Organ dose conversion coefficients for 0.1-10 MeV electrons calculated for the VIP-Man tomographic model

A whole-body tomographic model, called VIP-Man, was recently developed at Rensselaer Polytechnic Institute from the high-resolution color photographic images of the National Library of Medicine's Visible Human Project. An EGS4-based Monte Carlo user code, named EGS4-VLSI, was developed to efficiently transport electrons using the large image data set for VIP-Man. VIP-Man has been used to calculate doses for neutrons and photons. This paper presents a new set of fluence-to-absorbed-dose conversion coefficients for monoenergetic electron beams between 100 keV and 10 MeV for VIP-Man. Irradiation conditions include anterior-posterior, posterior-anterior, right lateral, left lateral, rotational, and isotropic source geometries. Comparisons between organ doses from VIP-Man, which is taller and heavier than the Reference Man, and existing data from mathematical models show significant discrepancies. It appears that even slight differences between body models can cause dramatic dosimetric deviations for low penetrating electron irradiation. This suggests that a single standard body model may poorly represent a large population and may not be acceptable for electron dosimetry.     

The development and application of the visible Chinese human model for Monte Carlo dose calculations

A new whole-body computational phantom, the Visible Chinese Human (VCH), was developed using high-resolution transversal photographs of a Chinese adult male cadaver. Following the segmentation and tridimensional reconstruction, a voxel-based model that faithfully represented the average anatomical characteristics of the Chinese population was established for radiation dosimetry. The vascular system of VCH was fully preserved, and the cadaver specimen was processed in the standing posture. A total of 8,920 slices were obtained by continuous sectioning at 0.2 mm intervals, and 48 organs and tissues were segmented from the tomographic color images at 5440 x 4080 pixel resolution, corresponding to a voxel size of 0.1 x 0.1 x 0.2 mm3. The resulting VCH computational phantom, consisting of 230 x 120 x 892 voxels with a unit volume of 2 x 2 x 2 mm3, was ported into Monte Carlo code MCNPX2.5 to calculate the conversion coefficients from kerma free-in-air to absorbed dose and to effective dose for external monoenergetic photon beams from 15 keV to 10 MeV under six idealized external irradiation geometries (anterior-posterior, posterior-anterior, left lateral, right lateral, rotational, and isotropic). Organ masses of the VCH model are fairly different from other human phantoms. Differences of up to 300% are observed between doses from ICRP 74 data and those of VIP-Man. Detailed information from the VCH model is able to improve the radiological datasets, particular for the Chinese population, and provide insights into the research of various computational phantoms.     

Fluence-to-dose conversion coefficients based on the VIP-Man anatomical model and MCNPX code for monoenergetic neutrons above 20 MeV

A new set of fluence-to-absorbed dose and fluence-to-effective dose conversion coefficients has been calculated for high-energy neutrons using a whole-body anatomical model, VIP-Man, developed from the high-resolution transversal color photographic images of the National Library of Medicine's Visible Human Project. Organ dose calculations were performed using the Monte Carlo code MCNPX for 20 monoenergetic neutron beams between 20 MeV and 10,000 MeV under 6 different irradiation geometries: anterior-posterior, posterior-anterior, left lateral, right lateral, isotropic, and rotational. For neutron Monte Carlo calculations, results based on an image-based whole-body model were not available in the literature. The absorbed dose results for 24 major organs of VIP-Man are presented in the form of tables and selected figures that compare with those based on simplified mathematical phantoms reported in the literature. VIP-Man yields up to 40% larger values of effective dose and many organ doses, thus suggesting that the results reported in the past may not be conservative.     

Photon dose conversion coefficients for human teeth in standard irradiation geometries

Photon dose conversion coefficients for human tooth materials are computed in energy range from 0.01 to 10 MeV by the Monte Carlo method. The voxel phantom "Golem" of the human body with newly defined tooth region and a modified version of the EGS4 code have been used to compute the coefficients for 30 tooth cells with different locations and materials. The dose responses are calculated for cells representing buccal and lingual enamel layers. The computed coefficients demonstrate a strong dependence on energy and geometry of the radiation source and a weaker dependence on location of the enamel voxels. For isotropic and rotational radiation fields, the enamel dose does not show a significant dependence on tooth sample locations. The computed coefficients are used to convert from absorbed dose in teeth to organ dose or to integral air kerma. Examples of integral conversion factors from enamel dose to air kerma are given for several photon fluences specific for the Mayak reprocessing plant in Russia. The integral conversion factors are strongly affected by the energy and angular distributions of photon fluence, which are important characteristics of an exposure scenario for reconstruction of individual occupational doses.     

