title: A semi-automated 2D/3D marker-based registration algorithm modelling prostate shrinkage during radiotherapy for prostate cancer.
abstract: OBJECTIVE
  Currently, most available patient alignment tools based on implanted markers use manual marker matching and rigid registration transformations to measure the needed translational shifts. To quantify the particular effect of prostate gland shrinkage, implanted gold markers were tracked during a course of radiotherapy including an isotropic scaling factor to model prostate shrinkage.
  
  
  METHODS
  Eight patients with prostate cancer had gold markers implanted transrectally and seven were treated with (neo) adjuvant androgen deprivation therapy. After patient alignment to skin tattoos, orthogonal electronic portal images (EPIs) were taken. A semi-automated 2D/3D marker-based registration was performed to calculate the necessary couch shifts. The registration consists of a rigid transformation combined with an isotropic scaling to model prostate shrinkage.
  
  
  RESULTS
  The inclusion of an isotropic shrinkage model in the registration algorithm cancelled the corresponding increase in registration error. The mean scaling factor was 0.89+/-0.09. For all but two patients, a decrease of the isotropic scaling factor during treatment was observed. However, there was almost no difference in the translation offset between the manual matching of the EPIs to the digitally reconstructed radiographs and the semi-automated 2D/3D registration. A decrease in the intermarker distance was found correlating with prostate shrinkage rather than with random marker migration.
  
  
  CONCLUSIONS
  Inclusion of shrinkage in the registration process reduces registration errors during a course of radiotherapy. Nevertheless, this did not lead to a clinically significant change in the proposed table translations when compared to translations obtained with manual marker matching without a scaling correction.
authors.affiliation: Department of Radiotherapy, University Hospitals Leuven, Belgium. tom.budiharto@uz.kuleuven.ac.be
authors.forename: Tom
authors.initials: T
authors.lastname: Budiharto
authors.forename: Pieter
authors.initials: P
authors.lastname: Slagmolen
authors.forename: Jeroen
authors.initials: J
authors.lastname: Hermans
authors.forename: Frederik
authors.initials: F
authors.lastname: Maes
authors.forename: Jan
authors.initials: J
authors.lastname: Verstraete
authors.forename: Frank Van den
authors.initials: FV
authors.lastname: Heuvel
authors.forename: Tom
authors.initials: T
authors.lastname: Depuydt
authors.forename: Raymond
authors.initials: R
authors.lastname: Oyen
authors.forename: Karin
authors.initials: K
authors.lastname: Haustermans
mesh_terms: D000230:Adenocarcinoma / Q000188:drug therapy / Q000532:radiotherapy*; D000465:Algorithms; D000726:Androgen Antagonists / Q000627:therapeutic use; D003131:Combined Modality Therapy; D006046:Gold; D006801:Humans; D021621:Imaging, Three-Dimensional; D008297:Male; D008962:Models, Theoretical; D011467:Prostate / Q000473:pathology* / Q000528:radiation effects; D011471:Prostatic Neoplasms / Q000188:drug therapy / Q000532:radiotherapy*
chemical_list: D000726:Androgen Antagonists; D006046:Gold