Designing an improved model of the adult female ICRP reference phantom dedicated to mammography procedure.

Mammography is an x-ray-based imaging method to examine breast abnormalities. Since low-energy photons are used in mammography, doses to different organs would depend strongly on the phantom posture and anatomy. Until now, a few studies have been performed on doses delivered to different organs during mammography. However, in none of them, the correct posture of the patient has been considered. In the present study, the effect of accurate patient positioning, on doses to organs in the chest region were investigated through Monte Carlo simulations. The results show the rotation of the phantom head, may affect organ doses up to 60%. Also, ignoring the head in dosimetry calculations changes scattering effects and causes dose uncertainty of about 8% for these organs. Moreover, according to the obtained results, not compressing the breast causes serious dose misestimation. Finally, using developed phantoms dedicated for mammography, total doses received by different organs have been calculated for the tube voltages of 25, 28, 30 and 35 kVp and for craniocaudal and mediolateral oblique views.

Investigation of the effect of using radiation protective glasses on the photon fluence-to-dose conversion coefficients of eye substructures.

The use of radiation protective glasses is common in radiation-contaminated environments. However, the effect of these glasses has not yet been investigated on the fluence-to-dose conversion coefficients (DCCs). The aim of this study is to investigate the effect of five types of gamma ray protective glasses on the photon fluence-to-DCCs of different eye substructures. For this purpose, a real eye model has been used and its conversion coefficients have been calculated in the presence of five types eye protective glasses with chemical formulae of ZnO-PbO-B2O3, Bi2O3-PbO-B2O3, PbO-B2O3, PbO-BaO-Na2O-MgO-B2O3and BaO-Nb2O3-P2O5. Calculations were performed for monoenergetic photon sources, whose energy ranges from 0.02 to 10 MeV, with different polar and azimuthal angles. The results indicate that the use of radiation protective glasses has acceptable effects on reducing the fluence-to-DCCs only at low photon energies up to 500 keV. At medium energy levels up to about 1 MeV, the effect of the glasses is negligible. However, at high energies it increases the fluence-to-DCCs for sensitive parts of the eye.

An age-dependent series of eye models for radiation dosimetry.

Several studies have been appeared, up to now, that indicate the effect of accurate simulation on dose received by different substructures of the eye, irradiated by external beams of short-range radiation such as electron. Different representations of the eye have consequently been introduced. Their accuracies depend on the purpose for which models are described or the degree of accuracy of information that models are based upon. All of developed models have presented the eye of an adult human, while the size, shape, and thickness of the eye substructures change with age. This work offers a series of age-dependent eye models including five models of different ages to investigate the influence of the individual age on doses to eye substructures. The absorbed dose conversion coefficients were calculated using Monte Carlo MCNP code for 17 monoenergetic broad electron beams in the range of 100 keV to 10 MeV and for 0°, 30° and 45° angles of incidence. Results showed the strong dependency of sclera and iris doses to the age (PDD above 105), for low-energies electrons. Increasing the electron energy, the PDD decreases severely for these substructure which means dose fluctuation in the age-dependent eye models reduces. For higher energies, dose fluctuations due to age become small and for some substructures could be negligible. Highlights - A series of age-dependent eye models was developed based on the realistic anatomy. - Dose conversion coefficients of eye substructures were calculated for each models. - 17 monoenergetic electron beams and three angles of incidence were considered. - Fluctuations of doses to eye sections due to the age were obtained.

Studying the lung dose uncertainty during chest CT scans using phantoms with statistical lung volumes and shapes.

In recent years, there has been increasing interest in constructing a series of deformable phantoms which follow the statistical distributions of some anatomical variations, known as 'statistical phantoms'. The main purpose of this study was to develop statistical phantoms by considering the variations in lung volume and shape, in order to evaluate the lung dose uncertainty for individuals undergoing chest computed tomography. Calculations were performed for 100 statistical lung volume phantoms and 70 statistical lung shape phantoms at tube voltages of 80 and 120 kVp, with the use of Monte Carlo MCNP code. The obtained results indicate that dose fluctuations for low tube voltage (80 kVp) are higher than those at 120 kVp. Moreover, it shows that the impact of statistical variations in lung volume on dose discrepancy (5% to 7%) is higher than the impact of statistical lung shape variations (around 2%).

Dosimetric comparison between realistic ocular model and other models for COMS plaque brachytherapy with 103Pd, 131Cs, and 125I radioisotopes.

Nowadays, Monte Carlo calculations are commonly used for the evaluation of dose distributions and dose volume histograms in eye brachytherapy. However, currently available eye models have simple geometries, and main substructures of the eye are either not defined in details or not distinguished at all. In this work absorbed doses of eye substructures have been estimated for eye plaque brachytherapy using the most realistic eye model available, and compared with absorbed doses obtained with other available eye models. For this, a medium-sized tumour on the left sides of the right eye was considered. Dosimetry calculations were performed for four different eye models developed based on a literature review, and using a 12 mm Collaborative Ocular Melanoma Study plaque containing 131Cs, 103Pd, and 125I sources. Obtained results illustrate that the estimated doses received by different eye substructures strongly depend on the model used to represent the eye. It is shown here that using a non-realistic eye model leads to a wrong estimation of doses for some eye substructures. For example, dose differences of up to 35% were observed between the models proposed by Nogueira and co-workers and Yoriyaz and co-workers, while doses obtained by use of the models proposed by Lesperance and co-workers, and Behrens and co-workers differed up to 100 and 63% as compared to the situation when a realistic model was used, respectively. Moreover, comparing different radionuclides showed that the most uniform dose distribution in the considered tumour region was that from 131Cs, with a coefficient of variation of 33%. In addition, considering the realistic eye model, it was found that the radiosensitive region of the lens received more than the threshold dose of cataract induction (0.5 Gy), for all investigated radionuclides.

