title: Low Z target switching to increase tumor endothelial cell dose enhancement during gold nanoparticle-aided radiation therapy.
abstract: OBJECTIVE
  Previous studies have introduced gold nanoparticles as vascular-disrupting agents during radiation therapy. Crucial to this concept is the low energy photon content of the therapy radiation beam. The authors introduce a new mode of delivery including a linear accelerator target that can toggle between low Z and high Z targets during beam delivery. In this study, the authors examine the potential increase in tumor blood vessel endothelial cell radiation dose enhancement with the low Z target.
  
  
  METHODS
  The authors use Monte Carlo methods to simulate delivery of three different clinical photon beams: (1) a 6 MV standard (Cu/W) beam, (2) a 6 MV flattening filter free (Cu/W), and (3) a 6 MV (carbon) beam. The photon energy spectra for each scenario are generated for depths in tissue-equivalent material: 2, 10, and 20 cm. The endothelial dose enhancement for each target and depth is calculated using a previously published analytic method.
  
  
  RESULTS
  It is found that the carbon target increases the proportion of low energy (<150 keV) photons at 10 cm depth to 28% from 8% for the 6 MV standard (Cu/W) beam. This nearly quadrupling of the low energy photon content incident on a gold nanoparticle results in 7.7 times the endothelial dose enhancement as a 6 MV standard (Cu/W) beam at this depth. Increased surface dose from the low Z target can be mitigated by well-spaced beam arrangements.
  
  
  CONCLUSIONS
  By using the fast-switching target, one can modulate the photon beam during delivery, producing a customized photon energy spectrum for each specific situation.
authors.affiliation: Department of Radiation Oncology, Brigham and Women's Hospital, Dana-Farber Cancer Institute and Harvard Medical School, Boston, Massachusetts 02115.
authors.forename: Ross I
authors.initials: RI
authors.lastname: Berbeco
authors.affiliation: Department of Radiation Oncology, Brigham and Women's Hospital, Dana-Farber Cancer Institute and Harvard Medical School, Boston, Massachusetts 02115.
authors.forename: Alexandre
authors.initials: A
authors.lastname: Detappe
authors.affiliation: Department of Radiation Oncology, St. Jude Children's Hospital, Memphis, Tennessee 38105.
authors.forename: Panogiotis
authors.initials: P
authors.lastname: Tsiamas
authors.affiliation: Department of Radiation Oncology and Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia B3H 1V7, Canada.
authors.forename: David
authors.initials: D
authors.lastname: Parsons
authors.affiliation: Department of Radiation Oncology and Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia B3H 1V7, Canada.
authors.forename: Mammo
authors.initials: M
authors.lastname: Yewondwossen
authors.affiliation: Department of Radiation Oncology and Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia B3H 1V7, Canada.
authors.forename: James
authors.initials: J
authors.lastname: Robar
mesh_terms: D042783:Endothelial Cells / Q000187:drug effects* / Q000528:radiation effects*; D006046:Gold / Q000737:chemistry* / Q000494:pharmacology*; D006801:Humans; D053768:Metal Nanoparticles; D009010:Monte Carlo Method; D010315:Particle Accelerators; D017785:Photons / Q000627:therapeutic use; D011829:Radiation Dosage; D011879:Radiotherapy Dosage; D011881:Radiotherapy, Computer-Assisted / Q000295:instrumentation / Q000379:methods*
chemical_list: D006046:Gold