinico-pathological characteristics of each tumor type as outlined in the WHO classification [6, 8, 30, 34].

#### **5.2. Genetic classification and subgroups**

Research in the molecular study has led to understanding of embryonal tumors of the CNS. This reflection has basically been driven by genomic studies characterizing prevalent genetic profiles and biological features. These have led to tumor reclassification, sub-typing and detection of novel entities [10, 39, 40]. Over the last two decades, individual molecular subgroups have been identified for medulloblastoma based on cytogenetic profiles. Each group is named for the cellular pathway activation or genomic alterations it exhibits and each subgroup is associated with distinct prognostic character and survival outcomes [8]. In 2010, in Boston, an international group of medulloblastoma authorities, came to a consensus to categorize medulloblastomas into four distinct subgroups: Wingless (Wnt), Sonic hedgehog (Shh), Group 3, and Group 4 based on their unique set of demographic and clinical features, genetics, gene expression, genome-wide transcriptomic and DNA methylomic profiles [41, 42]. Prediction of outcome and clinical behavior is more precise with the known molecular subgroups than the histopathology or clinical staging. [6, 43–45]. These molecular subgroups are distinct from the histologic subtypes, albeit there are certain areas of considerable overlap (**Figure 1**).

*5.2.2. Sonic hedgehog pathway tumors (Shh pathway/Shh-MB)*

nearly 30% of medulloblastomas overall [42, 48, 50].

is fitting as the PTCH gene is located at chromosome 9q22 [42].

medulloblastomas [40].

The Shh group of medulloblastomas is named after the Sonic Hedgehog signaling pathway, which is thought to drive this type of tumor initiation [42]. Medulloblastomas categorized by activation of Shh signaling are heterogeneous and are linked with a variety of genetic aberrations and outcomes, unlike the Wnt-MB [50]. As a whole, Shh subgroup tumors comprises

**Figure 1.** WHO 2016 classification of medulloblastoma subtypes, characterized by genetic and histological features.

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Medulloblatomas of the Shh subgroup results from genetic predisposition of alteration of the sonic hedgehog pathway in the form of germline mutations in the patched-1 gene (PTCH1) or the suppressor of fused gene (SUFU). Loss of function PTCH1 and SUFU mutations lead to truncation of their associated protein products, leading to failure of their tumor suppressor effects and activation of Shh signaling with resultant tumorigenesis. Somatic mutations in PTCH1 and SUFU are also associated with sporadic medulloblastomas characterized by activation of the Shh pathway, along with PTCH2, MYC, SMO, GLI1 and GLI2 mutations [6, 42, 51, 52]. Deletion of chromosome 9q seems to be restricted to Shh medulloblastomas, which

Clinical outcomes of individuals with Shh-MB can be divided into favorable and poor survival groups depending on TP53 mutation status which is particularly critical [48]. Patients with Shh/TP53 mutant variety medulloblastomas have extremely poor outcome than those with Shh/TP53 wild-type tumors as mutant TP53 has been associated with high rate of anaplasia and MYCN amplification, which are equally disastrous cellular events [48, 53]. Because of its clinical impact, TP53 mutation status has been incorporated into the 2016 WHO classification for CNS tumors and is part of routine assessment in all Shh-activated

#### *5.2.1. Wingless pathway tumors (Wnt pathway/Wnt-MB)*

The Wingless pathway (also termed as the β-catenin pathway) comprises of secreted glycoproteins which are secreted to act through signal transduction to regulate various parts of embryonic development. Uncontrolled activation of Wnt pathway signaling results in accumulation of β-catenin, encoded by the CTNNB1 gene, leading to aberrant up regulation of transcription and ensues oncogenesis [46]. Wnt pathway tumors are the least common of the four molecular subgroups that represents merely about 10% of sporadic medulloblastomas [47]. Genetic features of this subgroup are characterized by the aberrations like monosomy 6, CTNNB1 mutations, and nuclear β-catenin positivity by immunohistochemistry [47]. This subgroup of medulloblastomas is more common in children and in adults than in infants. The outcomes of these medulloblastomas are outstanding and 5-year overall survival rate is 95% in children and 100% in adults [6, 10, 42]. TP53 mutations are invariably present in both the Wnt and Shh subgroup medulloblastomas [6, 42, 48]. As Wnt-MB has uniformly good prognosis, treatment regimen can be de-escalated with reduced dose of craniospinal radiation, reduced-intensity of chemotherapy, or a combination of both in patients without metastatic disease [49].

