Elekta Unity MR-linac commissioning: mechanical and dosimetry tests

Abstract

We report the commissioning results of Elekta Unity for the dosimetric performance and mechanical quality assurance (QA), and propose additional commissioning procedures. Mechanical tests included multi-leaf collimator (MLC) positional accuracy, radiation isocenter diameter at the center and off-center position, and coincidence between the magnetic resonance (MR) image center and radiation isocenter. Comparisons between the measurements and calculations of the simple irradiated field, intensity modulated radiation therapy (IMRT) commissioning, MLC output factor ratio, validation of independent dose calculation software and end-to-end testing were performed to evaluate dosimetric performance. The average values of the MLC positional accuracy for film- and imaging device-based analysis were −0.1 and 0.3 mm, respectively. The measured radiation isocenter size was 0.41 mm, and the off-center results were within 1 mm. The coincidence was −0.21, −1.19 and 0.49 mm along the x-, y- and z-axes, respectively. The calculated percent depth doses (PDD) and profiles agreed with the measurements. The results of independent dose calculation were within the action level recommended by American Associations of Physicist in Medicine. The gamma passing rate (GPR) for IMRT commissioning was 98.6 ± 0.9%, and end-to-end testing of adapted plans showed agreement within 2% between the measurement and calculation. We reported the results of mechanical and dosimetric performances of Elekta Unity, and proposed novel commissioning procedures. Our results should provide knowledge to the physics community for enhancing the QA programs.

Technical note: Cryostat transmission characterization for MR linac – temporal stability, clinical impact and change implementation

“…This confirms both the robustness of the method employed as well as long term stability of the output with varying gantry angle. It should be underlined that the curves reproduced in the literature, 2,4,10 while agree in terms of variability range, show significant variability in their shape as they incorporate manufacturing tolerances of the traversed cryostat components which are unique for each machine. As the ground truth is unknown during CTC acquisition, errors due to setup, machine output fluctuations or the presence of air gaps in the build‐up cap may affect the chamber reading 11,12 and are difficult to identify.…”

Multileaf collimator characterization and modeling for a 1.5 T MR-linac using static synchronous and asynchronous sweeping gaps

“…One might argue that a limitation of this study is that clinical treatment plans were not evaluated. However, clinical treatment plans consist of beam apertures with a variety of MLC configurations, and the PSQA results generally pass the acceptance criteria using MONACO (Snyder et al 2020, Powers et al 2022, Tsuneda et al 2023. This can be attributed to the relatively low complexity of the apertures in typical clinical plans, and to MLC parameter errors being compensated for by other MLC or beam modeling parameters.…”

A new workflow of the on-line 1.5-T MR-guided adaptive radiation therapy

“…After confirming the dose–volume histogram (DVH) based on the new treatment plan, a verification MRI is obtained just before the start of treatment to ensure the contours of the target and risk organs do not require further modification. Independent dose verification using MU2net (DOSIsoft, Cachan, France) is performed by medical physicists [ 15 ]. Two-dimensional (2D) cine images in three cross sections are acquired for motion monitoring to ensure proper irradiation of the target.…”