Factors affecting target motion in stereotactic body radiotherapy of liver cancer using CyberKnife

Introduction: In stereotactic body radiation therapy (SBRT) of solitary liver cancer, organ motion due to respiration is an important factor in the definition of planning target volume (PTV). This study evaluated the potential associations of target motion with gross tumour volume (GTV) size, tumour location, Child–Pugh score and intra-fraction treatment time in SBRT of liver cancer treated by CyberKnife. Methods: Translational motion data of 145 liver cancer patients, who were previously treated by CyberKnife with free breathing under tumour tracking, were recorded in the log files of the motion tracking system and analysed. The factors including target location based on liver segments, Child–Pugh score which was an indication of liver cirrhosis, GTV size and intra-fraction treatment time were recorded and their associations with the magnitude of target movement were evaluated. Results: Target location demonstrated significant association with the translational target motion in the supero-inferior (SI) and left–right (LR) directions but less in antero-posterior (AP) direction. Tumours located at the peripheral segments were more affected than the central segments. Child–Pugh score and GTV size were not significantly associated with target motion in any direction. Longer intra-fraction treatment time generally increased target motion in the SI and LR directions. Conclusion: In SBRT of liver cancer, the target motions in SI and LR directions were correlated with the location of target and treatment time, but not with Child–Pugh score and GTV size. These results should assist in deciding the GTV-PTV margin in SBRT treatment planning for solitary liver cancer.