title: Magnitude of residual internal anatomy motion on heavy charged particle dose distribution in respiratory gated lung therapy.
abstract: OBJECTIVE
  To assess the variation in carbon beam dose distribution due to residual motion in lung cancer patients undergoing respiratory-gated radiotherapy.
  
  
  METHODS
  A total of 11 lung cancer patients underwent four-dimensional computed tomography with a 256-multislice computed tomography scanner under free-breathing conditions. A compensating bolus was designed to cover the treatment beam for all planning target volumes during a 30% duty cycle centered on exhalation (gating window). This bolus was applied to the four-dimensional computed tomography data for one respiratory cycle, and then the carbon beam dose distribution was calculated.
  
  
  RESULTS
  A water equivalent pathlength variation of <5 mm was observed in the gating window, but this increased to </=20 mm on inhalation. As a result, beam overshoot/undershoot occurred around inhalation, which increased the excessive dosing to normal tissues and the organs at risk. The dose for >95% volume irradiation is dependent on the respiratory phase but not the gating window. However, the dose for >95% volume irradiation correlated well with the tumor displacement distance. More than 90% of the dose for >95% volume irradiation could be delivered in the gating window with <4-mm tumor displacement resulting from exhalation.
  
  
  CONCLUSIONS
  The results of our study have shown that even when the treatment beam delivery occurs outside the gating window, the prescribed dose to the target is not affected in patients with a tumor displacement of <4 mm. Thus, respiratory gating is not required in radiotherapy for patients with <4-mm tumor displacement in a respiratory cycle.
authors.affiliation: Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, Chiba, Japan. shinshin@nirs.go.jp
authors.forename: Shinichiro
authors.initials: S
authors.lastname: Mori
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Asakura
authors.forename: Susumu
authors.initials: S
authors.lastname: Kandatsu
authors.forename: Motoki
authors.initials: M
authors.lastname: Kumagai
authors.forename: Masayuki
authors.initials: M
authors.lastname: Baba
authors.forename: Masahiro
authors.initials: M
authors.lastname: Endo
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D000465:Algorithms; D006801:Humans; D001239:Inhalation; D008168:Lung / Q000000981:diagnostic imaging / Q000473:pathology; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy*; D008875:Middle Aged; D009068:Movement; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D012119:Respiration; D015898:Tomography Scanners, X-Ray Computed; D014057:Tomography, X-Ray Computed / Q000379:methods*