PNET (primitive neuroectodermal tumor) is a name used for tumors which appear identical under the microscope to medulloblastoma, but occur primarily in the cerebrum. PNET is used by some to refer to tumors such as the pineoblastoma, polar spongioblastoma, medulloblastoma, and medulloepithelioma. Except for medulloblastoma, these are all very rare tumors.
PNETs contain underdeveloped brain cells, are highly malignant, and tend to spread throughout the central nervous system. These tumors often contain areas of dead tumor cells (necrosis) and cysts. Fluid surrounding the tumor is not uncommon.