title: Beam distortion due to gold fiducial markers during salvage high-intensity focused ultrasound in the prostate.
abstract: OBJECTIVE
  High intensity focused ultrasound (HIFU) provides a non-invasive salvage treatment option for patients with recurrence after external beam radiation therapy (EBRT). As part of EBRT the prostate is frequently implanted with permanent fiducial markers. To date, the impact of these markers on subsequent HIFU treatment is unknown. The objective of this work was to systematically investigate, using computational simulations, how these fiducial markers affect the delivery of HIFU treatment.
  
  
  METHODS
  A series of simulations was performed modelling the propagation of ultrasound pressure waves in the prostate with a single spherical or cylindrical gold marker at different positions and orientations. For each marker configuration, a set of metrics (spatial-peak temporal-average intensity, focus shift, focal volume) was evaluated to quantify the distortion introduced at the focus. An analytical model was also developed describing the marker effect on the intensity at the focus. The model was used to examine the marker's impact in a clinical setting through case studies.
  
  
  RESULTS
  The simulations show that the presence of the marker in the pre-focal region causes reflections which induce a decrease in the focal intensity and focal volume, and a shift of the maximum pressure point away from the transducer's focus. These effects depend on the shape and orientation of the marker and become more pronounced as its distance from the transducer's focus decreases, with the distortion introduced by the marker greatly increasing when placed within 5 mm of the focus. The analytical model approximates the marker's effect and can be used as an alternative method to the computationally intensive and time consuming simulations for quickly estimating the intensity at the focus. A retrospective review of a small patient cohort selected for focal HIFU after failed EBRT indicates that the presence of the marker may affect HIFU treatment delivery.
  
  
  CONCLUSIONS
  The distortion introduced by the marker to the HIFU beam when positioned close to the focus may result in an undertreated region beyond the marker due to less energy arriving at the focus, and an overtreated region due to reflections. Further work is necessary to investigate whether the results presented here justify the revision of the patient selection criteria or the markers' placement protocol.
authors.affiliation: Department of Medical Physics and Biomedical Engineering, University College London, London, UK.
authors.forename: P S
authors.initials: PS
authors.lastname: Georgiou
authors.affiliation: Faculty of Information Technology, Brno University of Technology, Brno, Czech Republic.
authors.forename: J
authors.initials: J
authors.lastname: Jaros
authors.affiliation: Department of Medical Physics and Biomedical Engineering, University College London, London, UK.|Department of Oncology, University College London Hospitals, London, UK.
authors.forename: H
authors.initials: H
authors.lastname: Payne
authors.affiliation: Department of Oncology, University College London Hospitals, London, UK.
authors.forename: C
authors.initials: C
authors.lastname: Allen
authors.affiliation: Division of Surgery and Interventional Science, University College London, London, UK.
authors.forename: T T
authors.initials: TT
authors.lastname: Shah
authors.affiliation: Division of Surgery and Interventional Science, University College London, London, UK.
authors.forename: H U
authors.initials: HU
authors.lastname: Ahmed
authors.affiliation: Department of Medical Physics and Biomedical Engineering, University College London, London, UK.
authors.forename: E
authors.initials: E
authors.lastname: Gibson
authors.affiliation: Department of Medical Physics and Biomedical Engineering, University College London, London, UK.
authors.forename: D
authors.initials: D
authors.lastname: Barratt
authors.affiliation: Department of Medical Physics and Biomedical Engineering, University College London, London, UK.
authors.forename: B E
authors.initials: BE
authors.lastname: Treeby
mesh_terms: D016477:Artifacts; D057918:Fiducial Markers; D006046:Gold; D006801:Humans; D008297:Male; D011471:Prostatic Neoplasms / Q000628:therapy*; D016879:Salvage Therapy / Q000592:standards*; D036761:Ultrasound, High-Intensity Focused, Transrectal / Q000592:standards*
chemical_list: D006046:Gold
keywords: EBRT; fiducial marker; prostate cancer; salvage HIFU