Evaluation of the scattered radiations of lead and lead-free aprons in diagnostic radiology by MCNPX

How efficient are metal-polymer and dual-metals-polymer non-lead radiation shields?

INTRODUCTION

Lead shields are often used to attenuate ionising radiations. However, to make lighter, recyclable and more efficient shields compared to lead, combinations of new metallic compounds together with polymer, for example, flexible polyvinyl chloride (PVC) have been developed recently. In this study, the capabilities of non-lead radiation shields made of one or two metallic compounds and polymer were evaluated.

METHODS

Monte Carlo (MC)-based BEAMnrc code was used to build a functional model based on a Philips X-ray machine in the range of radiographic energies. The MC model was then verified by IPEM Report 78 as a standardised global reference. The MC model was then used to evaluate the efficiency of non-lead-based garments made of metallic compound and polymer (MCP) including BaSO4 -PVC, Bi2 O3 -PVC, Sn-PVC and W-PVC, as well as dual-metallic compounds and polymer (DMCP) including Bi2 O3 -BaSO4 -PVC, Bi2 O3 -Sn-PVC, W-Sn-PVC and W-BaSO4 -PVC. The absorbed doses were determined at the surface of a water phantom and compared directly with the doses obtained for 0.5 mm pure lead (Pb).

RESULTS

Bi2 O3 -BaSO4 -PVC and W-BaSO4 -PVC were found to be efficient shields for most of the energies. In addition to the above radiation shields, Bi2 O3 -Sn-PVC was also found to be effective for the spectrum of 60 keV. Bi2 O3 -BaSO4 -PVC as a non-lead dual metals-PVC shield was shown to be more efficient than pure lead in diagnostic X-ray range.

CONCLUSION

Combination of two metals-PVC, a low atomic number (Z) metal together with a high atomic number metal, and also single-metal-PVC shields were shown to be efficient enough to apply as radiation protection shields instead of lead-based garments.

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.