Adult Male Voxel Phantoms Research Articles

Dosimetric effects of different hip prosthesis materials during pelvic radiotherapy using high energy photons

This study investigates the effects of different hip prosthesis materials on the secondary neutron and electron contamination produced by high energy, i.e. 18 MV, photon beams during pelvic irradiation. Three different materials, including Titanium-alloy (Ti-alloy), Stainless Steel (SST), and Cobalt–Chromium–Molybdenum-alloy (Co–Cr–Mo), were evaluated in a Monte Carlo (MC) study. The Varian 2100 C/D linac head and an ICRP reference adult male voxel phantom with a realistic hip prosthesis were simulated using the MCNPX (ver. 2.6) MC code. As expected, results showed that secondary electron and neutron generation mainly depends on the shape and material type of the prosthesis. The mean dose increase factor (DIF) for electron contamination is 1.72, 1.53, and 1.35 for Co–Cr–Mo, SST, and Ti-alloy, respectively, at the tissue-prosthesis interface. Electron contamination also depends on the prosthesis thickness and it is increased by increasing the thickness. The maximum DIF for neutrons occurs at the center of the prosthesis stem and its mean DIF is 2.91, 2.49, and 2.34 underneath Co–Cr–Mo, SST, and Ti-alloy prostheses, respectively. The amount of neutron DIF is slightly reduced above prostheses. Furthermore, the results indicate that the most significant change in the electron dose distribution is observed at the tissue-prosthesis interface. The neutron dose equivalent is increased from 76% to 200% in the presence of the different materials of the prosthesis. Hip prostheses made of Ti-alloy can significantly decrease the dose due to neutron and electron contamination, compared to the other materials considered in the simulations.

Numerical estimation of calibration matrices for 241Am measurements using HPGe based in-vivo monitoring system

In-vivo measurements of 241Am using HPGe detectors become complicated when the active adjacent source organs interfere with the target organ measurements. It is important to calculate the contribution of confounding organs to estimate the activity of the target organ accurately. In the current study, numerical simulations were performed using FLUKA Monte Carlo code and International Commission on Radiological Protection (ICRP) computational reference phantoms to determine the calibration matrices consisting of the calibration and cross-talk coefficients for three interfering organs namely, lungs, liver and skeleton. It was found that the interference from adjacent organs contaminated with 241Am was found to be significant in the case of lungs and liver. Knee monitoring was least influenced by the activity possessed by other source organs due to their anatomical distance from the knee. A comparison between lung and liver coefficients obtained from the Lawrence Livermore National Laboratory (LLNL) torso phantom and thorax phantom derived from ICRP adult male voxel phantom was performed. It was found that variations in coefficients obtained from simulations and experiments range between 2% and 48%. The differences were attributed to the uncertainties arising from the composition of the phantoms and detectors, size and shape of organs, positional errors, and source distribution. A comparison of calibration matrices of adult male and adult female thorax voxel phantoms revealed that all the coefficients except knee as the target organ were larger for the female thorax phantom owing to the lesser bulk of attenuating tissues on its chest. The coefficients obtained from simulations for different phantoms also showed that the organ activity estimation can be significantly affected by the subject morphology.

Dose conversion coefficients for ICRP110 voxel phantom in the Geant4 Monte Carlo code

The reference adult male voxel phantom recommended by International Commission on Radiological Protection no. 110 was implemented in the Geant4 Monte Carlo code. Geant4 was used to calculate Dose Conversion Coefficients (DCCs) expressed as dose deposited in organs per air kerma for photons, electrons and neutrons in the Annals of the ICRP. In this work the AP and PA irradiation geometries of the ICRP male phantom were simulated for the purpose of benchmarking the Geant4 code. Monoenergetic photons were simulated between 15keV and 10MeV and the results were compared with ICRP 110, the VMC Monte Carlo code and the literature data available, presenting a good agreement.

Analysis of Organ Doses from Computed Tomography (CT) Examination by the Radiation Transport Calculation to Develop the Dosimetry System, WAZA-ARI

A new dosimetry system, named WAZA-ARI, is being developed to assess organ doses of a patient in a Computed Tomography (CT) examination. A Japanese adult male voxel phantom (JM phantom) is applied to derive the basic data for dose assessment in WAZA-ARI. The JM phantom defines outer and inner configurations of the human body more precisely than mathematical human models, such as the MIRD-type phantom. The radiation transport in CT examination was simulated with the Particle and Heavy Ion Transport code System, PHITS. A subroutine in PHITS described a source model to simulate emission of photons in a CT machine. This study clarified the relationships between the organ doses and X-ray scanning on a human body with some geometrical conditions.

