IGRT: Image guidance in radiation oncology. A review from planar x-ray based positioning to cone beam CT

In most clinical situations, the purpose of radiotherapy is to apply high doses to the target volume as well as the protection of organs at risk from the side effects of radiation. For biological reasons, radiation is usually administered as a fractionated therapy, which requires reproducibly precise immobilisation of the patient and positioning of the radiation isocenter in the correct location in relation to tumor geometry before each treatment. This allows a maximum reduction in the "safety margin" around the target volume. Positioning for extracranial regions is problematic because of the movement of the tumor in relation to bony structures, e.g. due to breathing or organ filling. This uncertainty leads to difficulties in establishing high precision techniques such as intensity modulated radiation therapy in the extracranial regions. This overview discusses image guided radiotherapy (IGRT) techniques. Immobilisation strategies (breath holding, gating) and position verification techniques based on 2D image-guidance (port films, EPID), 2D-3D guidance (bidirectional EPID localization in combination with implanted fiducial markers and optional supplementary infrared positioning) as well true 3D image guidance possibilities such as ultrasound-based positioning systems and in-room CTs are discussed. © Springer Medizin Verlag 2006.