Impact of Intra‐fractional respiratory motion on dose distribution in lattice radiotherapy for liver tumors

Abstract Purpose To quantify the detrimental impact of respiratory motion on the critical “peak‐and‐valley” dose distribution in lattice radiotherapy (LRT) for liver tumors, to thereby evaluate the necessity of motion management. Methods This study assessed the data of 24 patients with liver cancer who underwent free‐breathing 4D‐CT simulation, for which a 2×2×2 vertices LRT tree was constructed. Volumetric Modulated Arc Therapy (VMAT) plans were generated based on the results of free‐breathing CT. The delivered dose distribution under respiratory motion was then simulated by dividing the planned dose into 10 equal subcomponents, applying phase‐specific isocenter shifts derived from 4D‐CT displacement measurements, and summing the deformed doses. The metrics analyzed included the vertex dose deviation, valley‐to‐peak dose ratio (VPDR), and low‐dose bath volumes. Results The analysis revealed a mean 3D respiratory motion error of 8.59 mm, with predominant displacement in the superior‐inferior (SI) direction (6.78±3.15 mm). Respiratory motion significantly degraded the LRT dose distributions; specifically, the vertices mean dose (D mean ) decreased from 50.19 Gy to 42.26 Gy, while the maximum dose (D max ) from 58.94 Gy to 52.63 Gy. Crucially, the VPDR increased in all directions, with the most pronounced increase observed in the SI direction (0.14±0.04 to 0.18±0.06, representing a 28.6% increase), escalating exponentially when motion error exceeded 8 mm. Increases in the left‐right (LR) (0.41 to 0.44, increasing 7.3%) and antero‐posterior (AP) (0.41 to 0.45, increasing 9.8%) directions were comparatively smaller. The motion also paradoxically altered the low‐dose regions; while the absolute V 40% , V 20% , and V 10% volumes decreased by approximately 10 cm 3 in the delivered plan, normalization to match the prescription coverage revealed significant increases in these volumes (minimum increases: V 40% by 3.59 cm 3 , V 20% by 15.85 cm 3 and V 10% by 92.29 cm 3 ). Conclusion Respiratory motion severely degrades essential spatial fractionation during liver LRT, particularly when exceeding 8 mm and occurring predominantly in the SI direction. This motion reduces peak vertex doses, increases the VPDR (homogenizing the dose distribution), and disrupts the low‐dose bath volumes critical for normal tissue sparing and the bystander effect.