VIP-Man: an image-based whole-body adult male model constructed from color photographs of the Visible Human Project for multi-particle Monte Carlo calculations

Human anatomical models have been indispensable to radiation protection dosimetry using Monte Carlo calculations. Existing MIRD-based mathematical models are easy to compute and standardize, but they are simplified and crude compared to human anatomy. This article describes the development of an image-based whole-body model, called VIP-Man, using transversal color photographic images obtained from the National Library of Medicine's Visible Human Project for Monte Carlo organ dose calculations involving photons, electron, neutrons, and protons. As the first of a series of papers on dose calculations based on VIP-Man, this article provides detailed information about how to construct an image-based model, as well as how to adopt it into well-tested Monte Carlo codes, EGS4, MCNP4B, and MCNPX.     

Specific absorbed fractions for internal photon emitters calculated for a tomographic model of a pregnant woman

Specific absorbed fractions are essential for calculation of radiation dose from internal emitters. Existing specific absorbed fractions for pregnant women were calculated using the stylized models; in this work, a partial-body tomographic model for a pregnant woman was constructed from a rare set of CT images. Based on this tomographic model, the Monte Carlo code, EGS4-VLSI, was used to derive specific absorbed fractions. Monoenergetic, isotropic photon emitters from 15 keV to 4 MeV were distributed in different source organs, and doses were calculated to many target regions in the body. Even though the results showed general agreement with previous studies for higher energies, significant differences were also found, especially for lower energies. The main reasons for the differences are due to the variation of mass, geometry, and organ distances, and they demonstrate the influence of more realistic body models on dose calculations.     

New calculations of neutron kerma coefficients and dose equivalent.

For neutron energies ranging from 1 keV to 20 MeV, the kerma coefficients for elements H, C, N, O, light water, and ICRU tissue were deduced respectively from microscopic cross sections and Monte Carlo simulation (MCNP code). The results are consistent within admitted uncertainties with values evaluated by an international group (Chadwick et al 1999 Med. Phys. 26 974-91). The ambient dose equivalent generated in the ISO-recommended neutron field for an Am-Be neutron source (ISO 8529-1: 2001(E)) was obtained from the kerma coefficients and Monte Carlo calculation. In addition, it was calculated directly by multiplying the neutron fluence by the fluence-to-ambient dose conversion coefficients recommended by ICRP (ICRP 1996 ICRP Publication 74 (Oxford: Pergamon)). The two results agree well with each other. The main feature of this work is our Monte Carlo simulation design and the treatments differing from the work of others in the calculation of neutron energy transfer in non-elastic processes.     

Fluence-to-absorbed dose conversion coefficients for use in radiological protection of embryo and foetus against external exposure to electrons from 10 MeV TO 10 GeV

Electrons as primary and more often as secondary radiation exist commonly in the environment and workplaces. No conversion coefficients are yet available, in the literature, for use in radiological protection of embryo and foetus against external exposure to electrons. This study uses mathematical models developed by the Radiation Protection Bureau, Health Canada, for the embryo of 8 wk and for the foetus of 3, 6, and 9 mo. Monte Carlo code MCNPX is used to determine mean absorbed doses to the embryo and foetus when the mother is exposed to external electron fields. Monoenergetic electrons ranging from 10 MeV to 10 GeV were considered. The irradiation geometries include antero-posterior (AP), postero-anterior (PA), lateral (LAT), rotational (ROT), isotropic (ISO), and top-down (TOP). At each of these irradiation geometries, absorbed doses to the foetal brain and body were calculated for the embryo of 8 wk and the foetus of 3, 6, and 9 mo. Electron fluence-to-absorbed dose conversion coefficients were derived for the four prenatal ages.     

