ESTRO 2025 - Abstract Book

S3676

Physics - Quality assurance and auditing

ESTRO 2025

average offset across all centers was 0.85mm. No correlation between treatment accuracy and distance to isocenter was found. Gradient indices varied significantly across centers with a median of 7 (range 3 to 18). This variability is illustrated by dose profiles the figure. Conclusion: Our multi-center multi-platform benchmark study shows high variability in SIMT-SRS approaches across the 23 centers and a lack of standardization. On average, dose delivery is accurate and within tolerances. Clinical plan quality varies significantly. Our results will build the foundation for future recommendations and guidelines for SIMT-SRS implementation.

Keywords: Radiosurgery, Multi-Target

References: The study was founded by German Cancer Aid (Deutsche Krebshilfe), grant number 701143317.

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Digital Poster Clinical evaluation of quality assurance method using 4D moving phantom for Synchrony's lung tumor tracking Koji Sasaki 1 , Masatoshi Miura 2 , Yasuo Shiota 2 1 Graduate School of Radiological Technology, Gunma Prefectural College of Health Sciences, Maebashi, Japan. 2 Department of Medical Physics, Iwata City General Hospital, Iwata, Japan Purpose/Objective: To reduce the internal margin for targets that move due to respiration, lung tumor tracking irradiation using Synchrony equipped on Accuray's radiation therapy device Radixact (Radixact-Synchrony) has been highly evaluated. At ESTRO 2024, we reported tracking accuracy for a 3 cm φ spherical target. In this study, we used a 4D moving phantom (Apex Medical), a water-equivalent cubic phantom, and a radiochromic film to perform patient QA of tracking irradiation using Radixact-Synchrony for actual patient plans and evaluated its accuracy. Material/Methods: The tracking target was a 2 cm cubic bone density embedded in a house-made cubic phantom (14 cm on each side), and a radiochromic film EBT-XD (Ashland) was placed on the central sagittal section to obtain a 2-dimensional dose distribution. A cylindrical ionization chamber (PTW TN31022) was inserted into the center of the PTV. A clinical treatment plan for 10 actual lung cancer patients (48 Gy/4 fr. for 95% of the PTV of the lung tumor, field width 2.5 cm) was used. Using tumor coordinate data of 10 phases per respiratory cycle obtained from 4DCT of each of 10 patients, 3D coordinates for driving the phantom were created at 30 ms intervals using spline interpolation. First, we compared the dose distribution calculated by TPS with the dose distribution obtained without moving the phantom (Stationary) to confirm the measurement accuracy using film. The Stationary was used as the standard for evaluation of 2D dose distribution. Gamma analysis (DTA 2 mm, DD 3%, TH 10%) was performed by comparing the dose distributions under the phantom moving condition without tracking (No-tracking) and the phantom moving condition with tracking (With-tracking). Results: Although the pass rate obtained by comparing Stationary and No-tracking was quite low at 40.95% ± 10.20%, the pass rate of With-tracking was 99.95 ± 0.08%, allowing for a high accuracy evaluation in 2D verification. The target center dose difference was -23.12% ± 20.69% and -0.79% ± 0.94% in No-tracking compared to Stationary and With tracking to Stationary, respectively. In actual clinical plans, we confirmed that the dose distribution in the PTV changes significantly without tracking and cannot guarantee the center dose.