Evaluation of Respiratory Motion Effects In VMAT Lattice Spatially Fractionated Radiotherapy Using 4DCT
Abstract
Purpose
To evaluate the impact of respiratory motion on dose distribution and spatial modulation in VMAT-based Lattice Spatially Fractionated Radiotherapy(Lattice-SFRT), and to evaluate the stability of peak–valley dose characteristics using four-dimensional(4D) dose reconstruction.
Methods
A 4D dose reconstruction framework was developed integrating deformable registration, respiratory phase mapping, and control-point–resolved monitor unit(MU) allocation. A 10-phase 4DCT dataset was used with the 50% phase as the reference. VMAT-Lattice plans were decomposed at the control-point level, temporally synchronized with respiratory motion, and redistributed into phase-specific sub-plans. Dose was calculated on each phase CT and deformably accumulated to the reference anatomy. Dosimetry evaluation included dose–volume-histogram(DVH) analysis for targets and organs at risk, and quantitative assessment of peak-to-valley ratio(PVDR).
Results
Compared with the static 3D dose, the 4D accumulated dose demonstrated reduced coverage in the low-dose target region. For PTV_20, the D95 decreased from 20.01Gy to 19.10Gy, corresponding to a 0.91Gy reduction. Consistently, volume-based metrics were also degraded. In contrast, PTV_66.7 showed minimal sensitivity to respiratory motion, with the D95 remaining essentially unchanged between the 3D and 4D dose calculations(65.96Gy vs. 65.97Gy). In addition, the peak-to-valley dose ratio(PVDR) from 2.49 in 3D dose to 2.27 in 4D dose.
Conclusion
The observed reductions in D95 for PTV_20 are indicative of motion-induced dose blurring inherent to respiratory motion. Respiratory motion redistributes dose across breathing phases, resulting in a smoothing of dose gradients in the accumulated dose, which predominantly affects low-dose target regions with shallower dose gradients. Furthermore, the reduction in PVDR from 2.49 to 2.27 reflects diminished spatial dose heterogeneity in motion situation. This smoothing effect may have important implications for lattice-based strategies, while biological effectiveness is relevant to maintaining dose heterogeneity. Overall, these results highlight the importance of 4D dose-evaluation for characterizing motion-related dose degradation and assessing plan robustness in respiratory motion.