title: Experimental measurements and Monte Carlo simulations of dose perturbation around a nonradioactive brachytherapy seed due to 6- and 18-MV photons.
abstract: OBJECTIVE
  Radioactive seeds used in permanent prostate brachytherapy are composed of high-Z metals and may exceed 100 in a patient. If supplemental external beam treatment is administered afterward, the seeds may cause substantial dose perturbation, which is being investigated in this article.
  
  
  METHODS
  Film measurements using 6-MV beam were primarily carried out using Kodak XV2 film layered above and below a nonradioactive iodine-125 ((125)I) seed. Monte Carlo simulations were carried out using DOSXYZnrc. Other experimental comparisons looked at changing beam energy, depth, and field size, including two opposing fields' pair. Effect of multiple seeds spatially spaced 0.5cm vertically was also studied.
  
  
  RESULTS
  For a single (125)I seed, on XV film, there is a localized dose enhancement of 6.3% upstream and -10.9% downstream. With two opposing fields, a cold spot around the seed of ∼3% was noticed. Increasing beam energy and field size decreased the magnitude of this effect, whereas the effect was found to increase with the increasing Z of material. DOSXYZnrc and EBT-2 film verified maximum dose enhancement of +15% upstream and -20% downstream of the (125)I seed surface.
  
  
  CONCLUSIONS
  In general, the dose perturbation because of the seeds was spatially limited to ∼2mm upstream and ∼5mm downstream to the incident beam. Similar to other heterogeneities, the seeds perturbation depends on incident beam energy, field size, and its Z. With multiple seeds spatially apart and multiple radiation fields routinely used in external beam radiotherapy, the cumulative effect may not result in clinically significant dose perturbation.
authors: Department of Radiation Medicine, Roswell Park Cancer Institute, Buffalo, NY 14263, USA.
authors: James Paul
authors: JP
authors: Steinman
authors: Mohammad
authors: M
authors: Bakhtiari
authors: Harish Kumar
authors: HK
authors: Malhotra
mesh_terms: D001918:Brachytherapy / Q000379:methods*; D002586:Cesium; D003198:Computer Simulation; D006801:Humans; D007455:Iodine; D008297:Male; D008670:Metals / Q000627:therapeutic use*; D009010:Monte Carlo Method; D010165:Palladium; D017785:Photons / Q000627:therapeutic use*; D011471:Prostatic Neoplasms / Q000532:radiotherapy; D011829:Radiation Dosage; D011868:Radioisotopes / Q000627:therapeutic use*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods
chemical_list: D008670:Metals; D011868:Radioisotopes; D002586:Cesium; D010165:Palladium; D007455:Iodine