In modern day radiotherapy, the emphasis on reduction on volume exposed to high radiotherapy doses, improving treatment precision as well as reducing radiation-related normal tissue toxicity has increased, and thus there is greater importance given to accurate position verification and correction before delivering radiotherapy. At present, several techniques that accomplish these goals impeccably have been developed, though all of them have their limitations. There is no single method available that eliminates treatment-related uncertainties without considerably adding to the cost. However, delivering “high precision radiotherapy” without periodic image guidance would do more harm than treating large volumes to compensate for setup errors. In the present review, we discuss the concept of image guidance in radiotherapy, the current techniques available, and their expected benefits and pitfalls. 1. Introduction Radiotherapy has always required inputs from imaging for treatment planning as well as execution, when the treatment target is not located on the surface and inspection and visual confirmation are not feasible. Traditional radiotherapy practices incorporate use of anatomic surface landmarks as well as radiologic correlation with two-dimensional imaging in the form of port films or fluoroscopic imaging. Broadly, imaging has two major roles in radiotherapy:(a)Sophisticated imaging techniques such as contrast enhanced computed tomography (CECT) scans, magnetic resonance imaging (MRI), positron emission tomography (PET) scans, and angiography obtain three-dimensional (3D) structural and biologic information which is used to precisely define the target and thus enable precise and accurate treatment planning with shaped beams in isocentric or non-isocentric geometry.(b)“In-room” imaging methods (planar, volumetric, video, or ultrasound-based) obtain periodic information on target position and movement (within the same session or between consecutive sessions), compare it with reference imaging, and give feedback to correct the patient setup and optimize target localization. They also have the potential to provide feedback that may help to adapt subsequent treatment sessions according to tumor response. More specifically, modern day radiotherapy regards the latter application with “in-room” imaging as “image guided radiation therapy” (IGRT). Modern external beam radiotherapy techniques such as intensity modulated radiation therapy (IMRT), volumetric modulated arc therapy (VMAT), stereotactic radiosurgery (SRS), or stereotactic radiotherapy (SRT) have
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