At the center of pediatric oncology in Copenhagen the investigators experience that the children with brain tumor, more or less have a healthy body with normal skeletal muscle mass and are physical active to the same level as their friends at the same age. The treatment period for brain tumor is approximately two years. After the treatment period, the children are more sedative with less interest in coming out doing physical activities and thus the investigators suspect that they have altered hormonal response, low skeletal muscle mass and perhaps are in risk of developing metabolic syndrome. By comparing children with newly diagnosed CNS tumor with children finished treated for CNS tumor, we wish to describe the metabolic path during the approximately two years treatment period these children go through. These results will also be compared with results from healthy controls. The investigators aim to include 10 children (aged 6-18 years) with newly diagnosed CNS tumor, 10 children (aged 6-18 years) finished treated for CNS tumor and 10 healthy controls (aged 6-18 years). By using stable isotope technique the investigators will investigate systemic fat, glucose and protein metabolism together with liver protein degradation and glucose production. Furthermore, by using DXA scan the investigators will describe the quality and distribution of skeletal muscle. Lastly, the investigators will determine the skeletal muscle signal pathway and metabolism in skeletal muscle via the Bergström biopsy technique in vastus lateralis.