title: Dosimetric perturbations at high-Z interfaces with high dose rate (192)Ir source.
abstract: OBJECTIVE
  To investigate dose perturbations created by high-atomic number (Z) materials in high dose rate (HDR) Iridium-192 ((192)Ir) treatment region.
  
  
  METHODS
  A specially designed parallel plate ion chamber with 5 μm thick window was used to measure the dose rates from (192)Ir source downstream of the high-Z materials. A Monte Carlo (MC) code was employed to calculate the dose rates in both upstream and downstream of the high-Z interfaces at distances ranging from 0.01 to 2 mm. The dose perturbation factor (DPF) was defined as the ratio of dose rate with and without high-Z material in a water phantom. For verifying the Z dependence, both 0.1- and 1.0 mm-thick sheets of Pb, Au, Ta, Sn, Cu, Fe, Ti and Al were used.
  
  
  CONCLUSIONS
  The DPF depends on the Z and thickness of layer. At the downstream of a 0.1 mm layer of Pb, Au, Ta, Sn, Cu, Fe, Ti and Al, the DPF by MC were 3.73, 3.42, 3.04, 1.71, 1.04, 0.98, 0.92, or 0.94 respectively. When Z is greater than or equal to 50, the MC and experimental results disagree significantly (>20%) due to large DPF gradient but are in agreement for Z less than or equal to 29. Thin layers of Z greater than or equal to 50 near a (192)Ir source in water produce significant dose perturbations (i.e. increases) in the vicinity of the medium-high-Z interfaces and may thus cause local over-dose in (192)Ir brachytherapy. Conversely, this effect may potentially be used to deliver locally higher doses to targeted tissue.
authors.affiliation: Department of Radiation Oncology, Northwestern University Feinberg School of Medicine, Northwestern Memorial Hospital, Chicago, IL 60611, USA; Department of Radiation Oncology, Indiana University School of Medicine, Indianapolis, IN 46202, USA. Electronic address: hualinzhang@yahoo.com.
authors.forename: Hualin
authors.initials: H
authors.lastname: Zhang
authors.affiliation: Department of Radiation Oncology, Indiana University School of Medicine, Indianapolis, IN 46202, USA.
authors.forename: Indra J
authors.initials: IJ
authors.lastname: Das
mesh_terms: D001918:Brachytherapy; D007496:Iridium Radioisotopes / Q000627:therapeutic use*; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D011874:Radiometry / Q000295:instrumentation*; D011879:Radiotherapy Dosage; D014867:Water
chemical_list: D007496:Iridium Radioisotopes; D014867:Water
keywords: (192)Ir; Brachytherapy; Dose perturbation; High-Z interface