Medulloblastoma: Thus far you have seen three types of posterior fossa tumors, all of them most common in childhood. These were cerebellar astrocytomas, brain stem gliomas (usually astrocytomas) and fourth ventricle ependymomas. Another important posterior fossa tumor of childhood is the medulloblastoma. In almost all cases this arises from the vernal portion of the cerebellum. It is a rapidly growing tumor, and nearly always produces some degree of obstruction of the fourth ventricle. In this slide the tumor nearly obliterates the fourth ventricle, and contains areas of small to medium-size hemorrhage. Note the infiltrative character of the tumor and the difficulty in demarcating its outline. It appears to extend downward into the medulla just caudal to the fourth ventricle. The third ventricle and aqueduct are enlarged.