Medulloblastomas are Primitive NeuroEctodermal Tumors (PNETs) that occur in the posterior fossa. They arise below the tentorium and are referred to as "infratentorial PNETs.
PNETs that occur elsewhere in the brain - above the tentorium are called "Supratentorial Primitive Neuro-Ectodermal Tumors" or supratentorial PNETs.
These tumors have a worse outlook than medulloblastomas and are always placed in a high risk category - even if there is no metastatic disease at presentation.
A primitive neuroectodermal tumor (PNET), is derived from neuroepithelial stem cells, cells that are arrested in different stages of differentiation.
Incidence
Supratentorial PNETs are very rare - only 2.5 to 3% of all childhood brain tumors.
Very occasionally this tumor can occur in adults.
This tumor is more common in younger children (especially in children less than 5 years old).
Supratentorial PNETs are also slighly more common in males.
Table: Summary of Facts about Medulloblastoma Incidence
Proportion of pediatric brain tumors |
15-20% |
Common age of onset |
4-6 |
% of patients less than 2 years old |
20% |
% of patients less than 15 years old |
80% |
Gender bias |
3:2 male predominance |
Racial bias |
None |
