During the initiation of each actual treatment session, the patient was positioned based on body skin marks and aligned at the isocenter. Before the daily treatment fraction of radiotherapy, 4D-CBCT imaging based on skin marks (skin matching) was performed using the Elekta Symmetry 4D IGRT System (Elekta Oncology Systems, Crawley, UK). The projection data of 4D-CBCT were sorted into 10 respiratory-phase bins. The scan parameters were set to 120 kV, 20 mA, 16 ms per frame, and a slice thickness of 2 mm, with a gantry rotation speed (GRS) of 50° min−1 [17, 19]. Automatic registration between planning CT and 4D-CBCT images was performed based on the bone anatomy (bone matching) using the Elekta XVI software. Subsequently, the manual registration between planning CT and 4D-CBCT images was performed based on the stomach anatomy using the axial, coronal, and sagittal images until moving images of the stomach in all 10 phases of the 4D-CBCT images were symmetrically positioned within the PTV structure in the planning CT images (4D soft-tissue matching).

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