Novel Simulation for Dosimetry Impact of Diaphragm Respiratory Motion in 4D VMAT for Esophageal Cancer

Abstract

<h3>Purpose/Objective(s)</h3> Respiratory motion affects the robustness of the dose distribution. For esophageal cancer (EC) near the esophagogastric junction, respiratory motion causes unintentional over- or under- dose distributions for the target. Although four-dimensional computed tomography (4DCT) is useful for robust evaluation, it increases the exposure dose and acquisition time. The objective of the current study is to propose the framework of the virtual 4DCT based robust evaluation method. Moreover, the robust evaluation with the virtual 4DCT was applied to EC patients. <h3>Materials/Methods</h3> We retrospectively evaluated 10 superficial (T1a - T1b) EC patients, had real 4DCT scans (real-4DCT). Target delineation was as follows; GTV: the primary lesion, CTV: GTV + 10mm (horizontal direction (H)), and 30mm (cranial and caudal direction (CC)). PTV: CTV + 8mm (H), and 15mm (CC). Volumetric modulated arc therapy (VMAT) was planned using double arcs which rotated 360° including two 60° lateral avoidance sectors during delivery. The virtual 4DCT defined as simulated 4DCT images (sim-4DCT) was generated with the simulated-organ motion function in commercial treatment planning system. Each sim-4DCT of 10 phases was generated from the end-inhalation CT image (reference image) by moving the diaphragm with mathematical equation proposed by Lujan et al (Med Phys 2003). First, we validated the accumulated CTV dose (D_99%, D_mean and D_1%) of 4D dose distribution in the sim-4DCT by comparing with the real-4DCT. The diaphragm motions (max, min and 60-90% tile of value) in the real 4DCT were obtained from an in-house program and adapted to sim-4DCTs. Then, the robustness of the CTV (D_99%, D_mean and D_1%) was evaluated by moving diaphragm from 5 to 20 mm. The percentage dose difference (ΔDx) between sim 4D-CT and static 3D-CT images which were used for planning was investigated. <h3>Results</h3> The accumulated CTV dose difference between sim-4DCT and real-4DCT was within 1% at 70% tile of the diaphragm motion. The mean CTV value of ΔD_99%, ΔD_mean and ΔD_1%, increased from 0.2% to 2.3%, 0.1% to 0.7 % and 0.1% to 0.6%, respectively, along with an increase in the diaphragm amplitude. For the diaphragm amplitude up to 15 mm, the mean ΔD_99%, ΔD_mean and ΔD_1% of CTV were within 3%. The largest percentage change was ΔD_99%, which was 2.2%. For the diaphragm amplitude of 20 mm, ΔD_99% for one patient was over 5%. The ΔD_99% of CTV for the other patients was within 3.9%. <h3>Conclusion</h3> This study proposed a novel method to evaluate the robust by generating virtual 4DCT. For EC case near the esophagogastric junction, the robust was enough if the diaphragm motion was within the PTV margin. The treatment planning with virtual 4DCT based robust evaluation method could propose an appropriate margin for PTV.