ESTRO 2024 - Abstract Book

S3355

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

[4] Pinnaduwage, D. S., Srivastava, S. P., Yan, X., Jani, S. S., Jenkins, C., Barani, I. J., & Sorensen, S. (2021). Small ‐ field beam data acquisition, detector dependency, and film ‐ based validation for a novel self ‐ shielded stereotactic radiosurgery system. Medical Physics, 48(10), 6121-6136.

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MR-compatible Octavius detector and phantom systems are suitable for plan QA in a 1.5 T MRI-linac.

Viktoriia A. Gorobets 1 , J.H. Wilfred De Vries 2 , Nicole Brand 3 , Andre J.M. Wopereis 1 , Simon J. Woodings 1

1 University Medical Center Utrecht, Radiation Oncology, Utrecht, Netherlands. 2 PTW-Benelux, N/A, Capelle aan den Ijssel, Netherlands. 3 PTW, N/A, Freiburg, Germany

Purpose/Objective:

The use of PTW Octavius ion chamber arrays (1500 and SRS) and various phantoms (Hexagonal and 4D) has been well-established in conventional systems [1-4]. MR-compatible detector arrays, such as the 1500 MR with air-filled ion chambers and the 1600 MR with liquid-filled ion chambers, have also been tested in magnetic fields with the older static phantom with observed significant angular sensitivity [5; 6]. The MR-compatible Octavius 4D system is specifically designed to rotate, ensuring a constant and optimal alignment between detectors and each beam segment direction, which is of particular importance in a treatment environment within an MRI. The fully MR compatible systems are now available for testing and implementation in MR-Linacs. The first tests of these systems are reported here. The goal of this study is to evaluate the suitability of the MR-compatible systems for patient plan QA in a 1.5 T MR-linac.

Material/Methods:

Fundamental system tests were performed. The results were compared to gold-standard ion chamber measurements. For field-size dependency, the reference values were calculated in the Monaco treatment planning system on the basis of PTW microDiamond commissioning data. 25 clinical treatment plans across various target sizes and anatomical sites (liver/pancreas, rectum, prostate, lungs, and lymph nodes) were evaluated using Octavius 1500MR and Octavius 4D. For the evaluation, we employed 3D gamma comparison analysis with a 3%/3mm and 2%/2mm criterion [7-9], using VeriSoft software. The low dose threshold was set to 10%. Evaluations were also conducted on plans containing deliberate. These plans were made within Monaco by modifying the number of monitor units (MU) and shifting the position of the isocenter.

Results:

All fundamental characterization tests were successfully passed. The detailed results for both devices are shown in Table 1. Of particular note, the Octavius 1600MR dose rate dependence test showed deviations up to 0.8% within the range of up to 115 MU/min. This result is consistent with the expected charge recombination as observed by PTW.