title: Statistical modeling of the eye for multimodal treatment planning for external beam radiation therapy of intraocular tumors.
abstract: OBJECTIVE
  Ocular anatomy and radiation-associated toxicities provide unique challenges for external beam radiation therapy. For treatment planning, precise modeling of organs at risk and tumor volume are crucial. Development of a precise eye model and automatic adaptation of this model to patients' anatomy remain problematic because of organ shape variability. This work introduces the application of a 3-dimensional (3D) statistical shape model as a novel method for precise eye modeling for external beam radiation therapy of intraocular tumors.
  
  
  METHODS
  Manual and automatic segmentations were compared for 17 patients, based on head computed tomography (CT) volume scans. A 3D statistical shape model of the cornea, lens, and sclera as well as of the optic disc position was developed. Furthermore, an active shape model was built to enable automatic fitting of the eye model to CT slice stacks. Cross-validation was performed based on leave-one-out tests for all training shapes by measuring dice coefficients and mean segmentation errors between automatic segmentation and manual segmentation by an expert.
  
  
  RESULTS
  Cross-validation revealed a dice similarity of 95%±2% for the sclera and cornea and 91%±2% for the lens. Overall, mean segmentation error was found to be 0.3±0.1 mm. Average segmentation time was 14±2 s on a standard personal computer.
  
  
  CONCLUSIONS
  Our results show that the solution presented outperforms state-of-the-art methods in terms of accuracy, reliability, and robustness. Moreover, the eye model shape as well as its variability is learned from a training set rather than by making shape assumptions (eg, as with the spherical or elliptical model). Therefore, the model appears to be capable of modeling nonspherically and nonelliptically shaped eyes.
authors.affiliation: ARTORG Center for Biomedical Engineering Research, University of Bern, and Department of Radiation Oncology, Inselspital, Bern University Hospital, Bern, Switzerland.
authors.forename: Michael B
authors.initials: MB
authors.lastname: Rüegsegger
authors.forename: Meritxell
authors.initials: M
authors.lastname: Bach Cuadra
authors.forename: Alessia
authors.initials: A
authors.lastname: Pica
authors.forename: Christoph A
authors.initials: CA
authors.lastname: Amstutz
authors.forename: Tobias
authors.initials: T
authors.lastname: Rudolph
authors.forename: Daniel
authors.initials: D
authors.lastname: Aebersold
authors.forename: Jens H
authors.initials: JH
authors.lastname: Kowal
mesh_terms: D000328:Adult; D000368:Aged; D003198:Computer Simulation; D003315:Cornea / Q000000981:diagnostic imaging; D005123:Eye / Q000033:anatomy & histology* / Q000000981:diagnostic imaging; D005134:Eye Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D005260:Female; D006801:Humans; D007908:Lens, Crystalline / Q000000981:diagnostic imaging; D008297:Male; D008488:Medical Illustration; D008875:Middle Aged; D008954:Models, Biological; D015233:Models, Statistical; D009898:Optic Disk / Q000000981:diagnostic imaging; D058958:Organs at Risk / Q000000981:diagnostic imaging; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results; D012590:Sclera / Q000000981:diagnostic imaging; D014057:Tomography, X-Ray Computed / Q000379:methods