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Abstract: of the radiation emerged from the seed

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Abstract:

Objective: The objective of the current study was to estimate the energy spectrum of the photons emitted from 125I seed model 6711 and energy deposited by the seed with and without the presence of titanium capsule using Monte-Carlo N- Particle code (MCNP) in order to investigate whether the titanium capsule attenuates photons and how much would be the attenuation. Materials and Methods: Two models were built and simulated using MCNPX code, in the first model, the simulation was performed assuming the geometry of 125I seed provided by the manufacturers. Whereas in the second model, the simulation was performed assuming that 125I seed without titanium encapsulation. For both models, the energy and energy deposited by the photons were estimated. Results: MCNPX computations showed that the energy spectrum released from 125I seed with the presence of the capsule was lower than that released without the presence of the capsule for all energies in the spectrum by approximately 19 %.  Further, the energy deposition computed with the presence of titanium capsule was lower than that computed without the presence of titanium capsule. Conclusion: It can be concluded that the titanium capsule has an impact on the energy spectrum as well as energy deposition of the photons emitted by 125I seed. According to the MCNPX results, titanium capsule attenuates the energy of the radiation emerged from the seed by nearly 19%.

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Interstitial brachytherapy using permanent radioactive implants is a common choice for most patients with prostate cancer 1-3. Iodine-125 (125I ) has been widely used for permanent implants in prostate brachytherapy2, 4-6. The advantages of 125I over gold-198 and radon are; its lower photon energy results in requiring less shielding 2, 5, and its longer half-life (59.4 days) makes it an appropriate for storage. However, the dosimetry of 125I is more complicated than the conventional interstitial sources5. Iodine-125 seeds are classified according to the design of sealed radioactive sources7. Three commercial models of 125I seed have been manufactured, which are 6701, 6702, and 6711. These models are similar in encapsulation and size but different in the design of active source5. In the current study, model 6711 has been selected to estimate the energy spectrum of photons emitted from 125I seed. The encapsulation of this seed composed of titanium tube of 0.05mm thickness welded at both ends to shape a cylindrical capsule which has outer diameter of 0.8 mm and length of 4.5 mm 5, 7.  Model 6711 seed contains a silver rod (3 mm length) where 125I is adsorbed on its surface 7-8. Figure 1 shows the schematic diagram of 125I seed model 6711.  agram of 125I seed model 6711.  The geometry of this figure is not drawn to scale. The information of this figure is derived from references 5, 7, 9. Iodine-125 is one of the man-made radioisotopes produced in nuclear reactor. “125I is produced mainly in a neutron activation process, through xenon-124 (124Xe) gas target to give rise to 125Xe. In turn, 125Xe decays into 125I by electron-capture (EC) transition. The half-life of the 125Xe is 16.9 hours. 125I also decays by EC into an excited state 125Te*, producing the maximum photon energy of 35.5 keV by gamma decay (6.7% of the time) See figure 2. In addition, the transition leads to characteristic x-rays of energy between 27.2 to 31.7 keV (K-shells) as a result of internal conversion (93.3%). Note that a very low-energy x-ray of 3.8 keV is also possible (15%) but in practice very low energy photons are attenuated within the source capsule” 10. The production and decaying processes of 125I is shown in the equations below 10.  Figure 2: Decay scheme of 125I. It was drawn according to the information from (6). Iodine-125 emits gamma and x-photons with spectrum of energies ranges from 3.3 to 35.5 Kev11. In 125I seed used in brachytherapy, the radioactive source of 125I is encapsulated by titanium material with a thickness of 0.05 mm 5, 7, and so the photons released from 125I source must pass through the titanium material before reaching the tissue. The current study was designed to estimate the energy spectrum of the photons emitted from 125I source with and without the presence of titanium capsule using Monte-Carlo N- Particle code (MCNP) in order to investigate whether the titanium capsule attenuates photons.  

 

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