title: Investigation of the geometric accuracy of proton beam irradiation in the liver.
abstract: OBJECTIVE
  To investigate the geometric accuracy of proton beam irradiation to the liver by measuring the change in Hounsfield units (HUs) after irradiation.
  
  
  METHODS
  We examined 21 patients with liver tumors who were treated with respiratory-gated proton beam therapy (PBT). The radiation dose was 66 GyE in 12 patients and 72.6 GyE in 9 patients. Image registration and reslicing of the computed tomography (CT) results obtained within 1 month before and 3 months after PBT was performed, referring to the planning CT image. The resliced CT images obtained after PBT were subtracted from the images obtained before PBT. We investigated whether the area of the large HU change was consistent with the high-dose distribution area using the location of the largest change in HU around the tumor (peak) on the subtracted CT image and the 90% dose distribution area of the planning CT image.
  
  
  RESULTS
  The number of patients (n = 20) whose left-right peaks were within the 90% dose distribution area was significantly larger than the number of patients whose anterior-posterior peaks and superior-inferior peaks were within the 90% dose distribution area (n = 14, n = 13, p = 0.034, and p = 0.02, respectively). Twelve patients exhibited a peak within the 90% dose distribution area in all directions. Nine of the 11 patients with smaller 90% confidence intervals of the percent normalization of the beam cycle (BC; 90% BC) showed a peak within the 90% dose distribution area in six directions, and this percentage was higher than that among the patients with larger 90% BC (3/10, p = 0.03).
  
  
  CONCLUSIONS
  The geometric accuracy of proton beam irradiation to the liver was higher in the left-right direction than in the other directions. Patients with an irregular respiratory rhythm have a greater risk of a reduced geometric accuracy of PBT in the liver.
authors.affiliation: Proton Medical Research Center, Graduate School of Comprehensive Human Sciences, University of Tsukuba, Tsukuba, Japan. fukumitsun@yahoo.co.jp
authors.forename: Nobuyoshi
authors.initials: N
authors.lastname: Fukumitsu
authors.forename: Takayuki
authors.initials: T
authors.lastname: Hashimoto
authors.forename: Toshiyuki
authors.initials: T
authors.lastname: Okumura
authors.forename: Masashi
authors.initials: M
authors.lastname: Mizumoto
authors.forename: Eriko
authors.initials: E
authors.lastname: Tohno
authors.forename: Kuniaki
authors.initials: K
authors.lastname: Fukuda
authors.forename: Masato
authors.initials: M
authors.lastname: Abei
authors.forename: Takeji
authors.initials: T
authors.lastname: Sakae
authors.forename: Hideyuki
authors.initials: H
authors.lastname: Sakurai
mesh_terms: D000368:Aged; D016001:Confidence Intervals; D003720:Densitometry; D005260:Female; D006801:Humans; D008099:Liver / Q000000981:diagnostic imaging; D008113:Liver Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D008297:Male; D008875:Middle Aged; D061766:Proton Therapy; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D055425:Respiratory-Gated Imaging Techniques / Q000379:methods*; D013382:Subtraction Technique; D014057:Tomography, X-Ray Computed / Q000379:methods; D011522:Protons
chemical_list: D011522:Protons