**Figure 1.** WHO 2016 classification of medulloblastoma subtypes, characterized by genetic and histological features.

## *5.2.2. Sonic hedgehog pathway tumors (Shh pathway/Shh-MB)*

**4. Medulloblastoma with extensive nodularity (MBEN)**: Similar to nodular type but has an expanded lobular architecture with more prominent reticulin-free zones that are more elongated and rich in neutrophil-like tissue. Reticulin staining highlights internodular desmoplasia. Advanced neurocytic differentiation in islands with strong nuclear NeuN

Two other histologic types of medulloblastomas are also recognized but are not considered to be distinct variants. They are the Myogenic differentiation type and the Melanotic medulloblastoma. All the histopathological types have significant predictability of prognoses in accordance with the clinico-pathological characteristics of each tumor type as outlined in the

Research in the molecular study has led to understanding of embryonal tumors of the CNS. This reflection has basically been driven by genomic studies characterizing prevalent genetic profiles and biological features. These have led to tumor reclassification, sub-typing and detection of novel entities [10, 39, 40]. Over the last two decades, individual molecular subgroups have been identified for medulloblastoma based on cytogenetic profiles. Each group is named for the cellular pathway activation or genomic alterations it exhibits and each subgroup is associated with distinct prognostic character and survival outcomes [8]. In 2010, in Boston, an international group of medulloblastoma authorities, came to a consensus to categorize medulloblastomas into four distinct subgroups: Wingless (Wnt), Sonic hedgehog (Shh), Group 3, and Group 4 based on their unique set of demographic and clinical features, genetics, gene expression, genome-wide transcriptomic and DNA methylomic profiles [41, 42]. Prediction of outcome and clinical behavior is more precise with the known molecular subgroups than the histopathology or clinical staging. [6, 43–45]. These molecular subgroups are distinct from the

histologic subtypes, albeit there are certain areas of considerable overlap (**Figure 1**).

chemotherapy, or a combination of both in patients without metastatic disease [49].

The Wingless pathway (also termed as the β-catenin pathway) comprises of secreted glycoproteins which are secreted to act through signal transduction to regulate various parts of embryonic development. Uncontrolled activation of Wnt pathway signaling results in accumulation of β-catenin, encoded by the CTNNB1 gene, leading to aberrant up regulation of transcription and ensues oncogenesis [46]. Wnt pathway tumors are the least common of the four molecular subgroups that represents merely about 10% of sporadic medulloblastomas [47]. Genetic features of this subgroup are characterized by the aberrations like monosomy 6, CTNNB1 mutations, and nuclear β-catenin positivity by immunohistochemistry [47]. This subgroup of medulloblastomas is more common in children and in adults than in infants. The outcomes of these medulloblastomas are outstanding and 5-year overall survival rate is 95% in children and 100% in adults [6, 10, 42]. TP53 mutations are invariably present in both the Wnt and Shh subgroup medulloblastomas [6, 42, 48]. As Wnt-MB has uniformly good prognosis, treatment regimen can be de-escalated with reduced dose of craniospinal radiation, reduced-intensity of

expression is also seen.

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WHO classification [6, 8, 30, 34].

**5.2. Genetic classification and subgroups**

*5.2.1. Wingless pathway tumors (Wnt pathway/Wnt-MB)*

The Shh group of medulloblastomas is named after the Sonic Hedgehog signaling pathway, which is thought to drive this type of tumor initiation [42]. Medulloblastomas categorized by activation of Shh signaling are heterogeneous and are linked with a variety of genetic aberrations and outcomes, unlike the Wnt-MB [50]. As a whole, Shh subgroup tumors comprises nearly 30% of medulloblastomas overall [42, 48, 50].

