title: Accuracy of seed reconstruction in prostate postplanning studied with a CT- and MRI-compatible phantom.
abstract: OBJECTIVE
  Postimplant dosimetry of prostate seed implants is usually performed by seed localisation on transversal CT or MR images. In order to obtain reliable dosimetric evaluation data, it is important that seeds are reconstructed accurately. Currently, there is no comparative data available on seed localisation accuracy of CT-and MRI-based reconstructions, mainly due to the lack of a suitable QA tool. In this study, we developed a CT-and MRI compatible prostate phantom to investigate the intrinsic accuracy of seed detection for both imaging modalities.
  
  
  METHODS
  A 60 seed geometry was created according to a clinically meaningful plan, including rotated and shifted seeds. After implantation of the seeds in the phantom, CT and MRI scans with 3, 4 and 5mm slice thickness were performed. The seed locations were reconstructed in the treatment planning system and compared with the known reference positions.
  
  
  RESULTS
  Due to the comparable density and relaxation times of the phantom material to prostate tissue, the seeds are visualised similarly as on real patient images. The observed mean reconstruction uncertainties were in general smaller for CT (0.9+/-0.6, 0.9+/-0.6, 2.1+/-0.8 mm on 3, 4 and 5mm scans, respectively), than for MRI (Philips 1.5 T: 2.1+/-1.4, 1.6+/-1.2, 1.9+/-0.9 mm on 3, 4 and 5 mm scans, respectively, and Siemens 1.5 T: 2.3+/-0.8, 2.0+/-1.6, 1.6+/-0.8 mm on 3, 4 and 5mm scans, respectively).
  
  
  CONCLUSIONS
  For our clinical sequences of both CT and MRI, the mean deviation of the reconstructed seed positions were all within acceptable limits for clinical use (<2.3 mm). The phantom was found to be a suitable quality assurance tool to assess the reliability and accuracy of the seed reconstruction procedure. Moreover, as the phantom material has the same imaging characteristics as real prostate tissue, it is a useful device to define proper MRI sequences.
authors.affiliation: Department of Radiotherapy, University Hospital Leuven, Belgium. marisol.debrabandere@uz.kuleuven.ac.be
authors.forename: Marisol
authors.initials: M
authors.lastname: De Brabandere
authors.forename: Christian
authors.initials: C
authors.lastname: Kirisits
authors.forename: Ronald
authors.initials: R
authors.lastname: Peeters
authors.forename: Karin
authors.initials: K
authors.lastname: Haustermans
authors.forename: Frank
authors.initials: F
authors.lastname: Van den Heuvel
mesh_terms: D001918:Brachytherapy / Q000379:methods* / Q000592:standards*; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000295:instrumentation; D008297:Male; D019047:Phantoms, Imaging; D011467:Prostate / Q000033:anatomy & histology; D011471:Prostatic Neoplasms / Q000532:radiotherapy; D011786:Quality Control; D011834:Radiation Monitoring / Q000295:instrumentation / Q000379:methods*; D011881:Radiotherapy, Computer-Assisted; D014057:Tomography, X-Ray Computed / Q000295:instrumentation