Absorbed dose estimates to structures of the brain and head using a high-resolution voxel-based head phantom

CORRELATION BETWEEN ROUTINE PERSONAL DOSIMETRY READING AND THE DOSE TO THE BRAIN OF INTERVENTIONAL STAFF

This study aimed to evaluate the relationship between the brain absorbed dose and personal dosimetry readings in interventional cardiologists. Interventional procedures were replicated using Monte Carlo simulations (MCNP 6) with anthropomorphic phantoms representing both operator and patient. Absorbed doses were evaluated for 10 predefined regions of the operator's brain as well as for dosemeters at chest and neck level. One beam quality (HVL = 6.2 mm Al) and nine beam projections were considered. A significant bias in the laterality of brain dose was found with doses at the left side of the brain being up to 2.8 times higher compared with the right. The correlation between brain dose and dosemeter reading was found to be dependent on beam projection. Yet, a generalized conversion factor (brain dose normalized by Hp(10)), averaged over all considered beam projections, could be proposed for (retrospective) brain dose estimation from routinely measured dosimetry data.

Monte Carlo simulation of secondary radiation exposure from high-energy photon therapy using an anthropomorphic phantom

The development of intensity-modulated radiotherapy treatments delivering large amounts of monitor units (MUs) recently raised concern about higher risks for secondary malignancies. In this study, optimised combinations of several variance reduction techniques (VRTs) have been implemented in order to achieve a high precision in Monte Carlo (MC) radiation transport simulations and the calculation of in- and out-of-field photon and neutron dose-equivalent distributions in an anthropomorphic phantom using MCNPX, v.2.7. The computer model included a Varian Clinac 2100C treatment head and a high-resolution head phantom. By means of the applied VRTs, a relative uncertainty for the photon dose-equivalent distribution of <1 % in-field and 15 % in average over the rest of the phantom could be obtained. Neutron dose equivalent, caused by photonuclear reactions in the linear accelerator components at photon energies of approximately >8 MeV, has been calculated. Relative uncertainty, calculated for each voxel, could be kept below 5 % in average over all voxels of the phantom. Thus, a very detailed neutron dose distribution could be obtained. The achieved precision now allows a far better estimation of both photon and especially neutron doses out-of-field, where neutrons can become the predominant component of secondary radiation.

Monte Carlo simulation of stereotactic microbeam radiation therapy: evaluation of the usage of a linear accelerator as the x-ray source

The usage of linear accelerator-generated x-rays for the stereotactic microbeam radiation therapy technique was evaluated in this study. Dose distributions were calculated with the Monte Carlo code MCNPX. Unidirectional single beams and beam arrays were simulated in a cylindrical water phantom to observe the effects of x-ray energies and irradiation geometry on dose distributions. Beam arrays were formed with square pencil beams. Two orthogonally interlaced beam arrays were simulated in a detailed head phantom and dose distributions were compared with ones which had been calculated for a bidirectional interlaced microbeam therapy (BIMRT) technique that uses synchrotron-generated x-rays. A parallel pattern of the beams was preserved through the phantom; however an unsegmented dose region could not be formed at the target. Five orthogonally interlaced beam array pairs (ten beam arrays) were simulated in a mathematical head phantom and the unsegmented dose region was formed. However, the dose fall-off distance is longer than the one that had been calculated for the BIMRT technique. Besides, the peak-to-dose ratios between the phantom's outer surface and the target region are lower. Therefore, the advantages of the MRT technique may not be preserved with the usage of a linac as the x-ray source.

A detailed Monte Carlo accounting of radiation transport in the brain during BNCT

The collision type central to BNCT is (10)B(n, alpha)(7)Li, however, other types of nuclear reactions also take place in the patient. In addition to the major elements (H, C, N, O), minor elements such as Na, Mg, P, S, Cl, K, Ca and Fe present in body tissues also interact in neutron collisions. Detailed accounting of the above not only provides a better understanding of radiation transport in the human body during BNCT, but such knowledge affects the design of the facility, as well as treatment planning, imaging and verification for a given BNCT agent. Of the methods of investigation currently available, only Monte Carlo simulation could provide the detailed accounting and breakdown of the quantities required. We report Monte Carlo simulation of an anthropomorphic voxel phantom, the VIP-Man and show how these quantities change with different (10)B concentrations in the tumour, the blood and the remaining tissues. The (10)B biodistribution has been chosen to be the variable of interest, since it is not accurately known, is frequently approximated and is a crucial quantity upon which dose calculations are based.

S-values calculated from a tomographic head/brain model for brain imaging

A tomographic head/brain model was developed from the Visible Human images and used to calculate S-values for brain imaging procedures. This model contains 15 segmented sub-regions including caudate nucleus, cerebellum, cerebral cortex, cerebral white matter, corpus callosum, eyes, lateral ventricles, lenses, lentiform nucleus, optic chiasma, optic nerve, pons and middle cerebellar peduncle, skull CSF, thalamus and thyroid. S-values for C-11, O-15, F-18, Tc-99m and I-123 have been calculated using this model and a Monte Carlo code, EGS4. Comparison of the calculated S-values with those calculated from the MIRD (1999) stylized head/brain model shows significant differences. In many cases, the stylized head/brain model resulted in smaller S-values (as much as 88%), suggesting that the doses to a specific patient similar to the Visible Man could have been underestimated using the existing clinical dosimetry.