Mayak film dosimeter response studies, part II: response models

A study was performed of energy and angular responses of the film dosimeters that were used for worker monitoring at the Mayak Production Association (Mayak PA) in 1948-1992. The study used experimental data from tests with three types of individual film dosimeters, and the data were used to determine the dosimeters' energy and angular response characteristics in the range from 9 keV to Co energies, with the dosimeters exposed both free-in-air and on-phantom at horizontal and vertical rotation. Mathematical models of the dosimeters were developed to calculate the response characteristics of the dosimeters. The models of the film dosimeters were validated by comparing calculations to measurements. The models were then used as the basis for individual dose reconstruction in realistic photon spectra and worker exposure geometries at the Mayak PA workplaces. Reconstructed individual doses have been included in the Mayak worker database "Doses-2005" that is used for epidemiological studies of the Mayak workers' radiation exposures and subsequent health effects.     

Determining organ dose conversion coefficients for external neutron irradiation by using a voxel mouse model

A set of fluence-to-dose conversion coefficients has been calculated for neutrons with energies <20 MeV using a developed voxel mouse model and Monte Carlo N-particle code (MCNP), for the purpose of neutron radiation effect evaluation. The calculation used 37 monodirectional monoenergetic neutron beams in the energy range 10⁻⁹ MeV to 20 MeV, under five different source irradiation configurations: left lateral, right lateral, dorsal–ventral, ventral–dorsal, and isotropic. Neutron fluence-to-dose conversion coefficients for selected organs of the body were presented in the paper, and the effect of irradiation geometry conditions, neutron energy and the organ location on the organ dose was discussed. The results indicated that neutron dose conversion coefficients clearly show sensitivity to irradiation geometry at neutron energy below 1 MeV.     

Adult female voxel models of different stature and photon conversion coefficients for radiation protection

This paper describes the construction of three adult female voxel models, two whole-body and one from head to thighs, from computed tomographic data of 3 women of different stature. Voxel models (also called phantoms) are human models based on computed tomographic or magnetic resonance images obtained from high resolution continuous scans of a single individual. The gray-scale data or information content of the medical images are interpreted into tissues (i.e., organs), a process known as segmentation. The phantoms, consisting of millions of volume elements, called voxels, provide a three-dimensional representation of the human body and the spatial form of its constituent organs and structures. They were initially developed for radiation protection purposes to estimate the organ and effective doses and hence the risk to a person or population due to an irradiation. This paper also presents conversion coefficients for idealized geometries of external photon exposures of energies 10 keV-1 MeV for the three female models, calculated with a Monte Carlo code. Until now there were not any published data on conversion coefficients for explicit female voxel models. Such sets of conversion coefficients exist for voxel adult males or for MIRD-type male, female, and hermaphrodite models. Numerical differences of the calculated conversion coefficients for the voxel female models and MIRD-type models can amount up to 60% or more for external exposures and are due to the improved anatomical realism of the voxel models. The size of the model also has an effect on the conversion coefficients, particularly for deeper lying organs and energies below 200 keV. The three separate sets of conversion coefficients allow one to choose the most suitable model according to the size of the individual as well as to study the dosimetric variations due to the size of the model.     

Stereotactic synchrotron microbeam radiotherapy

Highly collimated synchrotron x-ray beams with high fluence rate may be used in stereotactic radiotherapy of brain tumours. Several monochromatic x-ray beams having uniform microscopic thickness ie (microplanar beams) are directed to the center of the tumour from varying directions, delivering lethal dose to the target volume while sparing normal cells. This proposed technique takes advantage of the hypothesised repair mechanism of capillaries between closely spaced microplanar beam zones. The sharply dropping lateral dose profile of a microplanar beam provides low scattered dose to the off-target interbeam volume. In close proximity to the target volume, relatively high secondary electron doses close to the edge of the beams overlap and produce a high dose region between angled beams. This allows precise targeting and prevents gradual blurring of the higher and lower dose margins in the target volume. The advantages of stereotactic microplanar beam radiotherapy will be lost as the dose between microplanar beams exceeds the tolerance dose of the dose limiting tissues. Therefore to minimize the risks of delayed radiation damage it is essential to optimize the interbeam doses inside a human head phantom. The EGS4 Monte Carlo code is used to calculate the lateral dose profiles and depth dose of a 100 keV single microplanar beam in the phantom. A general equation for absorbed dose as a function of depth and lateral distances is derived for the single beam. Several microplanar beams are directed into the target volume at the center of the phantom. Using the equation, maximum dose on the beam axis (primary + total scattered dose) and the minimum interbeam dose (total scattered dose) are calculated at different depths and an isodose map of the phantom is obtained. A stereotactic microplanar beam radiotherapy model is proposed for a 10 mm diameter (approximately spherical) tumour at the center of the phantom.     

