ESTRO 2025 - Abstract Book

S4130

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

ESTRO 2025

Material/Methods: This single-center retrospective study included 16 patients undergoing thoracic SBRT, divided into two groups based on the immobilization technique used: 11 patients were positioned with a vacuum mattress and a chin-chest thermoformed mask (Group D), and 5 patients with a vacuum mattress alone (Group E). All patients had 4DCT simulation with free breathing. Tumor motion was monitored using 4D CBCT during each fraction. Translational (x, y, z) and rotational (X, Y, Z) displacements were recorded for all 71 fractions. Clinical differences between groups were assessed using Fisher's exact test for categorical variables and Wilcoxon rank-sum tests for continuous variables. Results: Among the 71 fractions analyzed, Group D accounted for 50 fractions (70.4%) and Group E for 21 fractions (29.6%). Patient characteristics, including tumor location, age, and BMI, were balanced across groups. Group D had a higher proportion of patients with ECOG >1 (14% vs. 0%, p=0.03), while Group E reported significantly higher rates of pain (61.9% vs. 8.0%, p<0.001) and respiratory discomfort (71.4% vs. 6.0%, p<0.001). The average translational displacement was comparable between groups: median of 0.3 cm (IQR: 0.1–1.5) for the whole cohort, 0.3 cm (0.1– 1.5) in Group D, and 0.4 cm (0.1–0.9) in Group E ( Figure 1 ). Rotational displacement was similarly consistent: median of 1.2° (0–2.4) for the whole cohort, 1.2° (0–2.4) in Group D, and 1.0° (0–2.1) in Group E. However, Group D showed significantly lower z-direction displacement compared to Group E: median of 0.25 cm (01.1–0.4) versus 0.4 cm (0.3– 0.4) (p=0.008). Additionally, z-direction displacement was significantly lower for inferior lobe targets in Group D compared to Group E: median of 0.25 cm (IQR: 0.1–0.4) versus 0.81 cm (IQR: 0.7–1.6) (p = 0.005) ( Figure 2 ).