Organ dose conversion coefficients for external photon irradiation using the Chinese voxel phantom (CVP)

A set of conversion coefficients from kerma free-in-air to the organ absorbed dose are presented for external monoenergetic photon beams from 10 keV to 10 MeV based on a whole-body, Chinese adult male voxel phantom. This computational phantom, called the Chinese voxel phantom (CVP), including totally 23 organs, was developed from magnetic resonance imaging of a young healthy Chinese man at a resolution of 2 x 2 mm. Compared with the ICRP Reference Man, more than half of the organs or tissues in the CVP show mass differences of more than 20. Monte Carlo simulations with MCNP code were carried out to calculate the organ dose conversion coefficients. Irradiation conditions include anterior-posterior, posterior-anterior (PA), right-lateral, left-lateral, rotational and isotropic geometries. Organ dose conversion coefficients from this study are compared with the data from the Asian voxel phantoms Visible Chinese Human and KORMAN. These data sets agree with each other within 10% for photon energy >5 MeV. However, discrepancies of 34-63% were observed for organs of the alimentary tract, such as the oesophagus and stomach, those of the urinary system, such as the bladder wall and thyroid, especially at low photon energy range and PA geometry. These results suggest that the anatomical variation within the Chinese population, as represented by these adult male voxel phantoms, can lead to uncertainties when a standard phantom is used for the entire population.

CNMAN: a Chinese adult male voxel phantom constructed from color photographs of a visible anatomical data set

A voxel phantom of Chinese adult male called CNMAN was constructed from color photographs of the first Chinese visible human data set, for radiation protection purpose. This data set was obtained from a 35-y-old Chinese male cadaver by a medical university in China. The man, 170 cm in height and 65 kg in weight, was dead without any pathological changes. The image size for transversal anatomical photographs of the whole body was 3072 x 2048. After the photographs were semi-automatically segmented, the voxel phantom (CNMAN) with a voxel size of 0.16 mm x 0.16 mm x 1 mm, consisting of 29 tissues or organs was constructed. Combined with the MCNP Monte Carlo transport code, preliminary results for radiation protection dosimetry were obtained based on this Chinese voxel phantom.

On the need to revise the arm structure in stylized anthropomorphic phantoms in lateral photon irradiation geometry

Distributions of radiation absorbed dose within human anatomy have been estimated through Monte Carlo radiation transport techniques implemented for two different classes of computational anthropomorphic phantoms: (1) mathematical equation-based stylized phantoms and (2) tomographic image-based voxel phantoms. Voxel phantoms constructed from tomographic images of real human anatomy have been actively developed since the late 1980s to overcome the anatomical approximations necessary with stylized phantoms, which themselves have been utilized since the mid 1960s. However, revisions of stylized phantoms have also been pursued in parallel to the development of voxel phantoms since voxel phantoms (1) are initially restricted to the individual-specific anatomy of the person originally imaged, (2) must be restructured on an organ-by-organ basis to conform to reference individual anatomy and (3) cannot easily represent very fine anatomical structures and tissue layers that are thinner than the voxel dimensions of the overall phantom. Although efforts have been made to improve the anatomic realism of stylized phantoms, most of these efforts have been limited to attempts to alter internal organ structures. Aside from the internal organs, the exterior shapes, and especially the arm structures, of stylized phantoms are also far from realistic descriptions of human anatomy, and may cause dosimetry errors in the calculation of organ-absorbed doses for external irradiation scenarios. The present study was intended to highlight the need to revise the existing arm structure within stylized phantoms by comparing organ doses of stylized adult phantoms with those from three adult voxel phantoms in the lateral photon irradiation geometry. The representative stylized phantom, the adult phantom of the Oak Ridge National Laboratory (ORNL) series and two adult male voxel phantoms, KTMAN-2 and VOXTISS8, were employed for Monte Carlo dose calculation, and data from another voxel phantom, VIP-Man, were obtained from literature sources. The absorbed doses for lungs, oesophagus, liver and kidneys that could be affected by arm structures in the lateral irradiation geometry were obtained for both classes of phantoms in lateral monoenergetic photon irradiation geometries. As expected, those organs in the ORNL phantoms received apparently higher absorbed doses than those in the voxel phantoms. The overestimation is mainly attributed to the relatively poor representation of the arm structure in the ORNL phantom in which the arm bones are embedded within the regions describing the phantom's torso. The results of this study suggest that the overestimation of organ doses, due to unrealistic arm representation, should be taken into account when stylized phantoms are employed for equivalent or effective dose estimates, especially in the case of an irradiation scenario with dominating lateral exposure. For such a reason, the stylized phantom arm structure definition should be revised in order to obtain more realistic evaluations.

Japanese adult male voxel phantom constructed on the basis of CT images

A Japanese adult male voxel (volume pixel) phantom (hereinafter referred to as the JM phantom) was constructed on the basis of CT images of a healthy Japanese adult male volunteer. Body characteristics of the JM phantom were compared with those of a voxelised MIRD5 type phantom and a Japanese adult male voxel phantom which was previously developed. The voxel size of the JM phantom is 0.98 x 0.98 x 1 mm(3). The shapes of the organs of the JM phantom, even for small or complicated organs, such as thyroid and stomach, are more realistically reproduced as compared with the previous Japanese voxel phantom (voxel size: 0.98 x 0.98 x 10 mm(3)). Photon self-absorbed fractions (self-AFs) for brain, kidneys, spleen, pancreas, thyroid and urinary bladder wall of JM were evaluated and were compared with those of the other phantoms. In consequence, it was suggested that the mass, shape and thickness of organs are important factors for the determination of self-AFs.