title: Monitoring of breathing motion in image-guided PBS proton therapy: comparative analysis of optical and electromagnetic technologies.
abstract: BACKGROUND
  Motion monitoring is essential when treating non-static tumours with pencil beam scanned protons. 4D medical imaging typically relies on the detected body surface displacement, considered as a surrogate of the patient's anatomical changes, a concept similarly applied by most motion mitigation techniques. In this study, we investigate benefits and pitfalls of optical and electromagnetic tracking, key technologies for non-invasive surface motion monitoring, in the specific environment of image-guided, gantry-based proton therapy.
  
  
  METHODS
  Polaris SPECTRA optical tracking system and the Aurora V3 electromagnetic tracking system from Northern Digital Inc. (NDI, Waterloo, CA) have been compared both technically, by measuring tracking errors and system latencies under laboratory conditions, and clinically, by assessing their practicalities and sensitivities when used with imaging devices and PBS treatment gantries. Additionally, we investigated the impact of using different surrogate signals, from different systems, on the reconstructed 4D CT images.
  
  
  RESULTS
  Even though in controlled laboratory conditions both technologies allow for the localization of static fiducials with sub-millimetre jitter and low latency (31.6 ± 1 msec worst case), significant dynamic and environmental distortions limit the potential of the electromagnetic approach in a clinical setting. The measurement error in case of close proximity to a CT scanner is up to 10.5 mm and precludes its use for the monitoring of respiratory motion during 4DCT acquisitions. Similarly, the motion of the treatment gantry distorts up to 22 mm the tracking result.
  
  
  CONCLUSIONS
  Despite the line of sight requirement, the optical solution offers the best potential, being the most robust against environmental factors and providing the highest spatial accuracy. The significant difference in the temporal location of the reconstructed phase points is used to speculate on the need to apply the same monitoring system for imaging and treatment to ensure the consistency of detected phases.
authors.affiliation: Center for Proton Therapy, Paul Scherrer Institut, 5232, Villigen, PSI, Switzerland.
authors.forename: Giovanni
authors.initials: G
authors.lastname: Fattori
authors.affiliation: Center for Proton Therapy, Paul Scherrer Institut, 5232, Villigen, PSI, Switzerland.
authors.forename: Sairos
authors.initials: S
authors.lastname: Safai
authors.affiliation: Center for Proton Therapy, Paul Scherrer Institut, 5232, Villigen, PSI, Switzerland.
authors.forename: Pablo Fernández
authors.initials: PF
authors.lastname: Carmona
authors.affiliation: Center for Proton Therapy, Paul Scherrer Institut, 5232, Villigen, PSI, Switzerland.
authors.forename: Marta
authors.initials: M
authors.lastname: Peroni
authors.affiliation: Center for Proton Therapy, Paul Scherrer Institut, 5232, Villigen, PSI, Switzerland.
authors.forename: Rosalind
authors.initials: R
authors.lastname: Perrin
authors.affiliation: Center for Proton Therapy, Paul Scherrer Institut, 5232, Villigen, PSI, Switzerland.|Radiation Oncology Department, Inselspital Universitätsspital Bern, 3010, Bern, Switzerland.
authors.forename: Damien Charles
authors.initials: DC
authors.lastname: Weber
authors.affiliation: Center for Proton Therapy, Paul Scherrer Institut, 5232, Villigen, PSI, Switzerland. tony.lomax@psi.ch.|Department of Physics, ETH-Hönggerberg, 8093, Zurich, Switzerland. tony.lomax@psi.ch.
authors.forename: Antony John
authors.initials: AJ
authors.lastname: Lomax
mesh_terms: D016477:Artifacts; D055590:Electromagnetic Phenomena; D056973:Four-Dimensional Computed Tomography / Q000379:methods; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D009038:Motion; D009068:Movement; D019047:Phantoms, Imaging; D061766:Proton Therapy / Q000379:methods*; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D012119:Respiration
keywords: CT imaging; Electromagnetic tracking; Gantry; Optical tracking; Proton therapy; Respiratory motion