Author: Tianyuan Dai, Xiaoying Fan, Shuting Wang, Yong Yin 👨🔬
Affiliation: Department of Graduate, Shandong First Medical University, Shandong Academy of Medical Sciences, Department of Radiation Oncology Physics and Technology, Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences 🌍
Purpose: The purpose of this study was to investigate the interplay effect during intra-fractional motion and the effect of the robust optimization parameters for inter-fractional motion in the intensity modulation proton therapy (IMPT) for esophageal cancer.
Methods: 33 patients with esophageal cancer were retrospectively analyzed. The patients were divided into two groups. Twelve patients reconstructed 4DCT image sets of 10 respiratory phases (Gintra), twenty-one patients received a second CT scan after the first one (Ginter). The IMPT plans were created using the posterior fields. 4D dynamic dose (4DDD) was then calculated to access the interplay effect by considering respiratory motion and dynamic beam delivery for Gintra. Seven plans with different robust optimization parameters were designed for each patient. The setup uncertainties were set to ±0-0.6cm for Ginter. Plan quality and dosimetric parameters for the target and organs at risk (OARs) were then analyzed.
Results: For Gintra, 4DDD was slightly perturbated compared to the nominal plan dose. The mean value of D98 of nominal dose and 4DDD were 49.8 and 49.4 Gy(RBE), respectively, and the D95 were 50.4 and 50.1 Gy(RBE), respectively. For Ginter, the dosimetry parameters of the target area and the OARs showed a linear relationship with the corresponding robust optimization parameters in IMPT. With the larger the robust optimization parameters set, the better the dose coverage of the target area, while the dose of OARs increased. The D98 of the target for the seven plans (setup0.6-0) were 49.42±0.75, 48.95±1.21, 48.54±1.48, 47.55±2.31, 47.07±2.71, 44.58±4.20 and 44.02±4.44 Gy(RBE), respectively.
Conclusion: The result showed that the interplay effect has little effect for esophageal cancer due to the inherent robustness of posterior fields. Combined with the target dose coverage and organ protection, ±0.3-0.5cm setup uncertainties may be considered sufficient to reduce the impact of both intra-fractional and inter-fractional motion for most patients.