First described by Bailey and Cushing in 1925 [1], MB is a rare embryonic tumor of the posterior cranial fossa, grade 4 assigned by WHO classification, arising from the granule cell precursors in the external germinal layer of the developing cerebellum.

Tumor growth starts in the fourth ventricle and has a propensity to spread to the cerebellar vermis and the brainstem, seeding the craniospinal axis through the subarachnoid system (the so-called “drop metastases”) ^[2][3][2,3].

Examining data from the Surveillance, Epidemiology, and End-Results (SEER) database from 1973 through 2007, MB is the second most common of all pediatric central nervous system (CNS) tumors (accounting for about 20% of the total CNS tumors in the pediatric age group), with a bimodal peak at age 3 to 4 years and at age 8 to 10 years ^[3][4][3,4]. Age of MB onset is variable, from birth to adolescence, but the incidence rate dramatically declines after age 15 [5].

In adult neuro-oncology practice, therefore, MB is a very rare condition, representing only 0.4–1% of all adult CNS tumors, with an incidence of 0.6–1 case per million per year [4]: this population, which is the topic of this resviearchw and for readability purposes will be called “adult MB”, is an “orphan” entity, being that the main therapeutic protocols were extrapolated from experiences in children, while prospective randomized clinical trials specifically designed for adult patients are very rare ^{[5][6][7][8][9][10]}[5,6,7,8,9,10].

Current standard treatment involves a multimodal treatment comprising surgical resection followed by radio-chemotherapy, selected and adapted to patients on the basis of risk stratification [7]. This interdisciplinary management can achieve a 5-year survival rate between 50% to 90% ^{[10][11][12][13][14][15]}[10,11,12,13,14,15].

However, regardless of long-term survival, treatment-related adverse effects, including neurological toxicity, hematological toxicity, polyneuropathies, endocrinopathies, and cognitive disorders remain a relevant issue, involving up to 80% of patients, thus representing a strong incitement for the search for more innovative and less impactful therapeutic strategies [9].

MB is a heterogeneous disease, both clinically and molecularly. Current MB classification, according to the fourth (2016) and fifth (2021) edition of the WHO Classification of Tumors of the Central Nervous System (CNS) ^[16][17][16,17], includes both histological and molecular criteria, defining four molecular types (wingless-type (WNT)-activated, Sonic Hedgehog (SHH) activated TP53-wildtype, SHH-activated and TP53-mutant, and non-WNT/non-SHH) and four histological patterns (classic, desmoplastic/nodular, MB with extensive nodularity, and large cell/anaplastic).

The diverse molecular types show significantly different age, molecular and clinical profiles ^{[17][18][19][20][21][22][23][24][25][26][27]}[17,18,19,20,21,22,23,24,25,26,27]. WNT MB harbors deregulation of the WNT pathway, mostly caused by activating somatic mutations in the CTNNB1 gene, and exhibits the most favorable prognosis across all the subgroups, rarely being metastatic ^{[17][18][19][20][21][22]}[17,18,19,20,21,22]. It is characterized by a high tendency to bleed and intralesional hemorrhage. SHH-activated MB is the most common subgroup in the adult population, accounting about 60% of adult MBs, with a 5-year overall survival (OS) rate of about 50–70% in the absence of TP53 mutation ^{[22][23][24][25][26][27]}[22,23,24,25,26,27].

The clinically meaningful contribution provided by the latest editions of the WHO classification lies in having finally opened the way to the development of molecularly adapted treatment strategies for MB patients.

As a result, the landscape of clinical trials has significantly changed since the discovery of distinct molecular types, and multiple targeted agents are currently under investigation. SHH-activated adult MBs, particularly, have become a very appealing focus in research for the preliminary evidence of activity of the SHH-inhibitors vismodegib and sonidegib ^{[28][29][30][31][32][33]}[28,29,30,31,32,33]. These small molecules are effective against MBs carrying SHH mutations, that in adults are at the level of SMO or upstream the SMO [34], and are free from one of the most feared side effects, the premature growth plate fusion, in the adult population.

The challenges reswearchers encounter in clinical practice suggest that there are two primary research objectives for adult MB: first, to design clinical trials specifically stratified by age and tumor biology, to develop rationally personalized treatments; second, to identify new experimental agents with improved acute and chronic toxicity profiles.

