Design of linear anti-scatter grid geometry with optimum performance for screen-film and digital mammography systems.

A detailed 3D Monte Carlo simulation of the grid geometrical parameters in screen-film mammography (SFM) and digital mammography (DM) systems has been performed. A combination of IEC 60627:2013 international standard conditions and other more clinically relevant parameters were used for this simulation. Accuracy of our results has been benchmarked with previously published data and good agreement has been obtained. Calculations in a wide range of linear anti-scatter grid geometries have been carried out. The evaluated parameters for the SFM system were the Bucky factor (BF) and contrast improvement factor (CIF) and for the DM system it was signal difference-to-noise ratio improvement factor (SIF). The CIF parameter was chosen to be nearly the same as the commercial grade, the BF and SIF were significantly improved compared to commercial grids in use today. Our optimized grid parameters for the SFM system were lead strip thickness d = 12 µm, grid ratio r = 5 and strip density N = 65 lines/cm. And for the DM system these parameters were d = 5 µm, r = 3 and N = 100 lines/cm. Both optimized grid sets have thinner d and higher N compared to the commercial grids.

The effect of energy spectrum change on DNA damage in and out of field in 10-MV clinical photon beams.

The aim of this study was to quantify the DNA damage induced in a clinical megavoltage photon beam at various depths in and out of the field. MCNPX was used to simulate 10 × 10 and 20 × 20 cm(2) 10-MV photon beams from a clinical linear accelerator. Photon and electron spectra were collected in a water phantom at depths of 2.5, 12.5 and 22.5 cm on the central axis and at off-axis points out to 10 cm. These spectra were used as an input to a validated microdosimetric Monte Carlo code, MCDS, to calculate the RBE of induced DSB in DNA at points in and out of the primary radiation field at three depths. There was an observable difference in the energy spectra for photons and electrons for points in the primary radiation field and those points out of field. In the out-of-field region, the mean energy for the photon and electron spectra decreased by a factor of about six and three from the in-field mean energy, respectively. Despite the differences in spectra and mean energy, the change in RBE was <1 % from the in-field region to the out-of-field region at any depth. There was no significant change in RBE regardless of the location in the phantom. Although there are differences in both the photon and electron spectra, these changes do not correlate with a change in RBE in a clinical MV photon beam as the electron spectra are dominated by electrons with energies >20 keV.

Monte Carlo dose calculations for radiotherapy machines: Theratron 780-C teletherapy case study.

The Monte Carlo transport code MCNP was used to simulate the photon beam from a Theratronics 780-C cobalt therapy unit and to calculate some dose-dependent parameters as functions of field size. The simulation process has included the source capsule, collimators (fixed and adjustable), lead in the unit head, and the field sizes as ranged from 5 x 5 to 35 x 35 cm2. Calculations have been carried out in a water phantom at a fixed source-surface distance of 80 cm. Detailed simulation of the major components of the therapy unit made it possible to calculate the effects of each unit component on the photon spectrum at the phantom surface. Percentage depth dose and peak scatter factor were evaluated for various field sizes. And tissue-air ratios were also determined for a field size of 10 x 10 cm2, as a function of depth down to 30 cm. To test the accuracy of the calculated results, they were compared with the published data of the British Journal of Radiology (BJR) suppl. 25 and good agreement between measurements and calculations has been obtained. Deviations typically were found to be of the order of 1%.

Impact of different pollutant sources on Shiraz air pollution using SPM elemental analysis.

Weekly average suspended particulate matter (SPM) concentrations were measured in four locations in Shiraz, Iran. Sampling was carried out from July 1999 and continued until July 2000. Instrumental neutron activation analysis (INAA) and atomic absorption spectrometry (AAS) methods were employed to obtain the weekly concentrations of Pb, Br, V, Ca, Al, Fe, Cu, Cr, Mn, Sc and Zn. The mean annual concentrations (in microg/m(3)) of 11 elements were found to be: Pb 0.545 (+/- 33.8%), Br 0.413 (+/- 34.1%), V 0.009 (+/- 28.2%), Ca 13.36 (+/- 24.5%), Al 2.56 (+/- 46.3%), Fe 2.62 (+/- 38.7%), Cu 0.122 (+/- 41.4%), Cr 0.015 (+/- 35.8%), Mn 0.053 (+/- 35.5%), Sc 0.0008 (+/- 37.5%) and Zn 0.085 (+/- 25.2%). The results of the study show that vehicle (traffic) pollution in Shiraz is higher than WHO and EPA standards and natural and industrial pollution is exceeding international guidelines in some seasons. The findings of the study confirm that the nearby cement factory has a major pollution impact on Shiraz air.

Gamma irradiator dose mapping simulation using the MCNP code and benchmarking with dosimetry.

The Monte Carlo transport code, MCNP, has been applied in simulating dose rate distribution in the IR-136 gamma irradiator system. Isodose curves, cumulative dose values, and system design data such as throughputs, over-dose-ratios, and efficiencies have been simulated as functions of product density. Simulated isodose curves, and cumulative dose values were compared with dosimetry values obtained using polymethyle-methacrylate, Fricke, ethanol-chlorobenzene, and potassium dichromate dosimeters. The produced system design data were also found to agree quite favorably with those of the system manufacturer's data. MCNP has thus been found to be an effective transport code for handling of various dose mapping excercises for gamma irradiators.

Borehole parametric study for neutron induced capture gamma-ray spectrometry using the MCNP code.

The MCNP Monte Carlo code has been used to simulate neutron transport from an Am-Be source into a granite formation surrounding a borehole. The effects of the moisture and the neutron poison on the thermal neutron flux distribution and the capture by the absorbing elements has been calculated. Thermal and nonthermal captures for certain absorbers having resonance structures in the epithermal and fast energy regions such as W and Si were performed. It is shown that for those absorbers having large resonances in the epithermal regions when they are present in dry formation or when accompanied by neutron poisons the resonance captures may be significant compared to the thermal captures.

Estimation of the genetically significant dose from diagnostic X-ray procedures in the Islamic Republic of Iran.

The genetically significant dose from diagnostic X-ray sources has been estimated. Age and sex distribution was based on a survey of 13,100 patients. Dosimetry was performed on patients and on an equivalent-man Rando phantom. Child-expectancy factors were estimated from the census results of fertility and mortality rates. The GSD value for the diagnostic procedures to the Iranian population was estimated to be 9.2 mrem/yr in 1980.