Clinical Assessment of Potential Difference in Motion-Tracking Irradiation for Liver Tumors Using Radixact Synchrony®

This study aimed to advance the precision of motion-managed radiotherapy and provide robust information for clinical practice in terms of necessary margins using Radixact Synchrony® (Accuray, Sunnyvale, USA). A retrospective analysis was conducted on 88 irradiation sessions in nine cases of motion-tracking irradiation for liver tumors. Two gold markers were placed near the tumor, and a planning target volume (PTV) margin of 5 mm was set. The interruption threshold for potential difference (PD) (the statistical prediction value of the three-dimensional (3D) distance error from the predictive model) was set at 3.0 mm during treatment. PDvalues obtained before and during treatment, as well as the number of interruptions, were analyzed. A multiple regression analysis was performed to evaluate the influence of various planning parameters, which were obtained during four-dimensional (4D) treatment planning, on the 95th percentile value of PD (PD₉₅). The mean PD₉₅ was 2.9 ± 0.6 mm. In the four cases where interruptions occurred, the average ± SD number of interruptions per fraction was 7.5 ± 4.6, 3.6 ± 2.5, 2.5 ± 1.5, and 1.0 ± 1.1, respectively. The coefficient of determination (adjusted R²) between PD₉₅ and planning parameters was 0.802 (p < 0.01), showing a positive correlation with respiratory motion amplitude (p = 0.017) and a negative correlation with inter-marker distance (p = 0.049). No treatment interruptions were experienced in 83.0% of a total of 88 fractions. To conclude, setting a PDinterruption threshold of 3.0 mm for motion-tracking irradiation using Radixact Synchrony® under metallic marker guidance for liver tumors was appropriate for minimizing treatment interruptions with sufficient dosimetric precision. In addition, inter-marker distance and respiratory motion amplitude were associated withPD₉₅.