ESTRO 2024 - Abstract Book

S4908

Physics - Quality assurance and auditing

ESTRO 2024

Irradiation methods that track moving targets of lung tumors are attracting attention as a method that could reduce internal margins, and many systems have been developed. Among these, the lung tumor tracking irradiation using Synchrony, equipped on the Radixact, has been evaluated for its high accuracy and usefulness 1,2) . In addition, patient plan quality assurance(Patient QA) before irradiation is essential for tracking lung tumor irradiation with complex movements. In this study, we evaluate the usefulness of tracking accuracy evaluation using a four-dimensional moving phantom in Radixact-Synchrony tracking irradiation.

Material/Methods:

Radixact (Accuray Inc, Sunnyvale, CA) equipped with Synchrony was used as the radiation therapy device, and UniTraQ (Apex Medical, Inc., Tokyo, Japan) was used as the 3-axis moving phantom base that can be driven in 4 dimensions. The tracking target was an 8 cm diameter cylindrical wooden phantom (QUASAR: IBA dosimetry, Bayern, Germany) embedded with a 3 cm diameter water equivalent sphere, and a radiochromic film EBT-XD (Ashland Inc., Wayne, NJ, USA) was sandwiched between its sagittal cross-section to obtain the two-dimensional dose distribution. We used a flatbed scanner ES-10000G (Seiko Epson Corp., Nagano, Japan) and a DD-System (R-Tec Inc., Tokyo, Japan) for film reading. In addition, a cylindrical ionization chamber TN31022 (PTW, Freiburg, Germany) was inserted into the center of the target sphere to measure the dose at the center of the target. In the treatment plan, a 5 mm margin was added to the water equivalent sphere to form the PTV, and the prescribed dose was 48 Gy/4fr at 95% of the PTV. The main calculation conditions are Field width 2.5 cm, Delivery mode Dynamic, Pitch 0.2, Dose-volume spacing 0.98×1.25×0.98 mm3, Calculation grid 1.95×1.25×1.95 mm3, Gantry period 19.2 s. Using tumor coordinate data of 10 phases per respiratory cycle obtained from 4DCT of 10 patients, 3D coordinates for driving the phantom were created at 30 ms intervals using spline interpolation. The maximum vector length of tumor movement in patient data was 12.95 mm, and its movement distance in the Y direction was 11.34 mm. Furthermore, the patients with the most considerable maximum movement distance in each axis direction had a movement length in the X direction of 3.2 mm, Y direction movement length of 11.34 mm, and Z direction movement length of 5.82 mm. 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 2%, 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). The pass rate in the comparison between TPS and Stationary was 98.8%, showing good agreement. The pass rate obtained by comparing Stationary and No-tracking was low at 67.45 ± 11.47%, but the pass rate with tracking was 99.13 ± 0.64%, showing good agreement, except for one case where the tracking model needed to be rebuilt multiple times. We believe it was possible to evaluate with high accuracy in 2D verification using film. In addition, from the analyzed patient data, a correlation (correlation coefficient -0.57) was found between the tumor movement vector length and the pass rate without tracking. The target center dose difference was -0.14% ± 0.17% and 0.083% ± 0.27% in No-tracking compared to Stationary and With-tracking to Stationary, respectively. The reason that the difference between the two data was slight is thought to be because the ionization chamber was moving almost within the PTV, and the dose distribution within the PTV was flat. One of the reasons for the difference in dose between With-tracking and No-tracking is that many kV images are acquired during tracking. The primary purpose of this study is to evaluate the tracking accuracy for 4D-moving tumors, and the use of irradiation plans for the same target was a limitation. In the future, we plan to perform measurements using individual patient plans using a water-equivalent phantom. Results: