The detection of intrafraction organ motion, necessary for the minimization of treatment errors, is a remaining challenge in radiotherapy. A novel technology for the dynamic monitoring of tumor motion uses tumor-implanted electromagnetic (EM) transponders. In the present thesis, concepts and strategies for the use of the EM technology in image-guided radiotherapy (IGRT) are developed. First, the compatibility of the EM technology with the radiotherapy environment is investigated experimentally. Subsequently, a technique is developed that combines EM tumor localization with the x-ray imaging options of IGRT. This technique exploits the unique advantages of EM tumor localization (non-ionizating radiation, three-dimensional target localization) and those of x-ray imaging (volumetric information about organ deformation and rotation, localization of organs at risk). The technique has been applied successfully to the elimination of motion artifacts in cone-beam computed tomography. In addition, the real-time control of a dynamic multileaf collimator based on the EM transponders could be demonstrated. Finally, the EM tumor tracking technology is introduced clinically with a study on prostate motion. The concepts developed in this thesis improve the detection of intrafraction organ motion in IGRT and thus enable the treatment of dynamic target volumes with increased accuracy