Monte Carlo calculation for microplanar beam radiography

In radiography the scattered radiation from the off-target region decreases the contrast of the target image. We propose that a bundle of collimated, closely spaced, microplanar beams can reduce the scattered radiation and eliminate the effect of secondary electron dose, thus increasing the image dose contrast in the detector. The lateral and depth dose distributions of 20-200 keV microplanar beams are investigated using the EGS4 Monte Carlo code to calculate the depth doses and dose profiles in a 6 cm x 6 cm x 6 cm tissue phantom. The maximum dose on the primary beam axis (peak) and the minimum inter-beam scattered dose (valley) are compared at different photon energies and the optimum energy range for microbeam radiography is found. Results show that a bundle of closely spaced microplanar beams can give superior contrast imaging to a single macrobeam of the same overall area.     

Extended conversion coefficients for use in radiation protection of the embryo and fetus against external neutrons from 10 MeV to 100 GeV

External neutron exposure is of concern in the environment and in some workplaces. Dose assessments for neutrons frequently rely on fluence-to-absorbed dose conversion coefficients. A problem of concern in radiation protection is exposure of pregnant women to ionizing radiation because of the high radiosensitivity of the embryo and fetus. While neutron fluence-to-dose conversion coefficients for adults are recommended in ICRP publications and ICRU reports, conversion coefficients for embryos and fetuses are not given in the publications. This study uses the Monte Carlo code MCNPX to determine mean absorbed doses to the embryo and fetus when the mother is exposed to neutron fields. A previous study has dealt with neutrons from 1 eV to 10 MeV. In this study, monoenergetic neutrons ranging from 10 MeV to 100 GeV are considered. The irradiation geometries include antero-posterior, postero-anterior, lateral, rotational, and isotropic. At each of these standard irradiation geometries, absorbed doses to the fetal brain and body are calculated for the embryo of 8 wk and the fetus of 3, 6, or 9 mo. Neutron fluence-to-absorbed dose conversion coefficients are derived for the four prenatal ages. The results showed that the fetus at about 3 mo of prenatal age should receive more radiation protection to prevent long-term brain damage. During prenatal life, the fetus generally receives the highest absorbed dose per unit neutron fluence for antero-posterior irradiation. In cases where the irradiation geometry is not specified or not adequately known, conversion coefficients of AP-irradiation can therefore be used in a conservative dose assessment of fetus exposure to external neutrons.     

Optimisation of electron cone design in high energy radiotherapy using the Monte Carlo method

Cylindrical solid-walled steel electron collimators are used at the Royal Adelaide Hospital with a Siemens KD2 Mevatron accelerator to produce circular fields 2-8 cm in diameter. The cones are used in contact with the patient's skin. A flat treatment field is required at the treatment depth and the beam should also satisfy the uniformity standards as specified by the International Electrotechnical Commission (IEC). However, the seven and eight centimetre diameter cones provided by the manufacturer did not meet these specifications. In particular, the maximum dose relative to the depth-dose maximum on the central axis exceeded 126% as compared with the IEC recommended value of 109%, when used with a 21 MeV electron beam. Cone modifications were previously investigated by others with the results demonstrating some improvement in the 'horn' (as it appears on surface dose profiles) but still not satisfying IEC requirements. In the present paper the EGS4 code was used to model the existing treatment head geometry and cones, as well as new suggested modifications to the cone. The results of the simulation for the existing cone geometry corresponded closely to previously obtained measurements. The suggested collimator modifications involved a plastic insert along the internal wall of the collimator. Variations of the insert width and height were simulated for a 21 MeV electron beam and the results plotted to indicate the optimal insert dimensions. A plastic insert with the dimensions taken from one of the best models was produced and tested. The measurements showed close agreement with the simulation results (for the 'horn' height, dose within 1% and radial position within 2 mm) and improvement of the "maximum ratio of absorbed dose" from 126% before modification to 108% with the plastic insert. The tested insert was also simulated for a 12 MeV electron beam, to see whether permanent fitting of such an insert would have a deleterious effect at lower energies. Neither penetration nor flatness was significantly compromised, with a small penalty being a slight increase in the central axis dose near the surface.     

