Radiosurgery performed with a Gamma Knife or on a linac facility is a specific radiotherapy method to treat certain lesions in the brain with a high single radiation dose. Geometric miss of a radiosurgica target is considered the most likely serious error in a steroetactic radiosurgery treatment. Quality assurance in this field, therefore, is of the utmost importance. This book is designed to offer and describe a quality assurance programme. It identifies the sources of possible errors and suggests quality assurance tests that should be performed in order for a stereotactic system to be used clinically.