2. Molecular Genetics

While histopathological MB features are nearly indistinguishable from those observed in children, the distribution of molecular types is markedly different, with SHH-activated MB representing by far the most common molecular group in adults [35]. The recently released 2021 WHO Classification of Central Nervous System (CNS) Tumours [17] classifies MB primarily according to molecular features, recognizing histopathological patterns of clinical utility and thus promoting an integrated molecular and histopathological diagnostic approach. Four main genetically defined types, including SHH-activated TP53-wildtype, SHH-activated TP53-mutant, WNT-activated and non-WNT/non-SHH MB (including group 3, 4 MB) ^[17][36][17,36] are identified, which allow for an accurate risk stratification (Table 1, Table 2 and Table 3). Goschzik T et al. [37] reported a 5-year event free survival (EFS) of 60% and 5-year overall survival (OS) of 75.6%, in a cohort of 117 adult MBs. For SHH-TP53mut-MBs, EFS and OS were poor, whereas all other types had an outcome similar to published standard-risk patients with a 5-year EFS/OS between 60% and 80% each. No significant survival differences in log-rank tests were reported between WNT-MBs, SHH-TP53wt-MBs, and non-WNT/non-SHH-MBs (Table 1).

2.1. Medulloblastoma Histologically Defined

In absence of molecular information, MB can be diagnosed based on morphological features as MB histologically defined. Histopathological subtypes of MB include: classic, desmoplastic/nodular (D/N), MB with extensive nodularity (MBEN), and large cell/anaplastic (LC/A). Classic MB is a blue cell tumor without distinctive architectural features composed of cells with high NC ratio, a variable degree of nuclear anaplasia from slight to moderate and the frequent presence of Homer–Wright rosettes. Some cases show evidence of neuronal/neurocytic differentiation with the presence of nodules which, however, lack perinodular reticulin deposition and internodular desmoplasia. These features are typical of classic rather than D/N MB. Nodular/desmoplastic MB is characterized by the presence of varying degrees of neuronal differentiation, typically with a nodular appearance confirmed by the presence of a perinodular reticulin stain. The differentiating nodules characteristically show intense synaptophysin positivity and limited proliferation/Ki67 labeling. D/N features can be focal, and MB with extensive nodularity, as described in very young children, is not seen in adults. Large cell/anaplastic MB is characterized by severe nuclear anaplasia and/or large cell appearance, i.e., cells with large roundish nuclei and prominent nucleoli. Synaptophysin stain is diffusely positive. Rare MB examples show evidence of melanotic or myogenic differentiation at times easily identifiable on H&E slides, sometimes exclusively by appropriate immunohistochemical markers. Mitotic activity is variable but generally high in the poorly differentiated components of the tumor. Prominent cell apoptosis and necrosis are frequently present. By immunohistochemical stains, MB cells are typically synaptophysin positive and may express to some extent GFAP, highlighting cells with scant cytoplasm, while OLIG2 stain is negative or at most only focally positive. Vimentin stain is typically negative. A markedly infiltrative pattern of growth can be seen in MB especially in adults and may make it difficult to distinguish MB from a high grade diffusely infiltrating glioma occurring in the cerebellum. In adults, especially older adults, differentiating MB from metastatic poorly differentiated/small cell carcinoma to the cerebellum is also critical. MB does not express cytokeratin and TTF1, markers typically expressed in metastatic poorly differentiated/small cell carcinoma. Surrogate immunohistochemical markers are frequently used in clinical practice to predict molecular groups. In theiour experience, an immunohistochemical panel including YAP1, GAB1 and beta-catenin, simplified from the original panel described by Ellison et al., which also included filamin stain, can reliably identify in most cases MB SHH-activated (as well as distinguish TP53-wildtype and mutant MB with the addition of p53 immunostaining), WNT-activated, non-WNT/non-SHH MB. SHH-activated MB is typically GAB1 and YAP1 positive, with a characteristic pattern of expression in the primitive/poorly differentiated components and absent stain in differentiating nodules. Beta-catenin expression is cytoplasmic. WNT-activated tumors typically show nuclear expression of beta-catenin (at least focally), expression of YAP1, but not GAB1. Lymphoid-enhancing factor 1 (LEF1), a transcription factor mediating WNT/β-catenin signaling, is overexpressed in WNT-activated MB and can further solidify the diagnosis [38]. Non-WNT non-SHH MB are negative for YAP1 and GAB1 and show cytoplasmic expression of beta-catenin. Immunostaining for p53 protein is generally sufficient to assess TP53 mutational status of most SHH-activated MB, and TP 53 mutation results in strong and diffuse expression in most tumors. Additional markers have been proposed, including p75-NGFR and OTX2, to help with assignment to a specific MB entity and exclude other mimic histological tumors [17]. There are two major limitations of the immunohistochemical panel in “molecularly” classifying MB: (1) the inability to distinguish between groups 3 and 4, which is, however, not relevant in adults; (2) the occurrence of inconclusive immunohistochemical results, especially in rare histopathological subtypes (such as MB showing melanocytic/rhabdomyoblastic differentiation). Molecular analysis may, therefore, be critical in reaching a definitive diagnosis.