Organ and effective dose conversion coefficients for radiographic examinations of the pediatric skull estimated by Monte Carlo methods

The objective of the present work was to provide precise effective and organ dose data for radiographic examinations of the skull performed on pediatric patients. To accomplish this, the MCNP4C2 transport code was utilized and 18 mathematical anthropomorphic phantoms, representing ages from a newborn child to a 17-year-old adolescent, were developed. Three common projections, anterior-posterior, posterior-anterior and lateral, were considered. The results consist of effective and organ radiation doses normalized to the entrance surface dose. Normalized data are presented for a wide range of peak kilovoltages and beam filtration values so that doses for any X-ray unit can be calculated. Both organ and effective dose values showed an inverse correlation with age. Good agreement was observed between the normalized effective doses produced in this study and values derived from calculations based on the National Radiological Protection Board coefficients for specific mathematical phantoms representing 1-, 5-, 10- and 15-year-old children. In the present work, dose data for nine mathematical phantoms representing 0-, 1-, 2-, 3-, 4-, 5-, 6-, 9- and 14-year-old pediatric patients were provided for estimation of organ and effective doses delivered to pediatric patients from radiographic examinations of the skull.

A CT and MRI scan to MCNP input conversion program

We describe a new program to read a sequence of tomographic scans and prepare the geometry and material sections of an MCNP input file. Image processing techniques include contrast controls and mapping of grey scales to colour. The user interface provides several tools with which the user can associate a range of image intensities to an MCNP material. Materials are loaded from a library. A separate material assignment can be made to a pixel intensity or range of intensities when that intensity dominates the image boundaries; this material is assigned to all pixels with that intensity contiguous with the boundary. Material fractions are computed in a user-specified voxel grid overlaying the scans. New materials are defined by mixing the library materials using the fractions. The geometry can be written as an MCNP lattice or as individual cells. A combination algorithm can be used to join neighbouring cells with the same material.

A stylized computational model of the head for the reference Japanese male

Computational models of human anatomy, along with Monte Carlo radiation transport simulations, have been used by Snyder et al. [MIRD Pamphlet No. 5, revised (The Society of Nuclear Medicine, New York, 1978)], Cristy and Eckerman [ORNL/TM-8381/VI, Oak Ridge National Laboratory, Oak Ridge, TN (1987)] and Zubal et al. [Med. Phys. 21, 299-302 (1994)] to estimate internal organ doses from internal and external radiation sources. These were created using physiological data from Caucasoid subjects but not from other races. There is a need for research to determine whether the obvious differences from the Caucasoid anatomy make these models unsuitable for estimating the absorbed dose in other races such as the Mongoloid. We used the cranial region of the adult Japanese male to represent the Mongoloid race. This region contains organs that are highly sensitive to radiation. The cranial region of a physical phantom produced by KYOTO KAGAKU Co., LTD. using numerical data from a Japanese Reference Man [Tanaka, Nippon Acta. Radiol. 48, 509-513 (1988)] was used to supply the data for the geometry of a stylized computational model. Our computational model was constructed with equations rather than voxel-based, in order to deal with as small a number of parameters as possible in the computer simulation experiment. The accuracy of our computational model was checked by comparing simulated experimental results obtained with MCNP4C with actual doses measured with thermoluminescence dosimeters (TLDs) inside the physical phantom from which our computational model was constructed. The TLDs, whose margin of error is less than +/-10%, were arranged at six positions. Co-60 was used as the radiation source. The irradiated dose was 2 Gy in terms of air kerma. In the computer simulation experiments, we used our computational model and Cristy's computational model, whose component data are those of the tissue substitute materials and of the human body as published in ICRU Report 46. The observed absorbed dose values (Gy) at all six points were calculated as the percentage difference between MCNP4C simulation and the TLDs. In our computational model, the average values of all the percentage differences were 6.0+/-4.0% (tissue substitute materials) and 7.6+/-6.6% (ICRU Report 46), respectively. In Cristy's model, the corresponding values were 20.4+/-3.8% (tissue substitute materials) and 21.0+/-4.1% (ICRU Report 46), respectively. Considering the margin of error in the radiation sensitivity of the TLDs, this study validates our computational model as a test object for radiation dosimetry studies.

Synchrotron stereotactic radiotherapy: dosimetry by Fricke gel and Monte Carlo simulations

Synchrotron stereotactic radiotherapy (SSR) consists in loading the tumour with a high atomic number element (Z), and exposing it to monochromatic x-rays from a synchrotron source (50-100 keV), in stereotactic conditions. The dose distribution results from both the stereotactic monochromatic x-ray irradiation and the presence of the high Z element. The purpose of this preliminary study was to evaluate the two-dimensional dose distribution resulting solely from the irradiation geometry, using Monte Carlo simulations and a Fricke gel dosimeter. The verification of a Monte Carlo-based dosimetry was first assessed by depth dose measurements in a water tank. We thereafter used a Fricke dosimeter to compare Monte Carlo simulations with dose measurements. The Fricke dosimeter is a solution containing ferrous ions which are oxidized to ferric ions under ionizing radiation, proportionally to the absorbed dose. A cylindrical phantom filled with Fricke gel was irradiated in stereotactic conditions over several slices with a continuous beam (beam section = 0.1 x 1 cm2). The phantom and calibration vessels were then imaged by nuclear magnetic resonance. The measured doses were fairly consistent with those predicted by Monte Carlo simulations. However, the measured maximum absolute dose was 10% underestimated regarding calculation. The loss of information in the higher region of dose is explained by the diffusion of ferric ions. Monte Carlo simulation is the most accurate tool for dosimetry including complex geometries made of heterogeneous materials. Although the technique requires improvements, gel dosimetry remains an essential tool for the experimental verification of dose distribution in SSR with millimetre precision.