Medulloblastoma is the most common malignant brain tumour in childhood and accounts for 15–20% of central nervous system (CNS) neoplasms. It was first described in 1925 by Bailey and Cushing as a tumour of primitive origin arising in the posterior fossa of young children. Medulloblastoma has strong common clinical characteristics with histologically similar tumours occurring elsewhere in the CNS, leading to the concept of primitive neuroectodermal tumours (PNETs) in 1973.1 This term included both medulloblastoma (80% of PNETs) and tumours occurring in the supratentorial region of the brain. Although the concept of PNETs was generally accepted, there is growing doubt as to whether PNETs arise from a common cell of origin, with increasing evidence of differences between the molecular characteristics of medulloblastoma and supratentorial PNETs.2 In this respect, the term medulloblastoma describing a distinct entity is generally preferred to posterior fossa PNET.

The large majority of medulloblastomas occur within the first decade of life with a peak incidence between 4 and 7 years of age. However, cases in adolescence and indeed adulthood are well described. There is an approximately 1.7:1 male to female ratio. There is a suggestion that the incidence of PNETs in persons under 20 years of age is increasing to now lie at around five per 106 person years.3 This equates to around 90 patients diagnosed with medulloblastoma in the UK each year.

By definition, a medulloblastoma arises in the posterior fossa, usually from the cerebellar vermis in the roof of the fourth ventricle, thus presenting as a midline tumour (fig 1). A smaller proportion of cases arise in a cerebellar hemisphere, particularly in older children. Medulloblastomas have a marked propensity to seed within the cerebrospinal fluid (CSF) pathways, with evidence of such metastatic spread occurring in up to 35% of cases at …