title: Modeling a hypothetical 170Tm source for brachytherapy applications.
abstract: OBJECTIVE
  To perform absorbed dose calculations based on Monte Carlo simulations for a hypothetical (170)Tm source and to investigate the influence of encapsulating material on the energy spectrum of the emitted electrons and photons.
  
  
  METHODS
  GEANT4 Monte Carlo code version 9.2 patch 2 was used to simulate the decay process of (170)Tm and to calculate the absorbed dose distribution using the GEANT4 Penelope physics models. A hypothetical (170)Tm source based on the Flexisource brachytherapy design with the active core set as a pure thulium cylinder (length 3.5 mm and diameter 0.6 mm) and different cylindrical source encapsulations (length 5 mm and thickness 0.125 mm) constructed of titanium, stainless-steel, gold, or platinum were simulated. The radial dose function for the line source approximation was calculated following the TG-43U1 formalism for the stainless-steel encapsulation.
  
  
  RESULTS
  For the titanium and stainless-steel encapsulation, 94% of the total bremsstrahlung is produced inside the core, 4.8 and 5.5% in titanium and stainless-steel capsules, respectively, and less than 1% in water. For the gold capsule, 85% is produced inside the core, 14.2% inside the gold capsule, and a negligible amount (<1%) in water. Platinum encapsulation resulted in bremsstrahlung effects similar to those with the gold encapsulation. The range of the beta particles decreases by 1.1 mm with the stainless-steel encapsulation compared to the bare source but the tissue will still receive dose from the beta particles several millimeters from the source capsule. The gold and platinum capsules not only absorb most of the electrons but also attenuate low energy photons. The mean energy of the photons escaping the core and the stainless-steel capsule is 113 keV while for the gold and platinum the mean energy is 160 keV and 165 keV, respectively.
  
  
  CONCLUSIONS
  A (170)Tm source is primarily a bremsstrahlung source, with the majority of bremsstrahlung photons being generated in the source core and experiencing little attenuation in the source encapsulation. Electrons are efficiently absorbed by the gold and platinum encapsulations. However, for the stainless-steel capsule (or other lower Z encapsulations) electrons will escape. The dose from these electrons is dominant over the photon dose in the first few millimeter but is not taken into account by current standard treatment planning systems. The total energy spectrum of photons emerging from the source depends on the encapsulation composition and results in mean photon energies well above 100 keV. This is higher than the main gamma-ray energy peak at 84 keV. Based on our results, the use of (170)Tm as a brachytherapy source presents notable challenges.
authors: Département de Radio-Oncologie, Centre Hospitalier Universitaire de Québec, Québec, Canada.
authors: Shirin A
authors: SA
authors: Enger
authors: Michel
authors: M
authors: D'Amours
authors: Luc
authors: L
authors: Beaulieu
mesh_terms: D000465:Algorithms; D001918:Brachytherapy / Q000295:instrumentation* / Q000379:methods; D003198:Computer Simulation; D004583:Electrons; D004867:Equipment Design; D006046:Gold / Q000737:chemistry; D006801:Humans; D008962:Models, Theoretical; D009010:Monte Carlo Method; D017785:Photons; D010984:Platinum / Q000737:chemistry; D011868:Radioisotopes / Q000032:analysis*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D013193:Stainless Steel / Q000737:chemistry; D013932:Thulium / Q000032:analysis*; D014025:Titanium / Q000737:chemistry
chemical_list: D011868:Radioisotopes; D013193:Stainless Steel; D010984:Platinum; D006046:Gold; D013932:Thulium; D014025:Titanium