ESTRO 2025 - Abstract Book

S3310

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2025

Conclusion: Geometric accuracy in prostate radiotherapy delivery is a combination of multiple sources, including inter- and intra-fraction motion. Pre-treatment setup uncertainties are likely to affect the prostate coverage and should be mitigated to avoid geometric uncertainties. Intra-fraction motion is most pronounced in the AP-direction, indicating that margins adapted based on anatomical direction are useful. This analysis will be complemented with dosimetric analysis to investigate the dosimetric impact of intra-fractional prostate motion.

Keywords: Fiducial markers, prostate, geometric accuracy

References: [1] Poulsen P.R., Cho B. and Keall P.J., A method to estimate mean position, motion magnitude, motion correlation, and trajectory of a tumor from cone-beam CT projections for image-guided radiotherapy. International Journal of Radiation Oncology, Biology, Physics, 2008. 72(5): pp. 1587-96.

3398

Digital Poster Quantifying the Impact of Deformable Image Registration and Interplay Effect in 4D Dose Accumulation for Lung and Liver Tumors during Proton Therapy Alba Meneses-Felipe 1 , Pablo Cabello-García 2 , Javier Burguete 1,3 , Juan Diego Azcona 2,3 1 Department of Physics and Applied Mathematics, University of Navarra, Pamplona, Spain. 2 Service of Medical Physics and Radiation Protection, Clínica Universidad de Navarra, Madrid, Spain. 3 IdiSNA, Navarra Institute for Health Research, Pamplona, Spain Purpose/Objective: This study aims, first, to assess the dosimetric impact of DIR on 4D dose accumulation by evaluating geometric uncertainties in CT-to-CT images using an independent observer evaluation metric, the Distance Discordance Metric (DDM) [1]; second, to assess the dosimetric effects of respiration on 4D accumulated dose distributions; and third, to evaluate the interplay effect using a stochastic model [2] in a HITACHI synchrotron-based pencil beam scanning proton therapy (PBSPT) system [3] for lung and liver tumors. Material/Methods: A planning CT (pCT) and an eight-phase 4D-CT were acquired for three lung (20, 30, 35 fractions) and two liver cancer patients (3 fractions each). First, DDM was calculated per voxel on the pCT using ANACONDA hybrid deformable registration. Second, 100 realizations were simulated by perturbing the Deformation Vector Fields (DVFs), applying to each voxel a normal distribution with mean zero and a standard deviation equals to the DDM along the three spatial directions. The DIR dosimetric impact was evaluated by comparing the mean of the 4D accumulated perturbed dose with its range (4D 100 mean±SD ), while respiratory motion effects were assessed by comparing with the 3D planning dose from the pCT (3D pCT ). Third, the temporal structure of the irradiation sequence, which varies across different realizations, was determined for each treatment plan [2], with each spot assigned to a CT phase based on 12 or 20 rpm. Eight sub-plans were generated by summing the contributions from all spots in each phase. The 4D dynamic doses were computed by generating 100 randomized respiratory sequence distributions, changing the initial respiratory phase for each fraction. The interplay effect was evaluated by calculating the homogeneity index (HI) for the CTVs. Results: DDM values for target volumes were below the deformation grid voxel size (2.5 mm). The dosimetric impact of DIR was more significant in the CTV, with D99 fluctuations up to ±1.5% in the diaphragm (Figure 1, right), while respiratory motion resulted in D99 reductions of up to -5.65% for the CTV in the mediastinum (Figure 1, left), and generally near