title: Dosimetric optimization of a colpostat in a 60Co high-dose-rate brachytherapy unit for bladder sparing.
abstract: BACKGROUND
  Cervical cancer brachytherapy has an effective role on the tumor control probability as a boost and/or single treatment option. High-dose-rate (60)Co brachytherapy units are used in many radiation oncology centers in Iran. Rectum and bladder tissues are considered as organs at risk in radiation treatment of cervical cancers, and they should be spared from unwanted radiation risk.
  
  
  OBJECTIVE
  In the present study, the effect of additional tungsten shield was investigated in a new colpostat design for bladder protection against radiation.
  
  
  METHODS
  Monte Carlo (MC) simulation has been performed using the MC N-Particle eXtended version 2.4.0 transport code. The HDR GZP6 brachytherapy source applicator was simulated along with its colpostats. The f6 tally was used for absorbed dose calculation; and for model validation, we used from dosimetric features of the GZP6 treatment planning system.
  
  
  RESULTS
  Results calculated by MC simulation method showed that dose reduction at the end of the colpostat was 2.44% for the medium colpostat and a dose increase of 1.35% was obtained for the small colpostat. In the reference point of the bladder (at the distance 1cm from the end point of the colpostat), the percentages of dose reductions were also 25% and 15% for medium and small colpostats, respectively.
  
  
  CONCLUSIONS
  Results show that the absorbed dose in the bladder tissue can be reduced significantly using a shielded colpostat.
authors.affiliation: Department of Medical Physics and Biomedical Engineering, Faculty of Medicine, Tehran University of Medical Sciences, Tehran, Iran.
authors.forename: Seyed Salman
authors.initials: SS
authors.lastname: Zakariaee
authors.affiliation: Department of Medical Physics, Faculty of Medicine, Iran University of Medical Sciences, Tehran, Iran. Electronic address: srmahdavi@hotmail.com.
authors.forename: Seied Rabi
authors.initials: SR
authors.lastname: Mahdavi
mesh_terms: D001918:Brachytherapy / Q000009:adverse effects / Q000379:methods*; D003037:Cobalt Radioisotopes / Q000627:therapeutic use; D004867:Equipment Design; D005260:Female; D006801:Humans; D008962:Models, Theoretical; D009010:Monte Carlo Method; D059351:Organ Sparing Treatments / Q000379:methods*; D058958:Organs at Risk / Q000528:radiation effects; D011829:Radiation Dosage; D011832:Radiation Injuries / Q000517:prevention & control; D011835:Radiation Protection / Q000295:instrumentation / Q000379:methods; D011874:Radiometry / Q000379:methods; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D012007:Rectum / Q000528:radiation effects; D001743:Urinary Bladder / Q000528:radiation effects*; D002583:Uterine Cervical Neoplasms / Q000532:radiotherapy*
chemical_list: D003037:Cobalt Radioisotopes
keywords: Bladder sparing; GZP6 HDR system; HDR (60)Co brachytherapy source; Monte Carlo simulation