Voxel dosimetry: Comparison of MCNPX and DOSXYZnrc Monte Carlo codes in patient specific phantom calculations

USE OF ANTHROPOMORPHIC HETEROGENEOUS PHYSICAL PHANTOMS FOR VALIDATION OF COMPUTATIONAL DOSIMETRY OF MEDICAL PERSONNEL AND PATIENTS

In the dosimetry of ionizing radiation, the phantoms of the human body, which are used as a replacement for thehuman body in physical measurements and calculations, play an important, but sometimes underestimated, role.There are physical phantoms used directly for measurements, and mathematical phantoms for computationaldosimetry. Their complexity varies from simple geometry applied for calibration purposes up to very complex, whichsimulates in detail the shapes of organs and tissues of the human body. The use of physical anthropomorphic phantoms makes it possible to effectively optimize radiation doses by adjusting the parameters of CT-scanning (computed tomography) in accordance with the characteristics of the patient without compromising image quality. The useof phantoms is an indispensable approach to estimate the actual doses to the organs or to determine the effectivedose of workers - values that are regulated, but cannot be directly measured.The article contains an overview of types, designs and the fields of application of anthropomorphic heterogeneousphysical phantoms of a human with special emphasis on their use for validation of models and methods of computational dosimetry.

Evaluation of MLC Thickness and Composite Effects on Collimation Parameters using EGSnrc and IAEA Phase Space Data

Background

Recently, multileaf collimators (MLC) have become an important part of any LINAC collimation system because they reduce the treatment planning time and improve the conformity. Important factors that affect MLCs collimation performance are leaves material composition and their thickness.

Objective

In this study, we investigate main dosimetric parameters of a typical MLC including dose in the buildup point, physical penumbra as well as average and end leaf leakages. Effects of the leaves geometry and density on these parameters are evaluated.

Materials and Methods

In this analytical study, calculations were performed by using phase space data for Varian ix just above MLC and BEAMnrc/DOSXYZnrc for SSD=100cm and in a water phantom.

Results

Based on the results, a new MLC with improved dosimetric parameters is proposed. The physical penumbra for proposed MLC is 4.7mm was compared to 5.16 mm for Millennium 120 leaf. Average leakage in our design is reduced to 1.16% compared to 1.73% for Millennium 120 leaf, the end leaf leakage suggested design also reduced to 4.86% compared to 7.26% for Millennium 120 leaf.

Conclusion

The results show that the proposed MLC could improve the dosimetric parameters and conformity of treatment planning.

Effect of age-dependent bone electron density on the calculated dose distribution from kilovoltage and megavoltage photon and electron radiotherapy in paediatric MRI-only treatment planning

OBJECTIVE

MRI-only treatment planning (TP) can be advantageous in paediatric radiotherapy. However, electron density extraction is necessary for dose calculation. Normally, after bone segmentation, a bulk density is assigned. However, the variation of bone bulk density in patients makes the creation of pseudo CTs challenging. This study aims to assess the effects of bone density variations in children on radiation attenuation and dose calculation for MRI-only TP.

METHODS

Bone contents of <15-year-old children were calculated, and substituted in the Oak Ridge National Laboratory paediatric phantoms. The percentage depth dose and beam profile of 150 kVp and 6 MV photon and 6 MeV electron beams were then calculated using Xcom, MCNPX (Monte Carlo N-particle version X) and ORLN phantoms.

RESULTS

Using 150 kVp X-rays, the difference in attenuation coefficient was almost 5% between an 11-year-old child and a newborn, and ~8% between an adult and a newborn. With megavoltage radiation, the differences were smaller but still important. For an 18 MV photon beam, the difference of radiation attenuation between an 11-year-old child and a newborn was 4% and ~7.4% between an adult and a newborn. For 6 MeV electrons, dose differences were observed up to the 2 cm depth. The percentage depth dose difference between 1 and 10-year-olds was 18.5%, and between 10 and 15-year-olds was 24%.

CONCLUSION

The results suggest that for MRI-only TP of photon- or electron-beam radiotherapy, the bone densities of each age group should be defined separately for accurate dose calculation. Advances in knowledge: This study highlights the need for more age-specific determination of bone electron density for accurate dose calculations in paediatric MRI-only radiotherapy TP.