ESTRO 2025 - Abstract Book

S2580

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

Purpose/Objective: Stereotactic Ablative Radiotherapy (SABR) is a highly precise treatment modality for spinal metastases, requiring stringent patient-specific quality assurance (PSQA) protocols to ensure accurate dose delivery and patient safety [1]. In this study, we've developed a novel PSQA phantom, specifically tailored for Spinal SABR, enabling concurrent point-dose and film measurements in axial plane including target and cord dose. This study evaluates implementation of AAPM Task-Group 218 (TG-218) guidelines for statistical analysis of pre-treatment PSQA [2] for vertebra SABR cohort. Material/Methods: 103 consecutive patients with spinal metastases were administered over 5 years (Jan-2019 to Dec-2023) with SABR prescriptions of 20Gy/1fx or 24Gy/2fx. All patients were treated using TrueBeam STx with the High-Definition MLC system and Eclipse TPS (Varian Medical Systems, Palo Alto) with AcurosXB algorithm (v16.1) reporting dose-to medium for dose calculation. PSQA was performed using an in-house water-equivalent phantom, specifically designed for Spine-SABR, enabling concurrent measurements in axial plane with two dosimetry systems: i) micro diamond (mD) and Gafchromic EBT3 film via clinical IGRT process. Its plan complexity was analysed in terms of modulation factor (MF) in MU/cGy and average-leaf pair opening (ALPO) in mm. For PSQA measurements mD was analysed in terms of total percentage difference (%diff); and the films were analysed with 5%/1mm and 3%/1mm global gamma passing-rate criteria and 10% low-dose threshold. Statistical process control methods were applied to determine tolerance and action limits based on TG-218 guidelines for both detectors.

Results: The plan complexity shows average ALPO 16.3mm (ranging 9.5 to 35mm) and average MF 4.3 MU/cGy (ranging 3.0 to 6.1 MU/cGy). Tolerance and action limits based on the TG-218 process (with β-value = 6) were set out as 97.5% and 94.3%, respectively for 5%/1mm film analysis. This resulted in 101/103 plans (98%) and 97/103 plans (94%) were within the tolerance and action limits, respectively. A similar process was applied for mD with β-value 3.99 (A x 1.33 as the process capability metric with A = 3)[3], which set out TG-218 tolerance and action limits of ±1.3% and ±2.9%,