Primitive Neuroectodermal tumours (PNET) usually occur in the brain, although they arise out of embryonal tissue (cells of the developing fetus still in the womb). PNETs mostly affect young children.
They tend to be fairly large tumours, putting pressure on the brain and causing seizures, among other symptoms.
Primary treatment for PNETs is surgery, but since these tumours tend to spread and have a large blood supply, total removal is often not possible. In these cases, radiotherapy and chemotherapy might also be used.
Information taken from National Cancer Institute.