2.2. Medulloblastoma Molecularly Defined

The histopathological patterns have characteristic and specific associations with molecular subgroups. For instance, all morphologically defined nodular/desmoplastic MB correspond to SHH-activated MB and most WNT-activated MB have classic morphology. There is, however, no 100% correspondence between histopathology and molecular groups. Molecular classification of MB, including WNT-activated, SHH-activated and TP53-wildtype, SHH-activated and TP53-mutant, non-WNT/non-SHH MB groups, can be achieved using a variety of molecular techniques, including methods based on mRNA such as NanoString and RNA-Seq ^{[39][40][41][42][43]}[39,40,41,42,43], or based on DNA analysis such as copy number variations, epigenetic classification by whole-genome methylation array ^[44][45][44,45]. To date, whole-genome methylation profiling is considered the gold standard diagnostic method, has been obtained mainly in children while these methylation data in adults is limited ^[41][42][45][41,42,45]. Comparative studies are being added to MB clinical trials in adults to further define the optimal diagnostic approach [35]. MB molecularly defined in adults ^{[11][24][46][47]}[11,24,46,47] shows a predominance of SHH-MB, typically TP53-wildtype [37], while SHH TP53-mutant MB cases seem to occur predominantly in children and adolescents. The frequency of WNT-MB is similar to that published for children (15% vs. 10%), and non-WNT/non-SHH-MB is relatively rare (2.6%) in adults compared to childhood MB (25%) ^[37][46][47][37,46,47]. The group of non-WNT/non-SHH-MBs in adults nearly consists of Group 4 tumors only ^[11][24][48][11,24,48]. Molecular groups in adult MB are summarized in Table 1 according to their mutational pattern, copy number variations and global methylation profile. Each group is characterized by distinct clinicopathological and molecular features.

2.3. Medulloblastoma SHH-Activated and TP53-Wild Type

SHH-activated TP53-wildtype MB represents the most frequent type [11], accounting for approximately 60% of MB in adults [37]. While SHH activation is, predominantly, associated with pathogenic mutations of Smoothened (SMO) in adults, Patched (PTCH1) mutations can be found in all age groups. Other frequent genetic alterations have been reported, including ELP11, DDX3X, and KMT2D mutations. PPM1D amplification has also been described as a driving molecular event in adult SHH-activated TP53-wildtype MB [49]. Copy number variations. involving 9q loss and 9p gain, are the most frequent abnormalities observed in this group [17]. The majority of SHH-MB TP53-wildtype tumors show desmoplastic/nodular histological patterns, which may also be present in focal areas only, while MB with extensive nodularity is not observed in adults. Less frequently, SHH-MB TP53-wildtype has classic histology.