Medulloblatomas of the Shh subgroup results from genetic predisposition of alteration of the sonic hedgehog pathway in the form of germline mutations in the patched-1 gene (PTCH1) or the suppressor of fused gene (SUFU). Loss of function PTCH1 and SUFU mutations lead to truncation of their associated protein products, leading to failure of their tumor suppressor effects and activation of Shh signaling with resultant tumorigenesis. Somatic mutations in PTCH1 and SUFU are also associated with sporadic medulloblastomas characterized by activation of the Shh pathway, along with PTCH2, MYC, SMO, GLI1 and GLI2 mutations [6, 42, 51, 52]. Deletion of chromosome 9q seems to be restricted to Shh medulloblastomas, which is fitting as the PTCH gene is located at chromosome 9q22 [42].

Clinical outcomes of individuals with Shh-MB can be divided into favorable and poor survival groups depending on TP53 mutation status which is particularly critical [48]. Patients with Shh/TP53 mutant variety medulloblastomas have extremely poor outcome than those with Shh/TP53 wild-type tumors as mutant TP53 has been associated with high rate of anaplasia and MYCN amplification, which are equally disastrous cellular events [48, 53]. Because of its clinical impact, TP53 mutation status has been incorporated into the 2016 WHO classification for CNS tumors and is part of routine assessment in all Shh-activated medulloblastomas [40].

Survival for Shh subgroup medulloblastomas is similar to Group 4 medulloblastomas and intermediate between that of Wnt and Group 3 medulloblastomas and varies significantly based on age and histologic subtype [10, 42, 48].

information has considerably transformed the diagnostic work-up and reporting of tumors of the CNS. Nonetheless, this remains perplexing in embryonal tumors due to their de novo tumor heterogeneity that is being faced [10]. This integrated diagnosis is presented in a layered format; that includes the histological diagnosis, WHO grade, molecular genetic information and ultimately the integrated diagnosis. The value of this approach is clearly illustrated in embryonal tumors, in particular medulloblastoma, where the combination of molecular and histological data provides discrete diagnostic information [40]. In this new concept of a "layered diagnosis" brain tumors that are diagnosed purely morphologically are incorporated with molecular characteristics for an "integrated diagnosis" at the peak diagnostic level [60]. In the layered integrated classification, a patient would be labeled as having medulloblastoma

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of a definite histological subtype, the WHO grade and the molecular subgroup [61].

have long term survival rates of approximately 85% and 70%, respectively [6].

There is no well-established staging system for medulloblastoma for prediction from which treatment plan and outcome prediction can be made. In 1969, Chang et al. suggested an operative staging system for medulloblastomas adapting the TNM classification for other tumors [62]. According to the size and extent of primary tumor, T category was divided into four main groups with subdivisions of T3 (T1, T2, T3a, T3b or T4) and the M category had five groups (M0, M1, M2, M3 and M4) based on the degree of tumor spread in the CSF pathway or extra-CNS metastases [9]. Overall, M staging has remained more useful in prognostic evaluation, while the T staging is less valuable as a prognostic indicator [63]. Several studies have stratified the patients into "high-risk" and "average" or "standard-risk" groups depending on age of the patient, residual disease after surgery, pathologic variant, and M staging [8, 64]. This risk stratification is a good predictor of outcome. Standard-risk and high-risk categories

Because of its origin in the posterior fossa, the presenting symptoms of medulloblastoma are often vague complaints and understandably the diagnosis may be delayed. Presentation depends on various elements of the tumor subject to location, size, duration, compression on the surrounding structures. As these arise from the vicinity of cerebellum and brainstem, often the first feature to appear is instability of gait. Being a midline posterior fossa lesion, trunkal ataxia appears first and appendicular ataxia gradually ensues as the tumor grows bigger to compress the cerebellar hemispheres, and other common cerebellar signs follow with time. When the tumor is big enough to compress the brainstem, long tract signs begin to appear and add more difficulty in movement of the patients. As the tumor grows further, especially downwards, the lower cranial nerves start to get involved and lower cranial nerve palsies manifest. If the tumor grows bigger to occupy and block the Aqueduct of Sylvius, hydrocephalus ensues. Hydrocephalus may also result from blockage of the fourth ventricular outlets, by compression, by the growing tumor, individually or in combination. Hydrocephalus in turn may lead to features of raised Intracranial Pressure (ICP) resulting in

