title: A comparison of in-room computerized tomography options for detection of fiducial markers in prostate cancer radiotherapy.
abstract: OBJECTIVE
  To compare volumetric in-room computed tomography (CT) and kilovoltage (kV) cone-beam CT (CBCT) to planar imaging with respect to their ability to localize fiducial markers (FMs) for radiotherapy of prostate cancer.
  
  
  METHODS
  Image guidance options from two linear accelerators were compared in terms of identifying the center of gravity (COG) of FMs from the isocenter: a Siemens Primatom, where the couch is rotated 180 degrees from the treatment isocenter to the in-room CT vs. electronic portal imaging (EPI); and a Varian OBI system, where kV CBCT, EPI, and planar kV radiographs were compared. In all, 387 image pairs (CBCT = 133; CT = 254) from 18 patients were analyzed. A clinical tolerance of 3 mm was predefined as the acceptable threshold for agreement.
  
  
  RESULTS
  COG location on in-room CT and EPI was in agreement 96.9%, 85.8%, and 89.0% of the time in the left-right (LR), superior-inferior (SI), and anterior-posterior (AP) directions, respectively, vs. 99.2%, 91.7%, and 93.2% for the CBCT and EPI analysis. The CBCT vs. kV radiographs were in agreement 100% (LR), 85.4% (SI), and 88.5% (AP), and EPI vs. kV radiographs were in agreement 100% (LR), 94.6% (SI), and 91.5% (AP) of the time.
  
  
  CONCLUSIONS
  Identification of FMs on volumetric or planar images was found to be not equivalent (+/-3 mm) using either linear accelerator. Intrafraction prostate motion, interpretation of FM location, and spatial properties of images are contributing factors. Although in-room CT has superior image quality, the process of realigning the treatment couch to acquire a CT introduces an error, highlighting the benefits of a single isocentric system.
authors.affiliation: Radiation Therapy Services, Peter MacCallum Cancer Centre, Victoria, Australia. Rebecca_Owen@health.qld.gov.au
authors.forename: Rebecca
authors.initials: R
authors.lastname: Owen
authors.forename: Farshad
authors.initials: F
authors.lastname: Foroudi
authors.forename: Tomas
authors.initials: T
authors.lastname: Kron
authors.forename: Alvin
authors.initials: A
authors.lastname: Milner
authors.forename: Jennifer
authors.initials: J
authors.lastname: Cox
authors.forename: Jim
authors.initials: J
authors.lastname: Cramb
authors.forename: Li
authors.initials: L
authors.lastname: Zhu
authors.forename: Gillian
authors.initials: G
authors.lastname: Duchesne
mesh_terms: D000465:Algorithms; D054893:Cone-Beam Computed Tomography / Q000379:methods; D019583:Dose Fractionation, Radiation; D006801:Humans; D008297:Male; D009068:Movement; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D011467:Prostate / Q000000981:diagnostic imaging; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D019736:Prostheses and Implants; D011881:Radiotherapy, Computer-Assisted / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000379:methods; D014057:Tomography, X-Ray Computed / Q000379:methods*