Assuming that the anatomy of the treatment site remains unchanged throughout treatment, the treatment plan is usually calculated once initially, and the same plan is used throughout the entire treatment. However, there may be daily changes in the shape and location of the tumour. For example, there may be shrinkage or growth in the tumour, differences in the filling of organs with anatomical spaces or cavities, movements of organs, changes in the patient's weight during treatment, or differences due to hypoxic changes that may occur in the tumour. These differences may cause changes in doses planned to be delivered to the tumour and the organ at risk. In conventional radiotherapy (RT), a safety margin was routinely defined around the target volume to account for anatomical changes. As a result, as the margins increased, the volume of normal tissue exposed to radiation increased and thus side effects increased.

In modern radiotherapy, there is increased emphasis on reducing the volume exposed to high radiotherapy doses and, consequently, reducing radiation-induced normal tissue toxicity. Currently, various techniques have been developed that accomplish these goals, but they all have their limitations. This chapter describes the concept of image guidance in radiotherapy, its available techniques and their expected benefits.