Modeling a Hypothetical {sup 170}Tm Source for Brachytherapy Applications

Journal Article · Sat Oct 15 00:00:00 EDT 2011 · Medical Physics

DOI:https://doi.org/10.1118/1.3626482· OSTI ID:22098629

Enger, Shirin A.; D'Amours, Michel; Beaulieu, Luc ^[1]

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1. Departement de Radio-Oncologie et Centre de Recherche en Cancerologie de l'Universite Laval, Centre Hospitalier Universitaire de Quebec, Quebec, Quebec G1R 2J6, Canada and Departement de Physique, de Genie Physique et d'Optique, Universite Laval, Quebec, Quebec G1R 2J6 (Canada)

Purpose: To perform absorbed dose calculations based on Monte Carlo simulations for a hypothetical {sup 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 {sup 170}Tm and to calculate the absorbed dose distribution using the GEANT4 Penelope physics models. A hypothetical {sup 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 {sup 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 {sup 170}Tm as a brachytherapy source presents notable challenges.

• OSTI ID: 22098629
• Journal Information: Medical Physics, Vol. 38, Issue 10; Other Information: (c) 2011 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-2405
• Country of Publication: United States
• Language: English

Related Subjects

07 ISOTOPES AND RADIATION SOURCES
62 RADIOLOGY AND NUCLEAR MEDICINE
APPROXIMATIONS
BETA PARTICLES
BRACHYTHERAPY
BREMSSTRAHLUNG
CAPSULES
COMPUTERIZED SIMULATION
DISTRIBUTION
DOSIMETRY
ENCAPSULATION
ENERGY SPECTRA
GAMMA RADIATION
GOLD
KEV RANGE 10-100
MONTE CARLO METHOD
PLATINUM
RADIATION DOSES
STAINLESS STEELS
THULIUM 170
TITANIUM