PURPOSE: This study aimed to quantitatively measure the movement of tumors in real-time and evaluate the treatment accuracy, during the treatment of a liver tumor patient, who underwent radiosurgery with a Synchrony Respiratory motion tracking system of a robot CyberKnife. MATERIALS AND METHODS: The study subjects included 24 liver tumor patients who underwent CyberKnife treatment, which included 64 times of treatment with the Synchrony Respiratory motion tracking system (Synchrony(TM)). The treatment involved inserting 4 to 6 acupuncture needles into the vicinity of the liver tumor in all the patients using ultrasonography as a guide. A treatment plan was set up using the CT images for treatment planning uses. The position of the acupuncture needle was identified for every treatment time by Digitally Reconstructed Radiography (DRR) prepared at the time of treatment planning and X-ray images photographed in real-time. Subsequent results were stored through a Motion Tracking System (MTS) using the Mtsmain.log treatment file. In this way, movement of the tumor was measured. Besides, the accuracy of radiosurgery using CyberKnife was evaluated by the correlation errors between the real-time positions of the acupuncture needles and the predicted coordinates. RESULTS: The maximum and the average translational movement of the liver tumor were measured 23.5 mm and 13.9+/-5.5 mm, respectively from the superior to the inferior direction, 3.9 mm and 1.9+/-0.9 mm, respectively from left to right, and 8.3 mm and 4.9+/-1.9 mm, respectively from the anterior to the posterior direction. The maximum and the average rotational movement of the liver tumor were measured to be 3.3degrees and 2.6+/-1.3degrees, respectively for X (Left-Right) axis rotation, 4.8degrees and 2.3+/-1.0degrees, respectively for Y (Cranio-Caudal) axis rotation, 3.9degrees and 2.8+/-1.1degrees, respectively for Z (Anterior-Posterior) axis rotation. In addition, the average correlation error, which represents the treatment's accuracy was 1.1+/-0.7 mm. CONCLUSION: In this study real-time movement of a liver tumor during the radiosurgery could be verified quantitatively and the accuracy of the radiosurgery with the Synchrony Respiratory motion tracking system of robot could be evaluated. On this basis, the decision of treatment volume in radiosurgery or conventional radiotherapy and useful information on the movement of liver tumor are supposed to be provided.