title: Gantry angle-dependent correction of dose detection error due to panel position displacement in IMRT dose verification using EPIDs.
abstract: OBJECTIVE
  The purpose of this study was to measure the mechanical position displacement of three types of electronic portal image device (EPID) panels at different gantry angles and evaluate the impact of positional displacement on intensity modulated radiation therapy (IMRT) dose verification using an EPID.
  
  
  METHODS
  Three types of linear accelerators and EPIDs (aS500, aS1000 and iViewGT) were used. The portal images were taken every 10° within 360° range. The position coordinate difference between the panel center and the portal film center at different gantry angles was measured, then the mechanical position displacement of EPIDs dependent on the gantry angles was analyzed. For the three linear accelerators and EPIDs, five IMRT plans were measured using EPIDs at 0° gantry angel and at the actual treatment angles. The Gamma technique was used to evaluate the resulted dose difference before and after the corrections of the position displacement by a in-house software.
  
  
  RESULTS
  For aS500, aS1000 and iViewGT, the maximum mechanical position displacement was 2.9 ± 0.1 mm, 0.2 ± 0.1 mm and 0.1 ± 0.3 mm in the lateral direction and -4.2 ± 0.2 mm, -4.2 ± 0.1 mm and -2.2 ± 0.1 mm in the longitudinal direction, respectively. The position displacement in the longitudinal direction of the three EPIDs can be fitted well with a function. For aS500, aS1000 and iViewGT, the 3%/3 mm gamma pass rates were increased by 6.7%, 2.9% and 0.1% after displacement corrections; and while the 2%/2 mm gamma pass rates were increased by 11.2%, 8.1% and 1.6%. After the displacement correction, there was a slight gamma pass rate difference between the fixed zero degree gantry and the actual treatment angles.
  
  
  CONCLUSIONS
  When the EPIDs were used for IMRT dose verification, there was occasionally large EPID mechanical position displacement, which should be corrected.
authors.affiliation: School of Physics and Engineering of SUN Yat-sen University, Guangzhou 510060, China; Department of Radiation Oncology, SUN Yat-sen University Cancer Center, State Key Laboratory of Oncology in Southern China, Guangzhou 510060, China.
authors.forename: Guang-hua
authors.initials: GH
authors.lastname: Jin
authors.affiliation: School of Physics and Engineering of SUN Yat-sen University, Guangzhou 510060, China; Department of Radiation Oncology, SUN Yat-sen University Cancer Center, State Key Laboratory of Oncology in Southern China, Guangzhou 510060, China.
authors.forename: Jin-han
authors.initials: JH
authors.lastname: Zhu
authors.affiliation: Department of Radiation Oncology, SUN Yat-sen University Cancer Center, State Key Laboratory of Oncology in Southern China, Guangzhou 510060, China.
authors.forename: Li-xin
authors.initials: LX
authors.lastname: Chen
authors.affiliation: Department of Radiation Oncology, SUN Yat-sen University Cancer Center, State Key Laboratory of Oncology in Southern China, Guangzhou 510060, China.
authors.forename: Xiao-wu
authors.initials: XW
authors.lastname: Deng
authors.affiliation: Department of Radiation Oncology, SUN Yat-sen University Cancer Center, State Key Laboratory of Oncology in Southern China, Guangzhou 510060, China.
authors.forename: Bo-tian
authors.initials: BT
authors.lastname: Huang
authors.affiliation: Department of Radiation Oncology, SUN Yat-sen University Cancer Center, State Key Laboratory of Oncology in Southern China, Guangzhou 510060, China.
authors.forename: Ke
authors.initials: K
authors.lastname: Yuan
authors.affiliation: School of Physics and Engineering of SUN Yat-sen University, Guangzhou 510060, China. Electronic address: stslxw@mail.sysu.edu.cn.
authors.forename: Xiao-wei
authors.initials: XW
authors.lastname: Liu
mesh_terms: D016477:Artifacts; D055615:Electrical Equipment and Supplies; D055595:Mechanical Phenomena; D010315:Particle Accelerators; D011829:Radiation Dosage; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation*; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation*; D015203:Reproducibility of Results; D012399:Rotation
keywords: Dose; EPID; IMRT; Position displacement