[Congenital medulloblastoma associated with intracranial arachnoid cyst].

Arachnoid cysts are very common lesions in paediatric patients, with treatment depending on their location and symptomatology. They are usually solitary cysts but may be associated with other central nervous system diseases such as tumours and congenital deformities. We describe the case of a neonate diagnosed with an arachnoid cyst of the quadrigeminal cistern treated by endoscopy. After the operation, the child's condition worsened; a CT scan revealed a midline posterior fossa tumour not visible in the preoperative neuroradiological tests. The tumour, a medulloblastoma, was partially removed. Given the child's age and the poor prognosis, oncological treatment was not undertaken. The association between medulloblastoma and arachnoid cyst is very rare, and we could find only one such case in the literature.