title: Low-Z linac targets for low-MV gold nanoparticle radiation therapy.
abstract: OBJECTIVE
  To investigate the potential of low-Z/low-MV (low-Z) linac targets for gold nanoparticle radiotherapy (GNPT) and to determine the microscopic dose enhancement ratio (DER) due to GNP for the alternative beamlines. In addition, to evaluate the degradation of dose enhancement arising from the increased attenuation of x rays and larger skin dose in water for the low-MV beams compared to the standard linac.
  
  
  METHODS
  Monte Carlo simulations were used to compute dose and DER for various flattening-filter-free beams (2.5, 4, 6.5 MV). Target materials were beryllium, diamond, and tungsten-copper high-Z target. Target thicknesses were selected based on 20%, 60%, 70%, and 80% of the continuous slowing down approximation electron ranges for a given target material and energy. Evaluation of the microscopic DER was carried out for 100 nm GNP including the degradation factors due to beam attenuation.
  
  
  RESULTS
  The greatest increase in DER compared to the standard 6.5 MV linac was for a 2.5 MV Be-target (factor of ∼ 2). Skin dose ranged from ∼ 10% (Be, 6.5 MV-80%) to ∼ 85% (Be, 2.5 MV-20%) depending on the target case. Attenuation of 2.5 MV beams at 22 cm was higher by ∼ 75% compared with the standard beam. Taking into account the attenuation at 22 cm depth, the effective dose enhancement was up to ∼ 60% above the DER of the high-Z target. For these cases the effective DER ranged between ∼ 1.6 and 6 compared with the standard linac.
  
  
  CONCLUSIONS
  Low-Z (2.5 MV) GNPT is possible even after accounting for greater beam attenuation for deep-seated tumors (22 cm) and the increased skin dose. Further, it can lead to significant sparing of normal tissue while simultaneously escalating the dose in the tumor cells.
authors: Department of Radiation Oncology, Brigham and Women's Hospital, Dana-Farber Cancer Institute and Harvard Medical School, Boston, Massachusetts 02215.
authors: P
authors: P
authors: Tsiamas
authors: Department of Radiation Oncology, Brigham and Women's Hospital, Dana-Farber Cancer Institute and Harvard Medical School, Boston, Massachusetts 02215.
authors: P
authors: P
authors: Mishra
authors: Medical Physics Program, Department of Physics and Applied Physics, University of Massachusetts Lowell, Lowell, Massachusetts 01854.
authors: F
authors: F
authors: Cifter
authors: Department of Radiation Oncology, Brigham and Women's Hospital, Dana-Farber Cancer Institute and Harvard Medical School, Boston, Massachusetts 02215.
authors: R I
authors: RI
authors: Berbeco
authors: Department of Radiation Oncology, Brigham and Women's Hospital, Dana-Farber Cancer Institute and Harvard Medical School, Boston, Massachusetts 02215.
authors: K
authors: K
authors: Marcus
authors: Medical Physics Program, Department of Physics and Applied Physics, University of Massachusetts Lowell, Lowell, Massachusetts 01854.
authors: E
authors: E
authors: Sajo
authors: Department of Radiation Oncology, Brigham and Women's Hospital, Dana-Farber Cancer Institute and Harvard Medical School, Boston, Massachusetts 02215.
authors: P
authors: P
authors: Zygmanski
mesh_terms: D006046:Gold / Q000737:chemistry* / Q000627:therapeutic use*; D006801:Humans; D053768:Metal Nanoparticles; D009010:Monte Carlo Method; D011878:Radiotherapy / Q000379:methods*; D011879:Radiotherapy Dosage; D012867:Skin / Q000528:radiation effects
chemical_list: D006046:Gold