Measurements of neutron effective doses and attenuation lengths for shielding materials at the heavy-ion medical accelerator in Chiba

The effective doses and attenuation lengths for concrete and iron were measured for the design of heavy ion facilities. Neutrons were produced through the reaction of copper, carbon, and lead bombarded by carbon ions at 230 and 400 MeV.A, neon ions at 400 and 600 MeV.A, and silicon ions at 600 and 800 MeV.A. The detectors used were a Linus and a Andersson-Braun-type rem counter and a detector based on the activation of a plastic scintillator. Representative effective dose rates (in units of 10(-8) microSv h(-1) pps(-1) at 1 m from the incident target surface, where pps means particles per second) and the attenuation lengths (in units of m) were 9.4 x 10(4), 0.46 for carbon ions at 230 MeV.A; 8.9 x 10(5), 0.48 for carbon ions at 400 MeV.A; 9.3 x 10(5), 0.48 for neon ions at 400 MeV.A; 3.8 x 10(6), 0.50 for neon ions at 600 MeV.A; 3.9 x 10(6), 0.50 for silicon ions at 600 MeV.A; and 1.1 x 10(7), 0.51 for silicon ions at 800 MeV.A. The attenuation provided by an iron plate approximately 20 cm thick (nearly equal to the attenuation length) corresponded to that of a 50-cm block of concrete in the present energy range. Miscellaneous results, such as the angular distributions of the neutron effective dose, narrow beam attenuation experiments, decay of gamma-ray doses after the bombardment of targets, doses around an irradiation room, order effects in the multi-layer (concrete and iron) shielding, the doses from different targets, the doses measured with a scintillator activation detector, the gamma-ray doses out of walls and the ratio of the response between the Andersson-Braun-type and the Linus rem counters are also reported.     

A comparison between default EGS4 and EGS4 with bound Compton cross sections when scattering occurs in bone and fat

Computer simulation packages are important tools in understanding how radiation interacts with matter. EGS4 is a photon/electron Monte Carlo transport program that is employed in the health/medical physics field. Due to its high energy roots, the default version of EGS4 treats all electrons as unbound and therefore uses the Klein-Nishina cross section formula to determine Compton scattering angle distributions and the probability of Compton scattering through the branching ratio. Researchers have created improvements to EGS4 that account for the bound Compton cross section as well as other scattering properties. Numerical experiments were performed on both the default code and modified EGS4 to examine output differences in low Z materials such as fat and bone. Four incident photon energies were considered. At higher energies (500 keV and 1 MeV) the default and modified EGS4 codes produced results within 2sigma of one another. At 50 and 100 keV differences in scattering angle distribution and branching ratio values were found. In addition, the number of photoelectric absorptions and Compton scatters were also different at these energies.     

Superposition dose calculation in lung for 10MV photons

Currently available radiotherapy treatment planning systems employ scatter function models such as ETAR and Batho dSAR for dose calculation. Errors using these models for high energy photon irradiation occur in and beyond lung tissue for small fields. For larger fields, central axis dose is correctly predicted but penumbral broadening in lung is underestimated. The major source of error is the assumption that lateral electronic equilibrium is always established. A superposition algorithm has been developed for 10MV photons which calculates the dose by convolving the TERMA (Total Energy Released per unit MAss by primary photons) with a dose spread array formed using the EGS4 Monte Carlo code. TERMA and dose spread arrays are both generated using a 10 component photon energy spectrum. Dose in inhomogeneous media is calculated using dose spread arrays generated for different density media and by scaling dose spread arrays according to density variations. This method ensures that electronic disequilibrium is modelled in situations where it exists. Superposition results in a lung phantom for a 5 x 5 cm field agree with EGS4 Monte Carlo results to within 2% for p = 0.20 gcm-3 and p = 0.30 gcm-3 lung. Profiles generated by superposition for a 10 x 10 cm field at mid-lung and compared with film measurements show that penumbral broadening in low density material is also correctly predicted.     

