Determination of virtual source position using back projecting zero and convergent arcTAN method for scanning-passive scatter beam in carbon ion therapy
Affiliations: [a] Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, China
| [b] University of Chinese Academy of Sciences, Beijing, China
| [c] Heavy Ion Center of Wuwei Cancer Hospital; Gansu Wuwei Academy of Medical Sciences, Gansu Wuwei Tumor Hospital, Wuwei City, Gansu Province, China
| [d] Department of Medical Physics, Chengde Medical University, Chengde City, Hebei Province, China
| [e] Department of Radiation Oncology, Yee Zen General Hospital, Tao Yuan City, Taiwan
Correspondence:
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Correspondence author: Qiang Li, Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000,
China. E-mail: liqiang@impcas.ac.cn and Jia-Ming Wu, Heavy Ion Center of Wuwei Cancer Hospital, Gansu
Wuwei Academy of Medical Sciences, Wuwei City, Gansu Province, China. Tel.: +82 188 7358 8300; Fax: +82 0735 2343382; jiaming.wu@chmsc.com.
Abstract: OBJECTIVE:This study aims to develop and test a new technique by using the convergent arcTAN (cATAN) method capable of dealing with the virtual source position delivered by different carbon ion energies from the pattern of scanning-passive scatter beam. MATERIALS AND METHODS:A homemade large-format CMOS sensor and Gaf Chromic EBT3 films are used for the virtual source position measurement. The Gaf films are embedded in a self-designed rectangular plastic frame to tighten films and set up on a treatment couch for irradiation in air with the film perpendicular to the carbon ion beam at the nominal source-axis-distance (SAD) as well as upstream and downstream from the SAD. The horizontal carbon ion beam with 5 energies at a machine opening field size is carried out in this study. The virtual source position is determined by using the convergent arcTAN (cATAN) method and compared with a linear regression by back projecting FWHM to zero at a distance upstream from the various source-film-distance. RESULTS:The film FWHM measurement error of 0.5 mm leads to 0.001% deviation of α (cATAN) at every assumed textend. The overall uncertainty for the reproducibility of calculated virtual source position by the assumed textend in the vertical and horizontal directions amounts to 0.1%. The errors of calculated virtual source position by assumed textend with back projecting FWHM to zero methods are within 1.1±0.001, p = 0.033. CONCLUSION:We develop a new technique capable of dealing with the virtual source position with a convergent arcTAN method to avoid any manual measurement mistakes in scanning-passive scatter carbon ion beam. The readers are encouraged to conduct the proposed cATAN method in this study to investigate the virtual source position in the Linac-based external electron beams and the proton beams.
Keywords: Carbon ion beams, virtual source position, scanning-passive scatter beam
