الفهرس 
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Abstract
Radiation is a form of energy that travels through space or matter. This energy is carried by waves, as in case of x- and gamma radiation, or particles, as in the case of alpha, beta, protons, and neutron radiation.
Modern radiation therapy uses high energy linear accelerators, that produce high energy photons, i.e., x-rays in the MV energy range or high energy electrons. High energy photon beams from medical LINACs produce, besides the clinically useful electron and photon beams, other secondary particles such as neutrons. Neutrons are by far the most significant secondary particles in the dosimetry of treatments because of their high range in ordinary matter and the high LET of their interaction products.
Neutron production in medical LINACs arises from photonuclear interactions of high energy photons with high-Z material components in the accelerator head, especially the target, and depends greatly on their isotopic composition.
All tungsten isotopes have significant photonuclear cross sections while only Fe-56 produces neutrons in iron. On the other hand, the absorption cross sections of the materials present in the accelerator head are very low for the generated neutron energies. Therefore, neutrons are not shielded by the LINAC collimators and reach the patient, contributing an extra dose not taken into account in routine radiotherapy treatments.
Several studies showed that this extra dose does not increase substantially the overall dose, with neutron dose equivalence values below 10 mSv per Gy. But higher photoneutron production associated with larger beam-on times required by IMRT treatments, compared to conventional radiationtherapy, is of main concern both for the patient dosimetry and for the radiation protection of the technicians.
In this work, it was aimed to Calculate the neutron dose and activation in various components of biological tissue phantom simulating patient’s body irradiated by clinical LINACs using MCNPX Code, and calculating the imposed dose due to activated isotopes within the phantom.
Methods
For these purposes, the MCNPX code was used to carry out the study. The code was used to:
• Establish a model for the accelerator head.
• Establish a phantom model for the patient with a tumor inside guided by the ICRP,23 1971.
• Calculate the photoneutrons produced from the accelerator head.
• Calculate the neutron activation induced in the different nuclides in the patient (phantom).
Later, radiation induced dose from the activated nuclide in the phantom were calculated according to the activated nuclides taking into consideration the physical and biological half-lives and types and energy of produced radiation in each case.
The accelerator used in this work was SEIMENS Artiste operating at 14 MV energy.
Results
Twenty-six nuclides in the phantom were considered, from these only 16 nuclides were found activatable. However, only nine were found to be significantly activated, namely: Si-31, Cl-36, Cl-38, K-42, Fe-55, H-3, Fe-59.
The total dose of each of the activated isotopes in descending order, are
Si-31 ≈ 1.18 Gy> Cl-36 ≈ 0.14 Gy> Cl-38 ≈ 0.05 Gy> Na – 24 ≈ 0.1 Gy>K-42 ≈ 0.002 Gy> Fe-55 ≈ 1.69E-7 Gy> H-3 ≈ 6.11E-7 Gy> Fe-59 7.05E-7 Gy.
The contribution of the Silicon-31 isotope was found to be about 80 fold of all the other isotopes.
In case of normal persons (without any Silicon transplantation) no problem arises. However, with patients with Silicon transplantation (such as breast transplantation and other parts of the body) the problem will be fatal, where the contribution of doses to the body would be very high due to the increasing concentration of Silicon in the transplant.