Method for determining a reference radiograph, said reference radiograph allowing a later comparison with a radiograph taken during treatment in radiation therapy, said method comprising the steps of: i. providing a Four-Dimensional Cone Beam Computed Tomography (4D-CBCT) apparatus, comprising a source and a detector that can be positioned at different angular positions around a patient, said 4D-CBCT apparatus being able to generate a plurality of CBCT images corresponding to different times; ii. providing a radiographic apparatus comprising a radiographic source and a radiographic detector and configured for generating a sequence of radiographs corresponding to different times; iii. providing positioning means configured for positioning a patient for generating CBCT images of said patient with said 4D-CBCT apparatus and radiographs of said patient with said radiographic apparatus; iv. performing a 4D-CBCT scanning sequence with said 4D-CBCT apparatus for generating a plurality of CBCT images of a portion of the patient's body, said generated CBCT images corresponding to different temporal phases associated to said patient; characterized in that the method further comprises the steps of v. performing a radiographic sequence with said radiographic apparatus so as to generate a number of radiographs of said patient, said number of radiographs corresponding to different temporal phases associated to said patient; vi. providing a synchronization device configured for correlating a CBCT image corresponding to a given temporal phase with a radiograph corresponding to an equivalent temporal phase; vii. correlating with said synchronization device a radiograph obtained in step v with a designated CBCT image of said CBCT images obtained in step iv, said designated CBCT image corresponding to a designated temporal phase; viii. identifying the correlated radiograph obtained in step vii as said reference radiograph. i. providing a Four-Dimensional Cone Beam Computed Tomography (4D-CBCT) apparatus, comprising a source and a detector that can be positioned at different angular positions around a patient, said 4D-CBCT apparatus being able to generate a plurality of CBCT images corresponding to different times; ii. providing a radiographic apparatus comprising a radiographic source and a radiographic detector and configured for generating a sequence of radiographs corresponding to different times; iii. providing positioning means configured for positioning a patient for generating CBCT images of said patient with said 4D-CBCT apparatus and radiographs of said patient with said radiographic apparatus; iv. performing a 4D-CBCT scanning sequence with said 4D-CBCT apparatus for generating a plurality of CBCT images of a portion of the patient's body, said generated CBCT images corresponding to different temporal phases associated to said patient; characterized in that the method further comprises the steps of v. performing a radiographic sequence with said radiographic apparatus so as to generate a number of radiographs of said patient, said number of radiographs corresponding to different temporal phases associated to said patient; vi. providing a synchronization device configured for correlating a CBCT image corresponding to a given temporal phase with a radiograph corresponding to an equivalent temporal phase; vii. correlating with said synchronization device a radiograph obtained in step v with a designated CBCT image of said CBCT images obtained in step iv, said designated CBCT image corresponding to a designated temporal phase; viii. identifying the correlated radiograph obtained in step vii as said reference radiograph.