Lithium Treatment to Prevent Cognitive Impairment After Brain Radiotherapy

Inclusion Criteria: * Age \>5 years. * Age \<18 years at time of radiotherapy. * Has received cranial/craniospinal radiation treatment of brain tumor within the last 7 years. * Adequate contraceptive method to prevent pregnancy\* during the entire lithium treatment period and six months thereafter. * Negative pregnancy test\* at screening, at start of study treatment, and monthly thereafter. * Written informed consent from patient and/or caregiver. Exclusion Criteria: * Allergy/hypersensitivity to lithium or any of the excipients * Renal failure (Cystatin C derived Glomerular Filtration Rate \< 60). * Cardiac failure or heart disease, including Brugada syndrome (or family history thereof). * Uncontrolled hypothyroidism. * Pregnancy or breast feeding. * Severe fluid or electrolyte imbalance. * Karnofsky-Lansky score \< 60. * Other condition deemed incompatible with inclusion in this study (estimated 2 year survival prognosis less than 25 %, expected poor protocol compliance, inability to swallow tablets, language difficulties). * Inclusion in other study protocol precluding inclusion in this study.