A high-resolution anthropomorphic voxel-based tomographic phantom for proton therapy of the eye

MCNPX Estimation of Photoneutron Dose to Eye Voxel Anthropomorphic Phantom From 18 MV Linear Accelerator

The dose due to photoneutron contamination outside the field of irradiation can be significant when using high-energy linear accelerators. The eye is a radiation-sensitive organ, and this risk increases when high linear energy transfer neutron radiation is involved. This study aimed to provide a fast method to estimate photoneutron dose to the eye during radiotherapy. A typical high-energy linear accelerator operating at 18 MV was simulated using the Monte Carlo N-Particle Transport Code System extended version (MCNPX 2.5.0). The latest International Atomic Energy Agency photonuclear data library release was integrated into the code, accounting for the most known elements and isotopes used in typical linear accelerator construction. The photoneutron flux from a 5 × 5 cm² field size was scored at the treatment table plane and used as a new source for estimating the absorbed dose in a high-resolution eye voxel anthropomorphic phantom. In addition, common shielding media were tested to reduce the photoneutron dose to the eye using common shielding materials. Introducing a 2 cm thickness of common neutron shielding medium reduced the total dose received in the eye voxel anthropomorphic phantom by 54%. In conclusion, individualized treatment based on photoneutron dose assessment is essential to better estimate the secondary dose inside or outside the field of irradiation.

The effects of simulating a realistic eye model on the eye dose of an adult male undergoing head computed tomography

In head computed tomography, radiation upon the eye lens (as an organ with high radiosensitivity) may cause lenticular opacity and cataracts. Therefore, quantitative dose assessment due to exposure of the eye lens and surrounding tissue is a matter of concern. For this purpose, an accurate eye model with realistic geometry and shape, in which different eye substructures are considered, is needed. To calculate the absorbed radiation dose of visual organs during head computed tomography scans, in this study, an existing sophisticated eye model was inserted at the related location in the head of the reference adult male phantom recommended by the International Commission on Radiological Protection (ICRP). Then absorbed doses and distributions of energy deposition in different parts of this eye model were calculated and compared with those based on a previous simple eye model. All calculations were done using the Monte Carlo code MCNP4C for tube voltages of 80, 100, 120 and 140 kVp. In spite of the similarity of total dose to the eye lens for both eye models, the dose delivered to the sensitive zone, which plays an important role in the induction of cataracts, was on average 3% higher for the sophisticated model as compared to the simple model. By increasing the tube voltage, differences between the total dose to the eye lens between the two phantoms decrease to 1%. Due to this level of agreement, use of the sophisticated eye model for patient dosimetry is not necessary. However, it still helps for an estimation of doses received by different eye substructures separately.

Creation and use of adjustable 3D phantoms: application for the lung monitoring of female workers

In vivo counting measurements, used for the monitoring of workers with internal contamination risks, are based on the use of calibration physical phantoms. However, such phantoms do not exist for female subjects. Computational calibration using numerical representations, Mesh and non-uniform rational basis spline (NURBS) geometries, was thus considered. The study presented here is focused on the creation of different female thoracic phantoms with various breast sizes and chest girths. These 3D models are used to estimate the radiation attenuation with morphology and the resulting variation of the calibration coefficient of a typical 4-germanium in vivo counting system. A basic Mesh female thoracic phantom was created from the International Commission on Radiological Protection Adult Female Reference Computational Phantom. Using this basic phantom, different chest girths (85, 90, 100, 110, and 120) and cup sizes (A to F) were created representing the most common thoracic female morphologies, as recommended by the available and relevant literature. Variation of breast tissue composition and internal organ volumes with morphology were also considered. As a result, 34 thoracic female phantoms were created combining different cup sizes and chest girths. For the 85 chest girth, at very low energies (15 keV), a relative counting efficiency variation of about 85% was observed between the A and E cups. As a result of this study, breast size dependent calibration coefficients, between 15 keV and 1.4 MeV, were obtained and tabulated for a typical lung counting germanium system.