Central Nervous System Primitive Neuroectodermal Tumors (CNS PNET changed to ETMR in the new WHO classification)
CNS PNETs arise in the cerebral hemispheres, the brain stem, and the spinal cord  of children and mainly young adults. A variant of this clinically very aggressive entity according to a specific differentiation are cerebral neuroblastomas or ganglio- neuroblastomas, medulloepitheliomas, and ependymoblastomas. On imaging we see highly cellular tumors with a low T2 and restricted diffusion (Fig. 3.7a-c). Contrast enhancement is variable but more frequently sparse than intense or complete (Fig. 3.7d). It is often missing completely. Perifocal edema is mostly minor or absent (Fig. 3.7e), and in contradiction to a fast growth and malignant behavior the tumor seems to be surrounded by a capsule and usually is on imaging well delineated from the surrounding brain . Like MB also CNS PNET are prone to tumor dissemination in the leptomeninges prompting an MRI evaluation of the complete cranial and spinal CSF space.