**1. Introduction: medulloblastoma as a clinical problem**

Medulloblastoma is the most common malignant pediatric brain tumor, accounting for about 25% of pediatric brain tumor cases [1]. However, it is found in infants and adults as well. Arising from embryonal cells in the cerebellum, medulloblastoma is a highly invasive cancer which unfortunately manifests initially with subtle clinical symptoms [2]. While conventional treatments are able to control the tumor in the majority of patients, debilitating side effects along

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with drug resistance remain significant concerns. For the clinician, one of the challenges to treating medulloblastoma is its complexity as it may be grouped either histologically or molecularly. Currently, there are four molecular subgroups of medulloblastoma, each of which contains specific genetic or cytological backgrounds which may impact prognosis [3].

#### **1.1. Origin and epidemiology of medulloblastoma**

Medulloblastoma is classified as a primitive neuroectodermal tumor, typically occurring in the cerebellar vermis which is located in the posterior fossa of the skull (**Figure 1**) [1]. This tumor accounts for 40% of those arising from the posterior fossa [4]. Medulloblastoma is the most common malignant central nervous system (CNS) tumor of childhood, comprising 15– 30% of pediatric CNS tumors and 1–3% of adult CNS tumors [5]. Medulloblastomas can affect any age group, although the majority (85%) occurs in childhood, and of those half occur between the ages of 4–9 [6]. Tumors most often arise sporadically, although they may arise as part of inherited disorders such as Li-Fraumeni, Turcot, or Gorlin syndrome [7]. The cellular origins of medulloblastoma differ by the tumor subgroup (described below). For example, medulloblastomas of the sonic hedgehog (SHH) subgroup arise from granule neuron progenitors (GNPs) that reside in the outermost layer of the cerebellum [8]. Wnt-subgroup medulloblastomas, on the other hand, arise from lower rhombic lip precursors in the brain‐ stem [9].

**Figure 1.** Medulloblastoma is a primitive neuroectodermal tumor commonly arising in the cerebellar vermis. The left image is a sagittal view of an MRI for a pediatric patient. The right image is a horizontal view of an MRI showing tumor growth towards the right cerebellar hemisphere, with displacement of the vermis. Copyright © 2014 from *Ped‐ iatric medulloblastoma—update on molecular classification driving targeted therapies* (DeSouza, Jones, Lowis and Kurian, Front. Oncol. 2014).
