Medulloblastoma is the most common paediatric brain tumour. It is a malignant embryonal tumour of the cerebellum with predominantly neuronal differentiation and tendency to metastasises via CSF pathways. The WHO classification of CNS tumours (2007) distinguishes classic medulloblastoma and four variants: desmoplastic/nodular (D/N), medulloblastoma with extensive nodularity (MBEN), anaplastic medulloblastoma and large cell medulloblastoma. Classic medulloblastoma consists of sheets of small cells with prominent nucleus and scant cytoplasm. Neoplastic cells of classic medulloblastoma might be elongated with oval nuclei and display moderate nuclear pleomorphism. Some classic tumours contain Homer Wright rosettes or palisades. The main histopathological features of D/N medulloblastoma and MBEN are nodules of differentiated neurocytic cells and internodular desmoplasia. Anaplasia in medulloblastoma is defined as marked nuclear pleomorphism, cell molding, cell wrapping, high mitotic activity and apoptosis. The large cell medulloblastoma contains groups of large cells with round nuclei with a single nucleolus. Because of the rarity of the pure large cell medulloblastoma and mixture of large cell and anaplastic phenotypes in some medulloblastoma, it is suggested to combine the two variants into a single category of large cell/anaplastic medulloblastoma. MBENs and D/N medulloblastomas in infants have better prognosis than classic tumours. Large cell/anaplastic medulloblastomas are more aggressive tumours.

medulloblastoma, classification, histopathological variants, embryonal tumours, PNET