Development of a computer model using the EGS4 simulation code to calculate scattered X-rays through some materials.

In this paper a computer model based on the use of the well-known Monte Carlo simulation code EGS4 was developed to simulate the scattering of polyenergetic X-ray beams through some materials. These materials are: lucite, polyethylene, polypropylene and aluminium. In particular, the ratio of the scattered to total X-ray fluence (scatter fraction) has been calculated for X-ray beams in the energy region 30-120 keV. In addition scatter fractions have been determined experimentally using a polyenergetic superficial X-ray unit. Comparison of the measured and the calculated results has been performed. The Monte Carlo calculations have also been carried out for water, bakelite and bone to examine the dependence of scatter fraction on the density of the scatterer. Good agreement (estimated statistical error < 5%) was obtained between the measured and the calculated values of the scatter fractions for materials with Z < 20 that were studied in this paper.

A comparison between EGS4 and MCNP computer modeling of an in vivo X-ray fluorescence system.

The Monte Carlo computer codes EGS4 and MCNP were used to develop a theoretical model of a 180 degrees geometry in vivo X-ray fluorescence system for the measurement of platinum concentration in head and neck tumors. The model included specification of the photon source, collimators, phantoms and detector. Theoretical results were compared and evaluated against X-ray fluorescence data obtained experimentally from an existing system developed by the Swansea In Vivo Analysis and Cancer Research Group. The EGS4 results agreed well with the MCNP results. However, agreement between the measured spectral shape obtained using the experimental X-ray fluorescence system and the simulated spectral shape obtained using the two Monte Carlo codes was relatively poor. The main reason for the disagreement between the results arises from the basic assumptions which the two codes used in their calculations. Both codes assume a "free" electron model for Compton interactions. This assumption will underestimate the results and invalidates any predicted and experimental spectra when compared with each other.

Evaluation of 133Xe for x-ray fluorescence analysis of cadmium in vivo: a Monte Carlo study.

A Monte Carlo model has been developed to support the design of a 180 degrees geometry x-ray fluorescence system for the measurement of cadmium concentration in deep body organs such as the kidney. 133Xe was investigated as the excitation photon source. A total number of 15 x 10(6) simulated incident photons were used. Monte Carlo simulations were performed using the EGS4 Monte Carlo code system. The results showed that for distances between the skin and the kidney surface of 30-60 mm, respectively, cadmium concentrations of 15-60 micrograms/g kidney tissue could easily be detected. The mean skin and kidney doses during such measurements were estimated to be between 8 and 0.9 mGy, respectively.

A theoretical design of a flattening filter to improve field uniformity of a superficial therapeutic X-ray beam.

A Monte Carlo model has been developed using the MCNP code to aid the design of a flattening filter, to improve field uniformity of a superficial x-ray machine. The machine is operating at 90 kV and filtered with a 1.1 mm aluminium filter. In the theoretical simulation the original flat filter was replaced by a varying thickness filter to improve the uniformity across field sizes 20, 5 and 2 cm diameter as well as hardening the beam. Simulation results showed that flatness of the beam profile was improved for the 20 cm field size from +/- 7.3% to +/- 1.1% across the anode/cathode direction and from +/- 7.7% to +/- 3.2% in the anode/cathode direction. For the 5 cm field size the improvement was from +/- 4.6% to +/- 3.1% and from +/- 5.5% to +/- 3.4%, and for the 2 cm field size from +/- 3.1% to +/- 2.4% and from +/- 10.2% to +/- 9.5%, in the same directions, respectively. Beam quality simulations were made and the original half-value layer was reduced from 2.21 +/- 0.09 mm aluminium to 2.04 +/- 0.09 mm aluminium. The study demonstrated that it was possible to build a filter capable of flattening the beam profile for different sized applicators without significantly changing the penetrating ability of the beam.

Body composition measurements using DXA and other techniques in tamoxifen-treated patients.

Tamoxifen is an anti-oestrogenic drug which is widely used in the treatment of patients with breast cancer. There is increasing interest in using the drug both for benign breast disease and as a chemo-preventative agent of the drug in women at high risk of breast cancer. Despite the fact that the acute side-effects of the drug are few, its agonistic and antagonistic oestrogenic effects are not fully known and may have some undesirable effects for patients treated with the drug for several years. A number of studies carried out recently indicate a varying degree of change in bone mineral content following treatment with tamoxifen. These studies concentrated mainly on bone mineral density measurements only and non of them reported the effects of tamoxifen on lean body mass and fat mass. In this study we measured lean body mass and fat mass in tamoxifen-treated females and a comparison group to determine the difference between the two groups. Twenty-six women receiving tamoxifen (20 mg/d) have participated in this study. The control group comprised 31 healthy women of a similar age. Total body bone mineral (TBBM) was measured using a dual-energy X-ray absorptiometry (DXA) (Hologic INV., Waltham, U.S.A.). Similarly, regional and total body soft tissue (lean and fat tissue) were measured using the DXA system. In addition to DXA measurements, percentage body fat (%BF) was measured using total body potassium counting (TBK), skinfold anthropometry (SF), infrared interactance (i.r.) and bioelectric impedance analysis (BIA). Results from DXA alone showed that there were no significant differences between the two groups for TBBM, regional and total body lean tissue mass. However, there was a significant difference between the two groups (P < 0.05) for %BF measurement. Similarly there was a significant difference between the two groups (P < 0.05) for %BF measured by other body composition techniques. Although there is no other research reported on the effects of tamoxifen on %BF, this retrospective study indicates that tamoxifen may lead to increase in fact content in women who are subjected to this treatment. We conclude that this observation is probably related to the agonistic oestrogenic effect of Tamoxifen on body fat. To our knowledge this deleterious effect has not been reported before and it should be taken into considerable when comprising different types of anti-oestrogenic drugs. Furthermore, patients should be warned about this side-effect when they are prescribed Tamoxifen therapy.