Evaluation of Elekta SymmetryTM 4D IGRT system by using moving lung phantom

Purpose: 4D 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 have a precise treatment plan. However, there is a limitation that general CBCT images cannot perfectly help the sophisticated registration. On the other hand, SymmetryTM 4D IGRT system of Elekta can offer the 4D CBCT registration option. In this study, we intend to evaluate the usefulness of SymmetryTM. Method and Materials: Planning CT images of the CIRS moving lung phantom were acquired from 4D MDCT. And they are sorted as 10 phases from 0% phase to 90% phase. The thickness of CT images was 1 mm. Acquired MDCT images were transferred to the contouring software and a virtual target was generated. An one arc VMAT plan was performed by using the treatment planning system on the virtual target. Finally, the movement of the phantom was verified through XVI SymmetryTM system. Results: The physical movement of CIRS moving lung phantom was +/- 10.0 mm in superior-inferior direction, +/- 1.0 mm in lateral direction, and +/- 2.5 mm in anterior-posterior direction. The movement of the phantom was measured from 4D MDCT registration as +/- 10.2 mm in superior-inferior direction, +/- 0.9 mm in lateral direction, and +/- 2.45 mm in anterior-posterior direction. The movement of the phantom was measured from SymmetryTM registration as +/- 10.1 mm in superior-inferior direction, +/- 0.9 mm in lateral direction, and +/- 2.4 mm in anterior/posterior direction. Conclusion: It is confirmed that 4D CBCT is a beneficial tool for the treatment of movable tumors. Therefore, 4D registration of SymmetryTM can increase the precision of the registration when a movable tumor is a target of radiation treatment.

💡 Research Summary

The purpose of this study was to assess the accuracy and clinical utility of the Elekta Symmetry™ 4D Image‑Guided Radiation Therapy (IGRT) system, which provides four‑dimensional (4D) cone‑beam computed tomography (CBCT) registration, for treating moving lung tumours. A commercially available CIRS moving lung phantom was employed to simulate realistic tumour motion: superior‑inferior (SI) displacement of ±10 mm, lateral (LR) displacement of ±1 mm, and anterior‑posterior (AP) displacement of ±2.5 mm. First, a 4D multi‑detector CT (MDCT) scan was acquired, sorting the data into ten respiratory phases (0 % to 90 %) with a slice thickness of 1 mm. The phase‑sorted images were imported into contouring software, where a virtual target mimicking a tumour was delineated. Using the treatment planning system (TPS), a single‑arc volumetric modulated arc therapy (VMAT) plan was generated on this virtual target, thereby reproducing the full planning workflow.

Subsequently, the phantom was positioned on the Elekta linear accelerator equipped with the XVI Symmetry™ system. A 4D CBCT acquisition was performed, and both automatic and manual 4D registrations were executed. The measured displacements from the 4D MDCT registration were SI ±10.2 mm, LR ±0.9 mm, and AP ±2.45 mm, while the Symmetry™ 4D CBCT registration yielded SI ±10.1 mm, LR ±0.9 mm, and AP ±2.4 mm. Compared with the known physical motion of the phantom, the deviations were less than 0.2 mm for all directions, demonstrating that the Symmetry™ system can reproduce tumour motion with sub‑millimetre accuracy.

These findings confirm that 4D CBCT overcomes the blurring and registration limitations inherent to conventional 3D CBCT, especially for lesions subject to respiratory motion. By aligning images to specific respiratory phases, 4D CBCT provides a precise snapshot of tumour position, enabling more accurate image‑guided setup and the potential for adaptive radiotherapy (ART). The study also highlights practical considerations: the experiments were limited to a single phantom and a fixed, periodic motion pattern, which does not capture the irregular breathing patterns seen in patients. Moreover, the additional imaging dose associated with 4D CBCT was not quantified, an important factor for clinical implementation.

In conclusion, the Elekta Symmetry™ 4D IGRT platform delivers highly accurate 4D CBCT registration, with measured tumour motion agreeing with the ground‑truth phantom motion within 0.1 mm. This level of precision supports its use in the treatment of moving lung tumours, where accurate localisation directly translates into improved dose delivery and reduced margins. Future work should extend validation to multiple phantoms, incorporate non‑periodic respiratory waveforms, and evaluate the system in a clinical cohort to fully establish its role in routine adaptive lung radiotherapy.