1113 - Applying GD-EOB-DTPA Enhanced MR to Unveil the Radiobiological Effect of Proton and Photon Therapy Based on the Longitudinal Evolution of the Focal Liver Reaction

H. Wang^1,2, J. Zhu³, and J. Yue²; ¹Shandong University Cancer Center, Cheeloo College of Medicine, ShanDong University, Jinan, China, ²Department of Radiation Oncology, Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, Jinan, China, ³Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, Shandong Provincial Key Medical and Health Laboratory of Pediatric Cancer Precision Radiotherapy (Shandong Cancer Hospital), Jinan, China

Purpose/Objective(s): Focal Liver Reaction (FLR), a radiobiological effect detected by GD-EOB-DTPA enhanced MR in the healthy liver after irradiation, and the signal intensity (SI) of FLR in MR is dose related, may unveil the difference between proton and photon radiotherapy. This study aims to validate the efficacy of a standard proton relative biological effectiveness (RBE) of 1.1 and to investigate the longitudinal evolution of proton FLR. Materials/

Methods: Ten patients, five treated with intensity-modulated proton radiotherapy and five treated with intensity-modulated radiotherapy, were enrolled. Longitudinal follow-up MRI scans were performed at 1, 2, 3, 6, 9 and 12 months after radiotherapy. Dose distributions from the planning CT were transformed into MR images using deformable registrations. The MR images were segmented into dose bins from 10 to 60 Gy in increments of -5 Gy. The SI of each bin was then calculated. Assuming a proton RBE of 1.1, we will see consistent radiographic changes in the proton and photon groups. Therefore, we analysed the change in SI gradient of the dose bins of two groups between different months to verify this assumption. In addition, we investigated the evolution of FLR by measuring monthly changes in volume and maximum diameter (MD). Statistical analysis was facilitated using generalized estimating equations to account for repeated measures and potential attrition. So far, the proton group has completed all time points of MR, the photon group has only the statistics of the first three time points.
Results: Our analysis of the first three time points showed that the linear regression equation for the proton cohort had a slope 0.016 lower than that of the photon cohort (?² = 19.254, P < 0.001). Regarding the FLR evolution of the proton cohort, the FLR volume and MD at 6 months were 27.695 cm³ and 0.64 cm lower than those at 3 months (P <0.001, respectively), while the FLR and MD at 9 months were 13.780 cm³ and 0.435 cm lower than those at 6 months (P=0.015, P <0.001, respectively).
Conclusion: The lower slope of the proton group means that its RBE is overestimated, in other words, the proton RBE in healthy liver tissue is below 1.1. In the proton group, the FLR decreases over time like the ebb tide, indicating recovery from the low-dose irradiation area, while the high-dose irradiation area shows irreversible effects, as shown by the insignificant differences between 9 and 12 months.