Image-guided radiation therapy (IGRT) is the use of frequent imaging during a course of radiation therapy for the purpose of improving the precision and accuracy of the delivery of treatment. In IGRT, machines that deliver radiation, such as a linear accelerator (for x-ray or photon) or cyclotron/synchrotron (for proton), are equipped with special imaging technology that allow the physician to image the tumor immediately before or even during the time radiation is delivered, while the patient is positioned on the treatment table. Using specialized computer software, these images are then compared to the reference images taken during simulation. Any necessary adjustments are then made to the patient’s position and/or radiation beams in order to more precisely target radiation at the tumor and avoid healthy surrounding tissue. Computed tomography (CT), magnetic resonance imaging (MRI), ultrasound (US) and x-ray imaging may be used for IGRT by visualizing boney or soft-tissue anatomy. Other methods for IGRT use markers placed on the patient’s body surface or implanted within the patient’s body.