title: Moving toward focal therapy in prostate cancer: dual-isotope permanent seed implants as a possible solution.
abstract: OBJECTIVE
  To compare the ability of single- and dual-isotope prostate seed implants to escalate biologically effective dose (BED) to foci of disease while reducing prescription dose to the prostate.
  
  
  METHODS
  Nine plans, using 125I, 103Pd, and 131Cs alone and in combination were created retrospectively for 2 patients. Ultrasound and MRI/MRS datasets were used for treatment planning. Voxel-by-voxel BED was calculated for single- and dual-isotope plans. Equivalent uniform BED (EUBED) was used to compare plans. The MRS-positive planning target volumes (PTVi) were delineated along with PTV (prostate+5 mm), rectum, and urethra. Single-isotope implants, prescribed to conventional doses, were generated to achieve good PTV coverage. The PTVi were prospectively used to generate implants using mixtures of isotopes. For mixed-radioisotope implants, we also explored the impact on EUBED of lowering prescription doses by 15%.
  
  
  RESULTS
  The EUBED of PTVi in the setting of primary 125I implant increased 20-66% when 103Pd and 131Cs were used compared with 125I boost. Decreasing prescription dose by 15% in mixed-isotope implants results in a potential 10% reduction in urethral EUBED with preservation of PTV coverage while still boosting PTVi (up to 80%). When radiobiologic parameters corresponding to more-aggressive disease are assigned to foci, faster-decaying isotopes used in mixed implants have the potential to preserve the equivalent biological effect of mono-isotope implants considering less-aggressive disease distributed in the entire prostate.
  
  
  CONCLUSIONS
  This is a hypothesis-generating study proposing a treatment paradigm that could be the middle ground between whole-gland irradiation and focal-only treatment. The use of two isotopes concurrent with decreasing the minimal peripheral dose is shown to increase EUBED of selected subvolumes while preserving the therapeutic effect at the level of the gland.
authors.affiliation: Department of Radiation Oncology, Virginia Commonwealth University, Richmond, VA 23298-0058, USA. dtodor@mcvh-vcu.edu
authors.forename: Dorin A
authors.initials: DA
authors.lastname: Todor
authors.forename: Igor J
authors.initials: IJ
authors.lastname: Barani
authors.forename: Peck-Sun
authors.initials: PS
authors.lastname: Lin
authors.forename: Mitchell S
authors.initials: MS
authors.lastname: Anscher
mesh_terms: D001918:Brachytherapy / Q000009:adverse effects / Q000379:methods*; D002588:Cesium Radioisotopes / Q000627:therapeutic use; D005240:Feasibility Studies; D006207:Half-Life; D006801:Humans; D007457:Iodine Radioisotopes / Q000627:therapeutic use; D008279:Magnetic Resonance Imaging; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D008954:Models, Biological; D009929:Organ Size; D058958:Organs at Risk / Q000528:radiation effects; D010165:Palladium / Q000627:therapeutic use; D011467:Prostate / Q000000981:diagnostic imaging / Q000473:pathology / Q000528:radiation effects*; D011471:Prostatic Neoplasms / Q000175:diagnosis / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D011832:Radiation Injuries / Q000517:prevention & control*; D011868:Radioisotopes / Q000627:therapeutic use; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D012007:Rectum / Q000528:radiation effects; D012062:Relative Biological Effectiveness; D012189:Retrospective Studies; D047368:Tumor Burden; D014463:Ultrasonography; D014521:Urethra / Q000528:radiation effects
chemical_list: D002588:Cesium Radioisotopes; D007457:Iodine Radioisotopes; D011868:Radioisotopes; D010165:Palladium