title: Patient dose from megavoltage computed tomography imaging.
abstract: OBJECTIVE
  Megavoltage computed tomography (MVCT) can be used daily for imaging with a helical tomotherapy unit for patient alignment before treatment delivery. The purpose of this investigation was to show that the MVCT dose can be computed in phantoms, and further, that the dose can be reported for actual patients from MVCT on a helical tomotherapy unit.
  
  
  METHODS
  An MVCT beam model was commissioned and verified through a series of absorbed dose measurements in phantoms. This model was then used to retrospectively calculate the imaging doses to the patients. The MVCT dose was computed for five clinical cases: prostate, breast, head/neck, lung, and craniospinal axis.
  
  
  RESULTS
  Validation measurements in phantoms verified that the computed dose can be reported to within 5% of the measured dose delivered at the helical tomotherapy unit. The imaging dose scaled inversely with changes to the CT pitch. Relative to a normal pitch of 2.0, the organ dose can be scaled by 0.67 and 2.0 for scans done with a pitch of 3.0 and 1.0, respectively. Typical doses were in the range of 1.0-2.0 cGy, if imaged with a normal pitch. The maximal organ dose calculated was 3.6 cGy in the neck region of the craniospinal patient, if imaged with a pitch of 1.0.
  
  
  CONCLUSIONS
  Calculation of the MVCT dose has shown that the typical imaging dose is approximately 1.5 cGy per image. The uniform MVCT dose delivered using helical tomotherapy is greatest when the anatomic thickness is the smallest and the pitch is set to the lowest value.
authors.affiliation: Department of Radiation Oncology, M. D. Anderson Cancer Center Orlando, Orlando, FL 32806, USA. Amish.Shah@orhs.org
authors.forename: Amish P
authors.initials: AP
authors.lastname: Shah
authors.forename: Katja M
authors.initials: KM
authors.lastname: Langen
authors.forename: Kenneth J
authors.initials: KJ
authors.lastname: Ruchala
authors.forename: Andrea
authors.initials: A
authors.lastname: Cox
authors.forename: Patrick A
authors.initials: PA
authors.lastname: Kupelian
authors.forename: Sanford L
authors.initials: SL
authors.lastname: Meeks
mesh_terms: D001932:Brain Neoplasms / Q000000981:diagnostic imaging; D001943:Breast Neoplasms / Q000000981:diagnostic imaging; D005260:Female; D006258:Head and Neck Neoplasms / Q000000981:diagnostic imaging; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging; D008297:Male; D019047:Phantoms, Imaging; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging; D011829:Radiation Dosage; D011874:Radiometry; D012189:Retrospective Studies; D013125:Spinal Neoplasms / Q000000981:diagnostic imaging; D036542:Tomography, Spiral Computed / Q000295:instrumentation