An Efficiency Studying of an Ion Chamber Simulation Using Vriance Reduction Techniques with EGSnrc

BACKGROUND

Radiotherapy is an important technique of cancer treatment using ionizing radiation. The determination of total dose in reference conditions is an important contribution to uncertainty that could achieve 2%. The source of this uncertainty comes from cavity theory that relates the in-air cavity dose and the dose to water. These correction factors are determined from Monte Carlo calculations of ionization chambers. The main problem of this type of calculation is the extremely long computation time to achieve reasonable statistics.

OBJECTIVE

The main purpose of this work is to present a combination with variance reduction techniques for the case of an ionization chamber in water.

METHODS

The egs_chamber code allows for very efficient computation of ionization chamber doses and dose ratios by using various variance reduction techniques, and also permits realistic simulations of the experimental setup due to the use of EGSnrc C++ library. Russian roulette and Photon Cross Section Enhancement were used with egs_chamber code. Tests were performed to obtain the parameters of variance reduction techniques resulting in a maximum efficiency.

RESULTS

It can be seen that the parameters which result in improved Monte Carlo calculation of the efficiency values are XCSE 64 and Russian Roulette (RR) 128.

CONCLUSION

This study determines the parameters of variance reduction techniques that result in an optimal computational efficiency.

Evaluation of Gold Nanoparticle Size Effect on Dose Enhancement Factor in Megavoltage Beam Radiotherapy Using MAGICA Polymer Gel Dosimeter

BACKGROUND

Gold nanoparticles (GNPs) are among the most promising radiosensitive materials in radiotherapy. Studying the effective sensitizing factors such as nanoparticle size, concentration, surface features, radiation energy and cell type can help to optimize the effect and possible clinical application of GNPs in radiation therapy. In this study, the radiation sensitive polymer gel was used to investigate the dosimetric effect of GNP size in megavoltage (MV) photon beam radiotherapy.

MATERIAL AND METHODS

GNPs with the size of 30nm, 50nm and 100nm in diameter were used. Transmission electron microscope (TEM) and dynamic light scattering (DLS) were applied to analyze the size of nanoparticles. The MAGICA polymer gel was synthesized and impregnated with different sizes of GNPs. The samples were irradiated with 6MV photon beam and 24 hours after irradiation, they were read using a Magnetic Resonance Imaging (MRI) scanner. Macroscopic Dose Enhancement Factor (DEF) was measured to compare the effect of GNP size. The MAGICA response of the 6MV x-ray beam was verified comparing Percentage Depth Dose (PDD) curve extracted from polymer gel dosimetry and Treatment Planning System (TPS).

RESULTS

MAGICA polymer gel dose response curve was linear in the range of 0 to 10 Gy. DEFs by adding 30nm, 50nm and 100nm GNPs were 1.1, 1.17 and 1.12, respectively. PDD curves of polymer gel dosimeter and treatment planning system were in good agreement.

CONCLUSION

The results indicated a substantial increase in DEF uses a MV photon beam in combination with GNPs of different sizes and it was inconsistent with previous radiobiological studies. The maximum DEF was achieved for 50nm GNPs in comparison with 30nm and 100nm leading to the assumption of self-absorption effect by larger diameters. According to the outcomes of this work, MAGICA polymer gel can be recommended as a reliable dosimeter to investigate the dosimetric effect of GNP size and also a useful method to validate the current radiobiological and simulation studies.

Monte Carlo Simulation of Electron Beams produced by LIAC Intraoperative Radiation Therapy Accelerator

Background

One of the main problems of dedicated IORT accelerators is to determine dosimetric characteristics of the electron beams. Monte Carlo simulation of IORT accelerator head and produced beam will be useful to improve the accuracy of beam dosimetry.

Materials and Methods

Liac accelerator head was modeled using the BEAMnrcMonte Carlo simulation system. Phase-space files were generated at the bottom of the applicators. These phase-space files were used as an input source in DOSXYZnrc and BEAMDP codes for dose calculation and analysis of the characteristic of the electron beams in all applicators and energies.

Results

The results of Monte Carlo calculations are in very close agreement with the measurements. There is a decrease in the peak of the initial spectrum when electrons come from the end of accelerator wave guide to the end of applicator. By decreasing the applicator diameter, the mean energy of electron beam decreased. Using applicators and increasing their size, X-ray contamination will increase. The percentage of X-ray contamination increases by applicator diameter. This is related to the increase of the mean energy of electron beams.

Conclusion

Application of PMMA collimator leads to, although well below accepted level, the production of bremsstrahlung. The results of this study showed that special design of LIAC head accompanying by PMMA collimator system cause to produce an electron beam with an individual dosimetric characteristic making it a useful tool for intraoperative radiotherapy purposes.