The research presented in this thesis describes the potential and limitations of the cone-beam CT (CBCT)-based image-guided adaptive radiotherapy (IGART) in several aspects. As clinical implementation of the IGART besides the imaging information requires fast and precise dose calculation, several commercial treatment planning systems were compared in terms of dosimetric accuracy of the implemented dose calculation algorithms and calculation speed. Special emphasis in these studies was put on the investigation of the dosimetric accuracy of the recently available Monte Carlo dose calculation for the photon beams. Another aspect of the IGART that influences the plan adaptation is presented by interobserver variability in tumor delineation on CBCT images. Two interobserver studies for prostate and lung tumors were conducted for the evaluation of the magnitude of this uncertainty. It was demonstrated that in case of prostate the variability for the contouring is higher compared to conventional CT and MRI imaging, whereas for the lung metastasis contouring uncertainty introduced by CBCT imaging is minimal. Finally the possibility of the dose calculation on CBCT images was investigated for several clinical indications (prostate, lung and head-and-neck tumors) and the comparison of the density override techniques with other correction techniques for the grey values in CBCT images was performed.