Calculation of dose profiles in stereotactic Synchrotron microplanar beam radiotherapy in a tissue-lung phantom

Synchrotron x-ray beams with high fluence rate and highly collimated may be used in stereotactic radiotherapy of lung tumours. A bundle of converging monochromatic x-ray beams having uniform microscopic thickness i.e. (microplanar beams) are directed to the center of the tumour, delivering lethal dose to the target volume while sparing normal cells. The proposed technique takes advantage of the hypothesised repair mechanism of capillary cells between alternate microbeam zones, which regenerate the lethally irradiated endothelial cells. The sharply dropping lateral dose of a microbeam provides low scattered dose to the off-target interbeam volume. In the target volume the converging bundle of beams are closely spaced, and relatively high primary and secondary electron doses overlap and produce a high dose region between the beams. This higher and lower dose margins in the target volume allows precise targeting. The advantages of stereotactic microbeam radiotherapy will be lost as the dose between microbeams exceeds the tolerance dose of the dose limiting tissues. Therefore, it is essential to optimize the interbeam doses in off-target volume. The lateral and depth doses of 100 keV microplanar beams are investigated for a single beam and an array of converging microplanar beams in a tissue, lung and tissue-lung phantoms. The EGS5 Monte Carlo code is used to calculate dose profiles at different depths and bundles of beams. The maximum dose on the beam axis (peak) and the minimum interbeam dose (valley) are compared at different energies, depths, bundle sizes, heights, widths and beam spacings. The interbeam dose is calculated at different depths and an isodose map of the phantom is obtained. An acceptable energy region is found for tissue and lung microbeam radiotherapy and a stereotactic microbeam radiotherapy model is proposed for a 4 cm diameter and 1 cm thick tumour on the lung phantom.     

Relative biological effectiveness of 290 MeV/u carbon ions for the growth delay of a radioresistant murine fibrosarcoma

The relative biological effectiveness (RBE) for animal tumors treated with fractionated doses of 290 MeV/u carbon ions was studied. The growth delay of NFSa fibrosarcoma in mice was investigated following various daily doses given with carbon ions or those given with cesium gamma-rays, and the RBE was determined. Animal tumors were irradiated with carbon ions of various LET (linear energy transfer) in a 6-cm SOBP (spread-out Bragg peak), and the isoeffect doses; i.e. the dose necessary to induce a tumor growth delay of 15 days were studied. The iso-effect dose for carbon ions of 14 and 20 keV/microm increased with an increase in the number of fractions up to 4 fractions. The increase in the isoeffect dose with the fraction number was small for carbon ions of 44 keV/microm, and was not observed for 74 keV/microm. The alpha and beta values of the linear-quadratic model for the radiation dose-cell survival relationship were calculated by the Fe-plot analysis method. The alpha values increased linearly with an increase in the LET, while the beta values were independent of the LET. The alpha/beta ratio was 129 +/- 10 Gy for gamma-rays, and increased with an increase in the LET, reaching 475 +/- 168 Gy for 74 keV/microm carbon ions. The RBE for carbon ions relative to Cs-137 gamma-rays increased with the LET. The RBE values for 14 and 20 keV/microm carbon ions were 1.4 and independent of the number of fractions, while those for 44 and 74 keV/microm increased from 1.8 to 2.3 and from 2.4 to 3.0, respectively, when the number of fractions increased from 1 to 4. Increasing the number of fractions further from 4 to 6 was not associated with an increase in the RBE. These results together with our earlier study on the skin reaction support the use of an RBE of 3.0 in clinical trials of 80 keV/microm carbon beams. The RBE values for low doses of carbon beams were also considered.