title: Image-based dose planning of intracavitary brachytherapy: registration of serial-imaging studies using deformable anatomic templates.
abstract: OBJECTIVE
  To demonstrate that high-dimensional voxel-to-voxel transformations, derived from continuum mechanics models of the underlying pelvic tissues, can be used to register computed tomography (CT) serial examinations into a single anatomic frame of reference for cumulative dose calculations.
  
  
  METHODS
  Three patients with locally advanced cervix cancer were treated with CT-compatible intracavitary (ICT) applicators. Each patient underwent five volumetric CT examinations: before initiating treatment, and immediately before and after the first and second ICT insertions, respectively. Each serial examination was rigidly registered to the patient's first ICT examination by aligning the bony anatomy. Detailed nonrigid alignment for organs (or targets) of interest was subsequently achieved by deforming the CT exams as a viscous-fluid, described by the Navier-Stokes equation, until the coincidence with the corresponding targets on CT image was maximized. In cases where ICT insertion induced very large and topologically complex rearrangements of pelvic organs, e.g., extreme uterine canal reorientation following tandem insertion, a viscous-fluid-landmark transformation was used to produce an initial registration.
  
  
  RESULTS
  For all three patients, reasonable registrations for organs (or targets) of interest were achieved. Fluid-landmark initialization was required in 4 of the 11 registrations. Relative to the best rigid bony landmark alignment, the viscous-fluid registration resulted in average soft-tissue displacements from 2.8 to 28.1 mm, and improved organ coincidence from the range of 5.2% to 72.2% to the range of 90.6% to 100%. Compared to the viscous-fluid transformation, global registration of bony anatomy mismatched 5% or more of the contoured organ volumes by 15-25 mm.
  
  
  CONCLUSIONS
  Pelvic soft-tissue structures undergo large deformations and displacements during the external-beam and multiple-ICT course of radiation therapy for locally advanced cervix cancer. These changes cannot be modeled by the conventional rigid landmark transformation method. In the current study, we found that the deformable anatomic template registration method, based on continuum-mechanics models of deformation, successfully described these large anatomic shape changes before and after ICT. These promising modeling results indicate that realistic registration of the cumulative dose distribution to the organs (or targets) of interest for radiation therapy of cervical cancers is achievable.
authors.affiliation: Department of Electrical Engineering, University of Iowa, Iowa City, IA, USA.
authors.forename: G E
authors.initials: GE
authors.lastname: Christensen
authors.forename: B
authors.initials: B
authors.lastname: Carlson
authors.forename: K S
authors.initials: KS
authors.lastname: Chao
authors.forename: P
authors.initials: P
authors.lastname: Yin
authors.forename: P W
authors.initials: PW
authors.lastname: Grigsby
authors.forename: K
authors.initials: K
authors.lastname: Nguyen
authors.forename: J F
authors.initials: JF
authors.lastname: Dempsey
authors.forename: F A
authors.initials: FA
authors.lastname: Lerma
authors.forename: K T
authors.initials: KT
authors.lastname: Bae
authors.forename: M W
authors.initials: MW
authors.lastname: Vannier
authors.forename: J F
authors.initials: JF
authors.lastname: Williamson
mesh_terms: D000465:Algorithms; D001918:Brachytherapy / Q000379:methods*; D012809:Colon, Sigmoid / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D005260:Female; D006801:Humans; D007047:Hysterosalpingography; D008962:Models, Theoretical; D010388:Pelvis / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D011446:Prospective Studies; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000379:methods*; D012007:Rectum / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D014057:Tomography, X-Ray Computed / Q000379:methods*; D001743:Urinary Bladder / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D002583:Uterine Cervical Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy*; D014599:Uterus / Q000033:anatomy & histology; D014621:Vagina / Q000033:anatomy & histology / Q000000981:diagnostic imaging