방사선의학 웹진

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Abstract
Purpose: This study aims to investigate a star shot analysis using a three-dimensional (3D) gel dosimeter for the imaging and radiation isocenter verification of a magnetic resonance linear accelerator (MR-Linac).

Methods: A mixture of methacrylic acid, gelatin, and tetrakis (hydroxymethyl) phosphonium chloride, called MAGAT gel, was fabricated. One MAGAT gel for each Linac and MR-Linac was irradiated under six gantry angles. A 6 MV photon beam of Linac and a 6 MV flattening filter free beam of MR-Linac were delivered to two MAGAT gels and EBT3 films. MR images were acquired by MR-Linac with a clinical sequence (i.e., TrueFISP). The 3D star shot analysis for seven consecutive slices of the MR images with TrueFISP was performed. The 2D star shot analysis for the central plane of the gel was compared to the results from the EBT3 films. The radius of isocircle (ICr ) and the distance between the center of the circle and the center marked on the image (ICd ) were evaluated.

Results: For MR-Linac with MAGAT gel measurements, ICd at the central plane was 0.46 mm for TrueFISP. Compared to EBT3 film measurements, the differences in ICd and ICr for both Linac and MR-Linac were within 0.11 and 0.13 mm, respectively. For the 3D analysis, seven consecutive slices of TrueFISP images were analyzed and the maximum radii of isocircles (ICr_max ) were 0.18 mm for Linac and 0.73 mm for MR-Linac. The tilting angles of radiation axis were 0.31° for Linac and 0.10° for MR-Linac.

Conclusion: The accuracy of 3D star shot analysis using MAGAT gel was comparable to that of EBT3 film, having a capability for integrated analysis for imaging isocenter and radiation isocenter. 3D star shot analysis using MAGAT gel can provide 3D information of radiation isocenter, suggesting a quantitative extent of gantry-tilting.

Affiliations

Jeong Ho Kim  1 , Bitbyeol Kim  2 , Wook-Geun Shin  2 , Jaeman Son  2   3   4 , Chang Heon Choi  2   3   4 , Jong Min Park  2   3   4   5   6 , Ui-Jung Hwang  7 , Jung-In Kim  2   3   4 , Seongmoon Jung  2   3   4
1 Department of Radiation Oncology, Samsung Changwon Hospital, Sungkyunkwan University School of Medicine, Changwon, Republic of Korea.
2 Department of Radiation Oncology, Seoul National University Hospital, Seoul, Republic of Korea.
3 Institute of Radiation Medicine, Seoul National University Medical Research Center, Seoul, Republic of Korea.
4 Biomedical Research Institute, Seoul National University Hospital, Seoul, Republic of Korea.
5 Department of Radiation Oncology, Seoul National University College of Medicine, Seoul, Republic of Korea.
6 Robotics Research Laboratory for Extreme Environments, Advanced Institute of Convergence Technology, Suwon, Republic of Korea.
7 Department of Radiation Oncology, Chungnam National University Sejong Hospital, Sejong, Republic of Korea.