Evaluation of the Elekta Symmetry™ 4D IGRT System by Using a Moving Lung Phantom

Purpose: 4D cone-beam computed tomography (CBCT) is a beneficial tool for the treatment of movable tumors because it can help us to understand where the tumors are actually located and it has a precise treatment plan. However, general CBCT images have a limitation in that they cannot perfectly perform a sophisticated registration. On the other hand, the Symmetry (TM) 4D image-guided radiation therapy (IGRT) system of Elekta offers a 4D CBCT registration option. In this study, we evaluated the usefulness of Symmetry (TM). Method and Materials: Planning CT images of the CIRS moving lung phantom were acquired 4D multi-detector CT (MDCT), and the images were sorted as 10 phases from 0% phase to 90% phase. The thickness of the CT images was 1 mm. Acquired MDCT images were transferred to the contouring software, and a virtual target was generated. A one-arc volumetric-modulated arc therapy (VMAT) plan was performed by using the treatment planning system on the virtual target. Finally, the movement of the phantom was verified by using the XVI Symmetry (TM) system. Results: The physical movement of the CIRS moving lung phantom was +/- 10.0 mm in the superior-inferior direction, +/- 1.0 mm in the lateral direction, and +/- 2.5 mm in the anterior-posterior direction. The movement of the phantom was measured from the 4D MDCT registration as +/- 10.2 mm in the superior-inferior direction, +/- 0.9 mm in the lateral direction, and +/- 2.45 mm in the anterior-posterior direction. The movement of the phantom was measured from the Symmetry (TM) registration as +/- 10.1 mm in the superior-inferior direction, +/- 0.9 mm in the lateral direction, and +/- 2.4 mm in the anterior-posterior direction. Conclusion: We confirmed that 4D CBCT is a beneficial tool for the treatment of movable tumors, and that the 4D registration of SymmetryTM can increase the precision of the registration when a movable tumor is the target of radiation treatment.