S2225
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
Results: Eighty CBCTs of 40 treatment fractions were
Digital Poster 1976
Evaluation of Varian TrueBeam triggered kV imaging for fiducial tracking: Comparison with BrainLab ExacTrac Dynamic Lloyd Tan, Jun Hao Phua, Glen Mok, James Lee, Hong Qi Tan Division of Radiation Oncology, National Cancer Centre Singapore, Singapore, Singapore Purpose/Objective: This study aims to compare the IGRT shifts using TrueBeam (v2.7 MR3) kV trigger imaging vs ExacTrac dynamic (ETD) stereoscopic X-ray imaging for fiducials tracking. Material/Methods: A two-arc 10 MV treatment plan delivering 5 Gy per fraction was generated and delivered on a BrainLab pelvis phantom containing four implanted fiducial markers and a ball bearing representing the PTV. The phantom was set up using the ETD pre-positioning workflow, matching the surface contour of the phantom to plan.Following pre-positioning, a cone beam computed tomography (CBCT) was acquired and registered to planning CT with a focus on the four fiducial markers. Stereoscopic X-ray images from ETD were then obtained, and the detected shifts were recorded. The phantom was repositioned according to the CBCT registration, after which ETD shifts were reset to zero.The treatment plan was subsequently delivered with kV-triggered imaging on the Varian TrueBeam system, and ETD stereoscopic X-rays were acquired at cardinal gantry angles (270°, 0°, and 90°).To simulate motion, a 1cm translational and 1° rotational shifts were manually introduced to the phantom and verified using ETD surface imaging. A CBCT was then performed to establish the baseline positional offsets. 2-D images from TrueBeam kV arms and ETD stereoscopic X-rays were acquired at the same cardinal angles for comparison. A 2D-3D automatic registration was performed to determine the positional shifts derived from the trigger imaging. Results:
evaluated. The maximum motion (median) of CTV-t in all fractions was 7 mm (2 mm) in left and 6 mm (2 mm) in right direction, 8 mm (2 mm) in anterior and 8 mm (2 mm) in posterior direction, and 5 mm (2 mm) in superior and 7 mm (2 mm) in inferior direction (Figure 1). The mean time between CBCT1 and CBCT2 was 6 minutes (SD 4 minutes). No significant correlation was found between time and intrafractional motion. Taking into account anatomical motion and other variations (i.e. contouring), an isotropic margin of 8 mm was deemed clinically appropriate. This margin could potentially reduce dose to bladder and rectum compared to the isotropic IGRT margin of 1.5 cm (Figure 2).
Conclusion: An isotropic CTV–PTV margin of 8 mm in all directions was determined to incorporate intrafractional target motion. Future studies may elucidate whether margins can be safely reduced even further while maintaining adequate target coverage. The 8 mm isotropic margin is currently under clinical evaluation. References: 1. Ten Asbroek-Zwolsman L, Zwart L, Ligtenberg H, Dasselaar J. Online adaptive radiotherapy for postoperative patients with endometrial cancer. Radiother Oncol. 2025 May, Volume 206, S772 - S773. DOI: 10.1016/S0167-8140(25)00830-8 Keywords: Endometrial cancer, Online adaptive radiotherapy
Made with FlippingBook - Share PDF online