title: Dose reduction in LDR brachytherapy by implanted prostate gold fiducial markers.
abstract: OBJECTIVE
  The dosimetric impact of gold fiducial markers (FM) implanted prior to external beam radiotherapy of prostate cancer on low dose rate (LDR) brachytherapy seed implants performed in the context of combined therapy was investigated.
  
  
  METHODS
  A virtual water phantom was designed containing a single FM. Single and multi source scenarios were investigated by performing Monte Carlo dose calculations, along with the influence of varying orientation and distance of the FM with respect to the sources. Three prostate cancer patients treated with LDR brachytherapy for a recurrence following external beam radiotherapy with implanted FM were studied as surrogate cases to combined therapy. FM and brachytherapy seeds were identified on post implant CT scans and Monte Carlo dose calculations were performed with and without FM. The dosimetric impact of the FM was evaluated by quantifying the amplitude of dose shadows and the volume of cold spots. D(90) was reported based on the post implant CT prostate contour.
  
  
  RESULTS
  Large shadows are observed in the single source-FM scenarios. As expected from geometric considerations, the shadows are dependent on source-FM distance and orientation. Large dose reductions are observed at the distal side of FM, while at the proximal side a dose enhancement is observed. In multisource scenarios, the importance of shadows appears mitigated, although FM at the periphery of the seed distribution caused underdosage (<prescription dose). In clinical cases, the FM reduced the dose to some voxels by up to 50% and generated shadows with extents of the order of 4 mm. Within the prostate contour, cold spots (<95% prescription dose) of the order of 20 mm(3) were observed. D(90) proved insensitive to the presence of FM for the cases selected.
  
  
  CONCLUSIONS
  There is a major local impact of FM present in LDR brachytherapy seed implant dose distributions. Therefore, reduced tumor control could be expected from FM implanted in tumors, although our results are too limited to draw conclusions regarding clinical significance.
authors.affiliation: Department of Radiation Oncology, Maastricht University Medical Center, Maastricht, The Netherlands. guillaume.landry@maastro.nl
authors.forename: Guillaume
authors.initials: G
authors.lastname: Landry
authors.forename: Brigitte
authors.initials: B
authors.lastname: Reniers
authors.forename: Ludy
authors.initials: L
authors.lastname: Lutgens
authors.forename: Lars
authors.initials: L
authors.lastname: Murrer
authors.forename: Hossein
authors.initials: H
authors.lastname: Afsharpour
authors.forename: Danielle
authors.initials: D
authors.lastname: de Haas-Kock
authors.forename: Peter
authors.initials: P
authors.lastname: Visser
authors.forename: Francis
authors.initials: F
authors.lastname: van Gils
authors.forename: Frank
authors.initials: F
authors.lastname: Verhaegen
mesh_terms: D001918:Brachytherapy / Q000592:standards*; D057918:Fiducial Markers; D006801:Humans; D008297:Male; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011829:Radiation Dosage; D011879:Radiotherapy Dosage