2.4. Medulloblastoma SHH-Activated TP53-Mutant

TP53 mutation is rare (2.6%) in adult SHH-MB [37], but may occur de novo in recurrent tumors. TP53 gene sequencing is recommended in all SHH-MB by the World Health Organization [17]. Most instances of SHH-MB TP53-mutant in adults show somatic TERT promoter, DDX3X and U1 snRNA mutations [34]. Amplification of MYCN and GLI2 has been reported. In adults, MYCN amplification and TP53 mutation tend to occur together and are associated with a poor prognosis [17]. Loss of several chromosomes such as 17p, 3p, 10q, 14 q and gain of 3q has been reported in SHH-activated TP53-mutant MB. DNA methylation profile aligned with a SHH-activated MB, either TP53-wildtype or TP53 mutant, is considered the gold standard method for determining MB group status in children. Four molecular subgroups of SHH-activated MB are emerging ^{[23][37][44][50][51][52][53]}[23,37,44,50,51,52,53] (Table 1). A consensus regarding these SHH subgroups and their defining features has not yet been reached through an international cooperative meta-analysis. Of the four proposed subgroups, the main subgroup occurring in adults is SHH-4, which is associated with near-universal U1 and TERT mutations and frequent somatic PTCH1 or SMO alterations [54]. TERT promoter mutations have been reported in most adult MBs, but only rarely in childhood MB ^[34][46][55][34,46,55]. SHH-3 is the other subgroup that may arise in adult patients and is associated with TP53 and ELP1 mutations [54]. The last two molecular SHH-subgroups occur mainly in young children: one (SHH-1) is enriched with somatic and germline suppressor of fused (SUFU) mutations and chromosome 2 gain, and the other (SHH-2) is characterized by 9q loss and DNMB morphology.

2.5. Medulloblastoma Non-WNT/Non-SHH-MB

Non-WNT/non-SHH-MB represent 22.2% of adult MBs and are classified as group 3 and group 4 [37]. Group 3 is exceedingly rare in adults and the majority of adult Non-WNT/non-SHH-MB cases belong to subgroup 4 ^[11][21][24][11,21,24]. The most frequent genetic alterations in groups 3 and 4 involve KDM6A, OTX2, ZMYM3, KMT2D, TBR1 and PRDM6. Aberrant overexpression of PRDM6 is specific to group 4 MB. Cytogenetically, isochromosome 17q is present in most cases. In addition, some cases show amplifications such as CDK6, while amplifications of MYC or MYCN are rare in adults. DNA methylation profiles of non-WNT/non-SHH-MB align with group 3 and group 4 MB, among which eight molecular subgroups have recently been identified and classified in a spectrum of group 3/group 4 MBs by DNA methylation profiling ^[23][37][50][23,37,50] (Table 1). Based on analysis of DNA methylation and transcriptomic data, these molecularly heterogeneous subgroups among group 3 and group 4 MBs were identified with distinct clinical and genetic associations [56]. In an international meta-analysis using DNA methylation and transcriptome data, Sharma et al. [57] found eight robust subgroups within MB group 3/4. Group 3 MBs consist only of subgroups 2, 3, and 4, whereas group 4 MBs include uppermost subgroups 6, 7, and 8. Subgroups 1 and 5 are intermediate subgroups, showing the molecular and cellular features of both group 3 and group 4 MBs [56]. Classic morphology is observed in most non-WNT/non-SHH MBs, but subgroup 2 are more frequent large cell/anaplastic tumors. Metastatic disease at presentation is relatively frequent in subgroups 2–5. A rather poor outcome is associated with tumors in subgroups 2 and 3 where MYC/MYCN amplification are identified.

2.6. Medulloblastoma WNT-Activated

MB WNT-activated is the least common of the molecular groups in adults. In a recent study, 14.5% of adult MBs show WNT activation, mostly caused by activating somatic mutations in the CTNNB1 gene [37]. For a definite diagnosis of WNT-activated MBs sequencing is recommended to confirm the presence of a CTNNB1 mutation. Inactivating mutations in APC genes are rare and might suggest germline mutations (familial polyposis coli) [17]. As both TERT and IDH1 mutations are genetic hallmark events in subgroups of gliomas, their molecular presence alone, without integration with the histological and immunohistochemical data, does not permit the differential diagnosis between malignant glioma and MB in cerebellum. Monosomy of 6 is found in >80% of WNT-activated MB, cytogenetically characterizing this group, especially in children [17]. The DNA methylation profile aligned with WNT-activated MB. Two molecular subgroups of WNT-activated MB have been proposed, WNT alfa and WNT beta [17]. WNT alfa MB is characterized by a monosomy of 6 and arises in children. WNT beta MB is diploid for chromosome 6 and arises in older children and young adolescents. The majority of WNT-MB shows classic histology.