Some methods include obtaining (405) a projection image of a plurality of fiducials associated with a coordinate system irradiated by a radiotherapy radiation source at a plurality of discrete locations on a trajectory path model, determination (420) of a projection matrix from projection images of the fiducials irradiated by the radiotherapy radiation source at each of the discrete locations, determination (425) of the actual coordinate of the radiotherapy radiation source in the coordinate system associated with the fiducials at the plurality of discrete locations based on the determined projection matrices, and correlating (430) the trajectory path model of the radiotherapy radiation source to the determined actual position of the radiotherapy radiation source at the discrete locations.
