title: A light-field-based method to adjust on-axis rounded leaf end MLC position to predict off-axis MLC penumbra region dosimetric performance in a radiation therapy planning system.
abstract: OBJECTIVE
  An analytical and experimental study of split shape dose calculation correction by adjusting the position of the on-axis round leaf end position is presented. We use on-axis corrected results to predict off-axis penumbra region dosimetric performance in an intensity-modulated radiation therapy treatment planning system.
  
  
  METHODS
  The precise light-field edge position (X(tang.p)) was derived from the on-axis 50% dose position created by using the nominal light field for geometric and mathematical manipulation. Leaf position (X(mlc.p)) could be derived from X(tang.p) by defining in the treatment planning system for monitor unit calculation. On-axis offset (correction) could be obtained from the position corresponding to 50% of the central axis dose minus the X(mlc.p) position. The off-axis 50% dose position can then be derived from the on-axis 50% dose position.
  
  
  RESULTS
  The monitor unit calculation of the split shape using the on-axis rounded leaf end MLC penumbra region could provide an under-or overdose of 7.5% per millimeter without an offset correction. When using the on-axis rounded leaf end offset correction to predict the off-axis dose, the difference between the off- and on-axis 50% dose position is within ±1.5 mm.
  
  
  CONCLUSIONS
  It is possible to achieve a dose calculation within 0.5% error for an adjusted MLC leaf edge location in the treatment planning system with careful measurement and an accurate on-axis offset correction. Dose calculations located at an off-axis spilt shape region should be used carefully due to noncorrectable errors which were found to be up to 10%.
authors.affiliation: Department of Radiation Oncology, E-Da Hospital, Kaohsiung 824, Taiwan ; Department of Medical Imaging and Radiological Science, Central Taiwan University of Science and Technology, Taichung 411, Taiwan.
authors.forename: Jia-Ming
authors.initials: JM
authors.lastname: Wu
authors.forename: Tsair-Fwu
authors.initials: TF
authors.lastname: Lee
authors.forename: Shyh-An
authors.initials: SA
authors.lastname: Yeh
authors.forename: Kuan-Yin
authors.initials: KY
authors.lastname: Hsiao
authors.forename: Hsin-Hsiung
authors.initials: HH
authors.lastname: Chen
authors.forename: Pei-Ju
authors.initials: PJ
authors.lastname: Chao
authors.forename: Yi-Ting
authors.initials: YT
authors.lastname: Chen
mesh_terms: D006801:Humans; D008027:Light; D009369:Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D011829:Radiation Dosage; D011874:Radiometry / Q000379:methods*