**6. Staging**

**7. Presentation**

#### *5.2.3. Non-Wnt/Shh tumors*

The non-Wnt/Shh tumor subgroup encompasses Group 3 and Group 4, in which the underlying genetic predisposition of mutations have yet not been recognized [49]. These types are associated with a higher incidence of tumor dissemination and approximately 30% of patients have metastasis at the time of diagnosis [6]. Despite these similarities, features of Group 3 and Group 4 subgroup medulloblastomas vary regarding the demographic, clinical, transcriptional, and genetic differences between them and these advocate that they are actually distinct entities with molecular diversity [42, 54].

#### *5.2.3.1. Group 3 MB*

Nearly 30% of medulloblastomas are of the Group 3 subgroup and histologically mostly are of the "classic" variety of medulloblastomas [6, 55–57]. These tumors genetically are more likely to have high-level expression and amplification of MYC, MYCN and OTX2 with imbalance of chromosome 17 [42, 49, 51, 58]. Group 3 tumors show gain of chromosome 1q, and/ or loss of chromosome 5q and chromosome 10q more than Group 4 tumors [42]. They occur more frequently in infants and children, and has the worst outcome among all molecular subgroups, with 10-year overall survival of 39% in infants and 50% in children [6, 42, 49, 51, 59].

#### *5.2.3.2. Group 4 MB*

About 35% of medulloblastomas overall are of Group 4 subgroup and the peak incidence for this subgroup is in late childhood and early adolescents [6, 42]. The prognosis of Group 4 tumors is similar to Shh subgroups tumors and is intermediate between those of Wnt and Group3 subgroup tumors. Patients with metastatic disease or MYC amplifications in this Group have significantly poorer outcomes [49, 50]. Though isochromosome 17q is also seen in Group 3 tumors, it is more common in Group 4 tumors. Another prominent cytogenetic alteration among Group 4 tumors is loss of the X chromosome, which is found in 80% of females with Group 4 medulloblastoma [42].

Recently, the discovery of seven novel molecular subgroups has permitted to categorize patients further to envisage better disease subclassification and outcome predictions. These subgroup dependent grouping stratified patients into four clinical risk groups for 5-year progression-free survival: favorable risk (91% survival); standard risk (81% survival); highrisk (42% survival); and very high-risk (28% survival) [41]. Another study recommended total of 12 subgroups based on the integration of transcriptomic and methylation data. In that analysis medulloblastoma encompasses 12 subtypes; 2 Wnt, 4 Shh, 3 Group 3, and 3 Group 4 groups. These subtypes of each subgroup are clinically and biologically pertinent [54].

#### **5.3. Integrated classification**

In 2016, for the first time, the molecular characteristics was incorporated into the WHO classification for the diagnosis of CNS tumors. This integration of histological features with genetic information has considerably transformed the diagnostic work-up and reporting of tumors of the CNS. Nonetheless, this remains perplexing in embryonal tumors due to their de novo tumor heterogeneity that is being faced [10]. This integrated diagnosis is presented in a layered format; that includes the histological diagnosis, WHO grade, molecular genetic information and ultimately the integrated diagnosis. The value of this approach is clearly illustrated in embryonal tumors, in particular medulloblastoma, where the combination of molecular and histological data provides discrete diagnostic information [40]. In this new concept of a "layered diagnosis" brain tumors that are diagnosed purely morphologically are incorporated with molecular characteristics for an "integrated diagnosis" at the peak diagnostic level [60]. In the layered integrated classification, a patient would be labeled as having medulloblastoma of a definite histological subtype, the WHO grade and the molecular subgroup [61].
