title: Implanted brachytherapy seed movement reflecting transrectal ultrasound probe-induced prostate deformation.
abstract: OBJECTIVE
  Compression of the prostate during transrectal ultrasound-guided permanent prostate brachytherapy is not accounted for during treatment planning. Dosimetry effects are expected to be small but have not been reported. The study aims to characterize the seed movement and prostate deformation due to probe pressure and to estimate the effects on dosimetry.
  
  
  METHODS
  C-arm fluoroscopy imaging was performed to reconstruct the implanted seed distributions (compressed and relaxed prostate) for 10 patients immediately after implantation. The compressed prostate was delineated on ultrasound and registered to the fluoroscopy-derived seed distribution via manual seed localization. Thin-plate spline mapping, generated with implanted seeds as control points, was used to characterize the deformation field and to infer the prostate contour in the absence of probe compression. Differences in TG-43 dosimetry for the compressed prostate and that on probe removal were calculated.
  
  
  RESULTS
  Systematic seed movement patterns were observed on probe removal. Elastic decompression was characterized by expansion in the anterior-posterior direction and contraction in the superior-inferior and lateral directions up to 4 mm. Bilateral shearing in the anterior direction was up to 6 mm, resulting in contraction of the 145 Gy prescription isodose line by 2 mm with potential consequences for the posterior-lateral margin. The average whole prostate D90 increased by 2% of prescription dose (6% max; p < 0.01).
  
  
  CONCLUSIONS
  The current investigation presents a novel study on ultrasound probe-induced deformation. Seed movements were characterized, and the associated dosimetry effects were nonnegligible, contrary to common expectation.
authors.affiliation: Department of Medical Physics, Cross Cancer Institute, Edmonton, Alberta, Canada; Department of Oncology, University of Alberta, Edmonton, Alberta, Canada. Electronic address: Derek.Liu@trilliumhealthpartners.ca.
authors.forename: Derek
authors.initials: D
authors.lastname: Liu
authors.affiliation: Department of Medical Physics, Tom Baker Cancer Centre, Calgary, Alberta, Canada; Department of Oncology, University of Calgary, Calgary, Alberta, Canada.
authors.forename: Tyler
authors.initials: T
authors.lastname: Meyer
authors.affiliation: Department of Oncology, University of Alberta, Edmonton, Alberta, Canada; Department of Radiation Oncology, Cross Cancer Institute, Edmonton, Alberta, Canada.
authors.forename: Nawaid
authors.initials: N
authors.lastname: Usmani
authors.affiliation: Department of Medical Physics and Bioengineering, Canterbury District Health Board, Christchurch, New Zealand.
authors.forename: Ian
authors.initials: I
authors.lastname: Kay
authors.affiliation: Department of Oncology, University of Calgary, Calgary, Alberta, Canada; Department of Radiation Oncology, Tom Baker Cancer Centre, Calgary, Alberta, Canada.
authors.forename: Siraj
authors.initials: S
authors.lastname: Husain
authors.affiliation: Department of Oncology, University of Calgary, Calgary, Alberta, Canada; Department of Radiation Oncology, Tom Baker Cancer Centre, Calgary, Alberta, Canada.
authors.forename: Steve
authors.initials: S
authors.lastname: Angyalfi
authors.affiliation: Department of Medical Physics, Cross Cancer Institute, Edmonton, Alberta, Canada; Department of Oncology, University of Alberta, Edmonton, Alberta, Canada.
authors.forename: Ron
authors.initials: R
authors.lastname: Sloboda
mesh_terms: D001918:Brachytherapy; D019160:Endosonography / Q000295:instrumentation / Q000379:methods*; D006801:Humans; D008297:Male; D009038:Motion; D011312:Pressure; D011467:Prostate; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D019919:Prosthesis Implantation / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D013314:Stress, Mechanical
keywords: C-arm fluoroscopy; Dosimetry; Prostate brachytherapy; Transrectal ultrasound-induced deformation