title: Megavoltage cone beam computed tomography dose and the necessity of reoptimization for imaging dose-integrated intensity-modulated radiotherapy for prostate cancer.
abstract: OBJECTIVE
  Megavoltage cone beam computed tomography (MV-CBCT) dose can be integrated with the patient's prescription. Here, we investigated the effects of imaging dose and the necessity for additional optimization when using intensity-modulated radiotherapy (IMRT) to treat prostate cancer.
  
  
  METHODS
  An arc beam mimicking MV-CBCT was generated using XiO (version 4.50; Elekta, Stockholm, Sweden). The monitor units (MU) for dose calculation were determined by conforming the calculated dose to the dose measured using an ionization chamber. IMRT treatment plans of 22 patients with prostate cancer were retrospectively analyzed. Arc beams of 3, 5, 8, and 15 MU were added to the IMRT plans, and the dose covering 95% of the planning target volume (PTV) was normalized to the prescribed dose with (reoptimization) or without optimization (compensation).
  
  
  RESULTS
  PTV homogeneity and conformality changed negligibly with MV-CBCT integration. For critical organs, an imaging dose-dependent increase was observed for the mean rectal/bladder dose (D(mean)), and reoptimization effectively suppressed the D(mean) elevations. The bladder generalized equivalent uniform dose (gEUD) increased with imaging dose, and reoptimization suppressed the gEUD elevation when 5- to 15-MU CBCT were added, although rectal gEUD changed negligibly with any imaging dose. Whereas the dose elevation from the simple addition of the imaging dose uniformly increased rectal and bladder dose, the rectal D(mean) increase of compensation plans was due mainly to low-dose volumes. In contrast, bladder high-dose volumes were increased by integrating the CBCT dose, and reoptimization reduced them when 5- to 15-MU CBCT were added.
  
  
  CONCLUSIONS
  Reoptimization is clearly beneficial for reducing dose to critical organs, elevated by addition of high-MU CBCT, especially for the bladder. For low-MU CBCT aimed at bony structure visualization, compensation is sufficient.
authors.affiliation: Department of Radiation Oncology, Osaka University Graduate School of Medicine, Suita, Osaka, Japan.
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Akino
authors.forename: Masahiko
authors.initials: M
authors.lastname: Koizumi
authors.forename: Iori
authors.initials: I
authors.lastname: Sumida
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Takahashi
authors.forename: Toshiyuki
authors.initials: T
authors.lastname: Ogata
authors.forename: Seiichi
authors.initials: S
authors.lastname: Ota
authors.forename: Fumiaki
authors.initials: F
authors.lastname: Isohashi
authors.forename: Koji
authors.initials: K
authors.lastname: Konishi
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Yoshioka
mesh_terms: D054893:Cone-Beam Computed Tomography / Q000295:instrumentation / Q000379:methods*; D006801:Humans; D008297:Male; D058958:Organs at Risk / Q000000981:diagnostic imaging / Q000528:radiation effects; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D011829:Radiation Dosage; D011832:Radiation Injuries / Q000517:prevention & control*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D061089:Radiotherapy, Image-Guided / Q000295:instrumentation / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000379:methods*; D012007:Rectum / Q000000981:diagnostic imaging / Q000528:radiation effects; D012189:Retrospective Studies; D012669:Seminal Vesicles / Q000000981:diagnostic imaging / Q000528:radiation effects; D047368:Tumor Burden; D001743:Urinary Bladder / Q000000981:diagnostic imaging / Q000528:radiation effects