ESTRO 2025 - Abstract Book

S4116

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

ESTRO 2025

Purpose/Objective: Comparative data on modern immobilization techniques used in pelvic radiation therapy—including knee wedge, vacuum cushion, thermoplastic mold, belly board, and no immobilization—remain limited. Understanding which technique most effectively minimizes setup errors while optimizing setup times is critical for improving treatment accuracy and workflow efficiency, particularly in high-volume centers with constrained resources lacking cone-beam computed tomography (CBCT) or six degrees of freedom (6-DoF) couch capabilities. Material/Methods: This single-institution, retrospective study analyzed X-ray portal images of patients who underwent external beam radiation therapy for pelvic malignancies (bladder, prostate, cervical, endometrial, and rectal cancers) between December 2020 and March 2023. Patients were treated using one of five techniques: knee wedge, vacuum cushion, thermoplastic mold, belly board, or no immobilization. Setup errors were calculated by comparing portal images with planning images, using bony landmarks to translational deviations (lateral, cranio-caudal, and antero-posterior) and rotational discrepancies (yaw). Translational errors were categorized into ≤2.0 mm, 2.1–3.0 mm, 3.1–5.0 mm, and >5.0 mm, while yaw errors were classified as ≤0.5°, 0.6°–1.0°, and >1.0°. Setup times, defined as the interval from the patient’s entry into the treatment room to the initiation of treatment, were recorded for the first and subsequent fractions. Results: A total of 150 patients and 750 portal images were analyzed. Only 10 and 25 images were available for the no immobilization and belly-board techniques, respectively, precluding robust analysis for these groups. Among the remaining techniques, thermoplastic mold demonstrated a statistically significant reduction in mean setup errors across all translational directions—lateral (p = 0.0007), cranio-caudal (p = 0.0001), and antero-posterior (p = 0.002)— as well as rotational discrepancies in yaw (p = 0.0009). Additionally, thermoplastic mold significantly reduced the frequency of deviations exceeding the 2.0 mm standard in the cranio-caudal (p = 0.012) and antero-posterior (p = 0.016) directions and >1.0° in yaw (p = 0.003), but not in the lateral direction (p = 0.603). This technique also achieved the shortest mean setup time in both first day (p = 0.0001) and subsequent treatments (p < 0.0001).