AB S TRACT purpose : To further investigate the effects of respiratory motions on CBCT imaging, as well as 4D-CT examinations, with a future goal of using obtained results to implement new methods for individual margins and daily matching procedures into routine clinical practice. background : Since the implementation of CBCT combined with modern accelerators, a higher degree of accuracy has been made possible in RT. However, due to the slow gantry speed of linear accelerators, the imaging procedure of CBCT is a slow process which is thereby degraded by internal motion such as respiration. material and methods : Attain patient specific respiratory motion patterns from CBCT projection data of previous examinations. Utilize this data to perform simulations for both CBCT and 4D-CT using a steering system which allows for arbitrary motion patterns in the longitudinal direction. results : Various imaging with CBCT showed that the resulting images during respiratory motion, can be described by the Probability Density Function of the motion for as long as it does not cause related distortions. This also meant that convolution could be implemented as a model to estimate the CBCT images during oscillation, knowing the object and motion pattern. The 4D-CT examinations using the steering system showed that irregular motion patterns were less accurately described than regular patterns, making the actual motion an important feature to combine together with the measured amplitude. conclusions : It was made clear that CBCT images can be described by the PDF, and thus can be seen as a Color Intensity Projection of the object position. Also it has been shown that the projection data of CBCT images contains valuable information about the respiratory motion of the patient. Another conclusion is that with the help of fiducials, the position of the target within the respiratory cycle can be determined relative to the 4D-CT examination, enabling further input data as to the daily matching procedure, proper applied margins as well as dose to the OAR.