Medulloblastoma

Tumour less than 3 cm in diameter and limited to the classic midline position in the vermis, the roof of the fourth ventricle, and less frequently to the cerebellar hemispheres.

Tumour more than 3 cm in diameter, further invading one adjacent structure or partially filling the fourth ventricle.

May be subdivided into T3a and T3b.

Tumour further invading two adjacent structures or completely filling the fourth ventricle with extension into the aqueduct of Sylvius, foramen of Magendie, or foramen of Luschka, thus producing marked internal hydrocephalus

Tumour arising from the floor of the fourth ventricle or brain stem and filling the fourth ventricle.

Tumour further spreading through the aqueduct of Sylvius to involve the third ventricle or midbrain, or tumour extending to the upper cervical cord.

No evidence of gross subarachnoid or haematogenous metastasis.

Microscopic tumour cells found in cerebrospinal fluid.

Gross nodular seedings demonstrated in the cerebellar, cerebral subarachnoid space, or in the third or lateral ventricles

Gross nodular seeding in spinal subarachnoid space

Extraneuroaxial metastasis.