The effects of simulating a realistic eye model on the eye dose of an adult male undergoing head computed tomography.

In head computed tomography, radiation upon the eye lens (as an organ with high radiosensitivity) may cause lenticular opacity and cataracts. Therefore, quantitative dose assessment due to exposure of the eye lens and surrounding tissue is a matter of concern. For this purpose, an accurate eye model with realistic geometry and shape, in which different eye substructures are considered, is needed. To calculate the absorbed radiation dose of visual organs during head computed tomography scans, in this study, an existing sophisticated eye model was inserted at the related location in the head of the reference adult male phantom recommended by the International Commission on Radiological Protection (ICRP). Then absorbed doses and distributions of energy deposition in different parts of this eye model were calculated and compared with those based on a previous simple eye model. All calculations were done using the Monte Carlo code MCNP4C for tube voltages of 80, 100, 120 and 140 kVp. In spite of the similarity of total dose to the eye lens for both eye models, the dose delivered to the sensitive zone, which plays an important role in the induction of cataracts, was on average 3% higher for the sophisticated model as compared to the simple model. By increasing the tube voltage, differences between the total dose to the eye lens between the two phantoms decrease to 1%. Due to this level of agreement, use of the sophisticated eye model for patient dosimetry is not necessary. However, it still helps for an estimation of doses received by different eye substructures separately.

Increasing the Personnel Safety of a Landmine Detection System using Proper Shielding Materials.

It has been proven that landmine detection systems based on prompt gamma neutron activation analysis method can be used effectively in detecting low metal content mines. Unfortunately, neutron and gamma radiations of these systems are inevitable. Therefore, the personnel safety of these devices is one of the major limitations in using these systems. In this work, a system composed of a moderated americium-beryllium neutron source was considered. We have investigated different thicknesses of proper neutron shielding materials such as polyethylene and boric acid around the system to shield neutrons and increase the safety conditions for the operators. The effect of using different thicknesses of a cylindrical lead shield (around the source) for shielding the gamma radiation of the source was also studied. Several experiments were done to measure neutron and gamma dose equivalent rate at different distances from the source. Most of the results were compared with the results obtained by Monte Carlo N-particle transport 4C code. Finally, we found a system that is safe for the operators who work at about a 1 m distance from the landmine detection system.

Improving the moderator geometry of an anti-personnel landmine detection system.

The optimum moderating geometry using an (241)Am-Be neutron source for detecting landmines has been investigated. The experimental setup composed of a Pb cylindrical shell enclosing the neutron source, embedded in a fixed-size high-density polyethylene cylinder with a variable thickness of the upper and lower moderator/reflector. According to the fact that the increased flux of thermal neutrons in the mine position yields increased prompt gamma rays, some groups of experiment have been done to measure several moderator configurations' responses, replacing a thermal neutron detector with the landmine and counting the neutron capture events.

Evaluation the nonlinear response function of a 3 x 3 in NaI scintillation detector for PGNAA applications.

Response functions of the 3 x 3 in NaI detector, which is mainly used in PGNAA applications, have been calculated by using MCNP-4C code. Calculated results are compared with measured data by using standard gamma-ray sources and prompt gamma-rays from pure element samples to check their accuracy. Prompt gamma-rays from pure element samples were used for this determination in the range from 1.942 to 10.829 MeV by use of (241)Am-Be neutron source and gamma-rays from radioisotope sources were used in the range from 0.081 to 4.438 MeV. Through the precise modeling of the detector structure, the agreement between both results has been improved. A surprising result is that in the PGNAA method the agreement between the MCNP simulation and experiment will be better by using a suitable neutron shield for NaI detector in order to prevent the activation of NaI (Tl) and a proper gamma-shield to attenuate the high-rate 4.438 MeV gamma-ray, (241)Am-Be gamma-ray component.

Improving the safety of a body composition analyser based on the PGNAA method.

The 252Cf radioisotope and 241Am-Be are intense neutron emitters that are readily encapsulated in compact, portable and sealed sources. Some features such as high flux of neutron emission and reliable neutron spectrum of these sources make them suitable for the prompt gamma neutron activation analysis (PGNAA) method. The PGNAA method can be used in medicine for neutron radiography and body chemical composition analysis. 252Cf and 241Am-Be sources generate not only neutrons but also are intense gamma emitters. Furthermore, the sample in medical treatments is a human body, so it may be exposed to the bombardments of these gamma-rays. Moreover, accumulations of these high-rate gamma-rays in the detector volume cause simultaneous pulses that can be piled up and distort the spectra in the region of interest (ROI). In order to remove these disadvantages in a practical way without being concerned about losing the thermal neutron flux, a gamma-ray filter made of Pb must be employed. The paper suggests a relatively safe body chemical composition analyser (BCCA) machine that uses a spherical Pb shield, enclosing the neutron source. Gamma-ray shielding effects and the optimum radius of the spherical Pb shield have been investigated, using the MCNP-4C code, and compared with the unfiltered case, the bare source. Finally, experimental results demonstrate that an optimised gamma-ray shield for the neutron source in a BCCA can reduce effectively the risk of exposure to the 252Cf and 241Am-Be sources.