Frameless Image Guidance in Stereotactic Radiosurgery

Stereotactic accuracy and target engagement largely rely on rigid immobilization of the skull. However, advanced imaging, image reconstructions, and adaptive treatment algorithms have made frameless image guidance without rigid immobilization a mainstay of stereotactic radiosurgery and radiotherapy deliveries. While these methods for achieving stereotactic precision require some immobilization, these obviate the need for an invasive head ring fixation. Because these approaches allow the target to still move to some degree and this movement must be compensated for, frameless image guidance requires intra-treatment imaging and target tracking and, critically, requires verification of accuracy. In this chapter, the different approaches are described, including key technological features of various frameless image-guided approaches. The anticipation is that these methods, which are primarily used for stereotactic radiosurgery, will continue to evolve to incorporate other imaging modalities, such as MRI, and eventually be generalized to other stereotactic procedures.