title: Prostate localization on daily cone-beam computed tomography images: accuracy assessment of similarity metrics.
abstract: OBJECTIVE
  To evaluate different similarity metrics (SM) using natural calcifications and observation-based measures to determine the most accurate prostate and seminal vesicle localization on daily cone-beam CT (CBCT) images.
  
  
  METHODS
  CBCT images of 29 patients were retrospectively analyzed; 14 patients with prostate calcifications (calcification data set) and 15 patients without calcifications (no-calcification data set). Three groups of test registrations were performed. Test 1: 70 CT/CBCT pairs from calcification dataset were registered using 17 SMs (6,580 registrations) and compared using the calcification mismatch error as an endpoint. Test 2: Using the four best SMs from Test 1, 75 CT/CBCT pairs in the no-calcification data set were registered (300 registrations). Accuracy of contour overlays was ranked visually. Test 3: For the best SM from Tests 1 and 2, accuracy was estimated using 356 CT/CBCT registrations. Additionally, target expansion margins were investigated for generating registration regions of interest.
  
  
  RESULTS
  Test 1-Incremental sign correlation (ISC), gradient correlation (GC), gradient difference (GD), and normalized cross correlation (NCC) showed the smallest errors (mu +/- sigma: 1.6 +/- 0.9 approximately 2.9 +/- 2.1 mm). Test 2-Two of the three reviewers ranked GC higher. Test 3-Using GC, 96% of registrations showed <3-mm error when calcifications were filtered. Errors were left/right: 0.1 +/- 0.5mm, anterior/posterior: 0.8 +/- 1.0mm, and superior/inferior: 0.5 +/- 1.1 mm. The existence of calcifications increased the success rate to 97%. Expansion margins of 4-10 mm were equally successful.
  
  
  CONCLUSIONS
  Gradient-based SMs were most accurate. Estimated error was found to be <3 mm (1.1 mm SD) in 96% of the registrations. Results suggest that the contour expansion margin should be no less than 4 mm.
authors.affiliation: Department of Radiation Oncology, Henry Ford Health System, Detroit, MI 48202, USA. jkim3@hfhs.org
authors.forename: Jinkoo
authors.initials: J
authors.lastname: Kim
authors.forename: Rabih
authors.initials: R
authors.lastname: Hammoud
authors.forename: Deepak
authors.initials: D
authors.lastname: Pradhan
authors.forename: Hualiang
authors.initials: H
authors.lastname: Zhong
authors.forename: Ryan Y
authors.initials: RY
authors.lastname: Jin
authors.forename: Benjamin
authors.initials: B
authors.lastname: Movsas
authors.forename: Indrin J
authors.initials: IJ
authors.lastname: Chetty
mesh_terms: D002114:Calcinosis / Q000000981:diagnostic imaging*; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D006801:Humans; D008297:Male; D011467:Prostate / Q000000981:diagnostic imaging*; D011469:Prostatic Diseases / Q000000981:diagnostic imaging*; D011881:Radiotherapy, Computer-Assisted / Q000379:methods*; D012189:Retrospective Studies; D012669:Seminal Vesicles / Q000000981:diagnostic imaging*; D014057:Tomography, X-Ray Computed / Q000379:methods