ABSTRACT
Objective To design and build a set of experimental equipment for neutron radiation irradiation by using the 241Am-Be neutron source. Methods In the preliminary work, the spatial distribution data of the neutron energy spectrum and the gamma energy spectrum inside and outside the device were simulated by Monte Carlo method, and the changes of radiation fluence rate with spatial distribution was studied. The model of the 241Am-Be neutron device was established, and the neutron transport process in the irradiation field was studied using the method of shadow cone, inverse square law and other data analysis methods. Results Based on the simulation results, the normalized effective does of fast neutron fluence at the measurement point is about 72.9 pSv/n, and the one of photon fluence is about 3.04 pSv/γ. The ratio of effective dose of photon fluence to neuteon is about 4.17%. Conclusion Using Monte Carlo method, a standard model of 241Am-Be neutron source was constructed, the shadow cone design was optimized, and the feasibility of using the shadow cone conversion method to establish a standard neutron source radiation device was discussed.
ABSTRACT
Objective The beam data is compared with those obtained from Monte Carlo (MC)simulation and measurement to investigate their feasibility and reliability for X-ray small fields.MethodsThe beam data,including the total scatter factor (Scp),percentage depth dose (PDD) was acquired byneasurement and calculation with the field size ranging from 0.5 cm × 0.5 cm to 10 cm x 10 cm.The resultswere compared and analyzed.Results All the data is most consistent for the fields size of ≥3.5 cmx 3.5cm,but they are obvious different for the fields size of ≤ 3.0 cm × 3.0 cm.The measurements seem toreliable using the chambers of CC04 and CC13 for the fields size of ≥2.0 cm x 2.0 cm.Conclusions It isdemonstrated that the accurate measurements and calculations of Scp and PDD can be obtained for the fieldssize of ≥2.0 cm ×2.0 cm,but they needed morc rcscarchcs for thc smaller fields.
ABSTRACT
Currently, the dose distribution calculation used by commercial treatment planning systems (TPSs) for high-dose rate (HDR) brachytherapy is derived from point and line source approximation method recommended by AAPM Task Group 43 (TG-43). However, the study of Monte Carlo (MC) simulation is required in order to assess the accuracy of dose calculation around three-dimensional Ir-192 source. In this study, geometry factor was calculated using segmented sources integration method by dividing microSelectron HDR Ir-192 source into smaller parts. The Monte Carlo code (MCNPX 2.5.0) was used to calculate the dose rate D(r,theta) at a point (r,theta) away from a HDR Ir-192 source in spherical water phantom with 30 cm diameter. Finally, anisotropy function and radial dose function were calculated from obtained results. The obtained geometry factor was compared with that calculated from line source approximation. Similarly, obtained anisotropy function and radial dose function were compared with those derived from MCPT results by Williamson. The geometry factor calculated from segmented sources integration method and line source approximation was within 0.2% for r> or =0.5 cm and 1.33% for r=0.1 cm, respectively. The relative-root mean square error (R-RMSE) of anisotropy function obtained by this study and Williamson was 2.33% for r=0.25 cm and within 1% for r>0.5 cm, respectively. The R-RMSE of radial dose function was 0.46% at radial distance from 0.1 to 14.0 cm. The geometry factor acquired from segmented sources integration method and line source approximation was in good agreement for r> or =0.1 cm. However, application of segmented sources integration method seems to be valid, since this method using three-dimensional Ir-192 source provides more realistic geometry factor. The anisotropy function and radial dose function estimated from MCNPX in this study and MCPT by Williamson are in good agreement within uncertainty of Monte Carlo codes except at radial distance of r=0.25 cm. It is expected that Monte Carlo code used in this study could be applied to other sources utilized for brachytherapy.
Subject(s)
Anisotropy , Brachytherapy , Organothiophosphorus Compounds , Uncertainty , WaterABSTRACT
In this study the dosimetric evaluation for a biological sample irradiated by gamma rays from Cs-137 irradiator (Gamma Irradiator, Chiyoda Technol Co., Japan) was performed for radiobiological experiment. A spherical water with a diameter of 3 cm was assumed as a biological sample. The absorbed dose were determined by the air kerma based dosimetric calculation system. The theoretical and Monte Carlo calculations (MCNPX) were performed and compared to evaluate measured air kerma and determined absorbed dose respectively. As a result of comparison with theoretical calculation, the measured air kerma was in good agreement within 3.1% at the distance of 100 and 200 cm from the source. In comparison with Monte Carlo results the determined absorbed dose along the central axis was in good agreement within 1.9% and 3.7% at 100 cm and 200 cm respectively. Although the preliminary results were obtained in this study these results were used as a basis of dosimetric evaluation for radiobiological experiment. Extended study will be performed to evaluate the dose in various conditions of biological samples.
Subject(s)
Axis, Cervical Vertebra , Gamma Rays , WaterABSTRACT
Monte Carlo calculations were performed to demonstrate the dose modulation with dynamic magnetic fields in phantom. The goal of this study is to obtain the uniform dose distributions at a depth region as a target on the central axis of photon beam under moving transverse magnetic field. We have calculated the depth dose curves for two cases of moving magnetic field along a depth line, constant speed and optimal speed. We introduced step-by-step shift and time factor of the position of the electromagnet as an approximations of continuous moving. The optimal time factors as a function of magnetic field position were calculated by least square methods using depth dose data for static magnetic field. We have verified that the flat depth dose is produced by varying the speed of magnetic field as a function of position as a results of Monte Carlo calculations. For 3 T magnetic field, the dose enhancement was 10.1% in comparison to without magnetic field at the center of the target.