Image-guided radiation therapy (IGRT) is the process of frequent two and three-dimensional imaging, during a course of radiation treatment, used to direct radiation therapy utilising the imaging coordinates of the actual radiation treatment plan. Image-guided radiation therapy (IGRT) uses advanced imaging technology to better define the tumour target and is the key to reducing and ultimately eliminating the uncertainties. The patient is localised in the treatment room in the same position as planned from the reference imaging dataset. An example of IGRT would include localisation of a cone-beam computed tomography (CBCT) dataset with the planning computed tomography (CT) dataset from planning.

This process is distinct from the use of imaging to delineate targets and organs in the planning process of radiation therapy. However, there is clearly a connection between the imaging processes as IGRT relies directly on the imaging modalities from planning as the reference coordinates for localising the patient. The variety of medical imaging technologies used in planning includes Computed Tomography (CT), Magnetic Resonance Imaging (MRI) and Positron Emission Tomography (PET) among others.