Automatic 3D Monte-Carlo-based secondary dose calculation for online verification of 1.5 T magnetic resonance imaging guided radiotherapy. (July 2021)

• Record Type: Journal Article
• Authors: Nachbar, Marcel; Mönnich, David; Dohm, Oliver; Friedlein, Melissa; Zips, Daniel; Thorwarth, Daniela
• Is Part Of: Physics and imaging in radiation oncology. Volume 19(2021)
• Page Start: 6
• Page End: 12
• Publication Date: 2021-07
• Subjects: MR-guided radiotherapy -- MR-Linac -- Online secondary dose calculation -- Adaptive radiotherapy -- Online plan quality assurance; Radiotherapy -- Periodicals; Radiation dosimetry -- Periodicals; Cancer -- Imaging -- Periodicals; Oncology -- Periodicals; 615.842
• DOI: 10.1016/j.phro.2021.05.002
• Languages: English
• ISSNs: 2405-6316

Abstract

Highlights: First implementation of an independent 3D-secondary dose calculation (3D-SDC). Validation of the 3D-SDC solution using patient plans and experimental plan QA. Online SDC of central targets is feasible with a median calculation time of 1:23 min. Peripheral targets with small beam numbers need alternative validation strategies. Abstract: Background and purpose: Hybrid magnetic resonance linear accelerator (MR-Linac) systems represent a novel technology for online adaptive radiotherapy. 3D secondary dose calculation (SDC) of online adapted plans is required to assure patient safety. Currently, no 3D-SDC solution is available for 1.5T MR-Linac systems. Therefore, the aim of this project was to develop and validate a method for online automatic 3D-SDC for adaptive MR-Linac treatments. Materials and methods: An accelerator head model was designed for an 1.5T MR-Linac system, neglecting the magnetic field. The use of this model for online 3D-SDC of MR-Linac plans was validated in a three-step process: (1) comparison to measured beam data, (2) investigation of performance and limitations in a planning phantom and (3) clinical validation using n = 100 patient plans from different tumor entities, comparing the developed 3D-SDC with experimental plan QA. Results: The developed model showed median gamma passing rates compared to MR-Linac base data of 84.7%, 100% and 99.1% for crossplane, inplane and depth-dose-profiles, respectively. Comparison of 3D-SDC and full dose calculation in a planning phantom revealed that with ⩾ 5 beams gamma passing rates > 95% can be achieved for central target locations. With a median calculation time of 1:23 min, 3D-SDC of online adapted clinical MR-Linac plans demonstrated a median gamma passing rate of 98.9% compared to full dose calculation, whereas experimental plan QA reached 99.5%. Conclusion: Here, we describe the first technical 3D-SDC solution for online adaptive MR-guided radiotherapy. For clinical situations with peripheral targets and a small number of beams additional verification appears necessary. Further improvement may include 3D-SDC with consideration of the magnetic field.