Assessment of two hemispherical and hemispherical-conical miniature sources used in electronic brachytherapy using Monte Carlo Simulation

Abstract

Introduction: Since the heart of the electronic brachytherapy system is a tube of a miniature x-ray and due to the increasing use of electronic brachytherapy, there is an urgent need for acquiring knowledge about the X-ray spectrum produced, and distribution of x-ray dose. This study aimed to assess the optimal target thickness (TT), the X-ray source spectrum, and the absorbed dose of two miniature sources of hemispherical and hemispherical-conical used in electronic brachytherapy systems, through a Monte Carlo simulation.

Methods: Considering the advantages of MCNPX Code (2.6.0), two input files corresponding to the characteristics of the investigated miniature sources were prepared for this code and then were used for simulation. The optimal thickness (OT) of gold and tungsten targets was determined for the energy levels of 40, 45, and 50 kilo-electron-volts.

Results: In this study, the values of the size of the optimal thickness of 0.92, 1.01 and 1.06 μ for gold target and values of 0.99, 1.08 and 1.34 μ for tungsten target were obtained for energies 40, 45 and 50 keV that using these values, the optimum thickness of 0.92, X-ray spectrum within and outside targets, axial and radial doses for the used energy were calculated for two miniature sources.

Conclusion: It was found that the energy of incident electron, target shape, cross-sectional area of the produced bremsstrahlung, atomic number of materials constituting of the target and output window are the factors with the greatest impacts on the produced X-ray spectrum and the absorbed dose.

Keywords: Dose calculation; Electronic Branchy therapy; Energy Spectrum; Mesh tally; Mont Carlo simulation.

Figure 1

A view of simulated miniature sources; electron beams path (green color)

Figure 2

Targets used for qualitative determination of the range of target thickness

Figure 3

Spectra of X-ray inside the target (A) and outside the target (B)

Figure 4

Spectra of x-ray in water (A) and air (B) after the beryllium output window

Figure 5

The plots obtained from the tallies used to determine the optimal size of gold and tungsten targets

Figure 6

The plots showing the axial dose with beryllium (A) and with water instead of beryllium (B)

Figure 7

The curves showing the radial dose

Figure 8

The plots showing the radial dose with mesh tally: hemispherical (A, B, and C) and hemispherical-conical (D, E, and F)

Figure 9

Diagrams obtained from section 2.2 that show electrons range