جستجوی مقالات مرتبط با کلیدواژه "توزیع دز" در نشریات گروه "علوم پایه"

Nowadays, in one hand, the potential clinical application of helium ion beams is being discussed in the literature. In other hand, MR-guided radiotherapy has been welcomed by the radiotherapy oncology community so far. The idea of ​​combining an MRI system with a helium-ion beam delivery gantry has challenges, one of which is the dose changes in the patient's body that occur due to the deviation of the initial beam under a magnetic field. In order to quantitatively evaluate these dose changes, in this study, variation in the depth- and transverse dose profile curves inside a water phantom at therapeutic energies under magnetic fields of 0.35, 1.5 and 3 T were calculated using the FLUKA Monte Carlo code. The percentage of dose fluctuation in the direction of the central axis due to the application of magnetic field at minimum, medium and maximum energies was calculated and it was shown that the maximum dose changes occur at 220.5 MeV / n energy and at a depth of 30.9 cm phantom. Also, the maximum longitudinal retraction and deflection of helium beams in the presence of 3 Tesla magnetic field and at 220 MeV energy were 5.2 mm and 4.7 mm, respectively. To evaluate the accuracy of the simulations, depth-dose curves of helium ion beams in a water phantom were compared with the experimental data without the magnetic field influence. The maximum difference between the simulated and the measured values was 0.962 mm which is less than the permissible limit for predicting the Bragg peak of the ion beam for clinical dosimetry purposes.

Todays gamma knife radiosurgery is used widely for treatment of very small brain tumors. In order to investigate accuracy of dosimetry and treatment planning calculations, using Monte Carlo simulation with dedicated code named as beamnrc including non-CT data and CT data options is necessary. The aim of this study is choosing the best options in order to have an accurate tools based on their advantages and disadvantages. In this study, gamma knife unit 4C along with standard water equivalent phantom and EBT3 films were used to obtain dose distributions. Monte Carlo simulation was done with non-CT data and CT data options of the code and their resulting dose were compared.Comparison the calculated and measured dose distributions at X, Y and Z axis showed gamma value below 1 which verified Monte Carlo simulations. Also comparing the dose distributions from both non-CT data and CT data with each other implies that there is no significant difference between two methods. Based on the obtained results using non-CT data and CT data results in the same dose distribution. So for simplicity, using non CT data for regular phantom shapes is preferred.

An important problem for samples irradiation in research reactors is determination of three dimensional dose distributions in the vicinity of reactor core. Polymer gel dosimeters can be used to measure complex three dimensional dose distributions as well as the integrated dose accurately with no dependency on the dose rate. Furthermore, as they are tissue-equivalent, they may be used as a phantom. So far, polymer gel dosimeters have been used for photon, electron, proton, neutron and heavy ions, but there is a lack of application of polymer gel dosimeters for dosimetry of the mixed field of radiation of different linear ionization concentration. In this research, PAGAT polymer gel dosimeters are fabricated in the laboratory and then were irradiated with the mixed neutron gamma field from the fission process of the Tehran Research Reactor. The gel response was determined by the nuclear magnetic resonance imaging technique as a change in the relaxation rate (R2) of the gel dosimeters. The gel response as a function of normalized dose was investigated and a bi-exponential fitting was adjusted to the dose-R2 data. The region with a linear response, is called dynamic range. The slope of the region as the sensitivity of PAGAT gels to the normalized dose resulted from the neutron-gamma mixed field, was estimated to be 1.695 s-1. The results of this research showed that PAGAT polymer gel dosimeter is a useful tool for 3D dose distribution to determine the neutron gamma mixed field.

In operation of nuclear power plants, significant amounts of radioactive wastes are produced annually so that it is necessary to determine special ways for transportation and disposal of the radioactive wastes. According to the related standards, containers for transportation of radioactive materials should be designed in such a way that the equivalent dose rates on the outer surface and at a distance of 2m from the container do not exceed 2mSv/hr and 0.1mSv/hr, respectively. The purpose of this research is to design a radiological shielding for containers to transport the group II radioactive wastes of Boushehr Nuclear Power Plant. The dose distribution calculations and the container design were implemented through the Monte Carlo method using MCNP5 code. The code was run by the use of 8 processors in a parallel way. The total activity of one drum and inventory density were estimated to be 4.248 Bq and 2000 kgr/m3, respectively. A steel drum with a dimension of 79.5cm in height, 28.55cm of radius and 0.3cm in thickness was filled with the cemented inventory. The dose distribution for the bottom rest wastes was calculated. The simulation result showed a value of 15.67mSv/hr for the equivalent dose rate on the surface of the drum. The result was 10% higher than the FSAR prediction. In order to decrease the dose rate, 3 leaden packages with 4 drums in each were put on the trailer for the transportation. The suitable lead thickness for reducing the equivalent dose rate in order to meet the required standards for the lateral parts, floor and top were 2.2cm, 2cm and 1.5cm, respectively. With this calculated thickness, the equivalent dose rates on the surface and at a distance of 2m from the surface were 550µSv/hr and 94µSv/hr, respectively.

One of the most important measures of therapeutic quality in Microbeam Radiation Therapy (MRT) is the Peak to Valley Dose Ratio (PVDR). This parameter is a criterion to evaluate ablation of cancerous cells and sparing of normal cells in tumor and in its surrounding region. The aim of this work is to study the influence of using gold and gadolinium nano-particles as contrast agents on dose distribution and PVDR when a phantom is irradiated by a typical micro-planar X-ray beam of European Synchrotron Radiation Facility (ESRF3). Using Geant4 computer code, a model has been designed to simulate depth dose in an intact phantom made of PMMA4 and dose distribution in a phantom containing assumed tumors in therapeutic techniques of MIMRT5 and BIMRT6. Comparison of simulated results in the intact phantom with the measured values of depth dose reveals the validity of our simulation with the designed model. To improve the efficiency of MRT, enhancement of absorbed dose in tumor tissues and sparing of normal tissues due to presence of contrast agents have been studied. The obtained results show that the enhancement is more noticeable for Au at the peak region and for Gd in the valley region. This approach of introducing contrast agents in MRT could hopefully prepare new treatment planning and improves the efficiency of tumor therapy.