ESTRO 2025 - Abstract Book

S4133

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2025

Material/Methods: Seventeen patients underwent WB-CT and WB-MRI simulations as part of the study. To ensure consistent positioning, four patients were immobilized using the same thermoplastic masks and frame for both imaging modalities, replicating the setup used during CT simulation. The remaining patients were positioned more comfortably on the WB-MRI's concave couch, given the extended scan duration and weight of the MRI coils. Image fusion quality was assessed through two complementary analyses. A qualitative analysis involved expert observers assigning alignment scores, ranging from 1 to 5, for six anatomical regions to evaluate the quality of rigid and elastic registrations. Quantitative analysis focused on measuring the clinical target volume of lymph nodes (CTV_LN) defined for treatment to identify differences attributable to the immobilization method. A t-test was applied to determine the statistical significance of differences observed in both qualitative and quantitative analyses. Results: Qualitative analysis demonstrated statistically significant improvements (p < 0.05) in alignment scores across all regions when thermoplastic masks were used (4.4±0.6 vs. 3.3±0.4), indicating superior CT-MRI fusion quality. Two representative examples are reported in Figure 1 and Figure 2. However, patient discomfort during prolonged MRI sessions occasionally occurred. Quantitative analysis on CTV_LN volumes showed no statistically significant differences (p > 0.05), likely due to sample size and inherent target volume variability (1728±371 cm 3 vs. 2173±536 cm 3 ).