Absorbed Dose Distribution Research Articles

Synthesis and Characterization of a Soft-tissue Substitute for Neutron Dosimetry

To develop a physical phantom for neutron dosimetry, a solid soft-tissue substitute was synthesized. The synthesized tissue substitute, NAN-JAERI, is improved in both hydrogen and oxygen elemental composition in comparison with existing tissue substitutes. To examine the radiation characteristics of the new soft-tissue substitute, absorbed dose distributions in NAN-JAERI were measured using a 252Cf neutron source. The measured absorbed dose distributions of neutrons and photons agree with those calculated by a Monte Carlo simulation code MCNP. The agreement between the experiment and the simulation verifies this method of evaluating the soft-tissue equivalence of NAN-JAERI for 252Cf neutrons. Similar simulations for some mono-energetic neutron sources showed that the newly developed tissue substitute has soft-tissue equivalent characteristics in the neutron energy range from 1 MeV up to 14 MeV, in terms of the absorbed dose distributions in a slab phantom.

514 Influence of initial electron beam characteristics on Monte Carlo calculated absorbed dose distribution for linac photon beam

The Gram-positive bacterium Corynebacterium glutamicum belongs to the order Corynebacteriales and is used as a producer of amino acids at industrial scales. Due to its economic importance, gene expression and particularly the regulation of amino acid biosynthesis has been investigated extensively. Applying the high-resolution technique of transcriptome sequencing (RNA-seq), recently a vast amount of data has been generated that was used to comprehensively analyze the C. glutamicum transcriptome. By analyzing RNA-seq data from a small RNA cDNA library of C. glutamicum, short transcripts in the known transcriptional attenuators sites of the trp operon, the ilvBNC operon and the leuA gene were verified. Furthermore, whole transcriptome RNA-seq data were used to elucidate the transcriptional organization of these three amino acid biosynthesis operons. In addition, we discovered and analyzed the novel attenuator aroR, located upstream of the aroF gene (cg1129). The DAHP synthase encoded by aroF catalyzes the first step in aromatic amino acid synthesis. The AroR leader peptide contains the amino acid sequence motif F-Y-F, indicating a regulatory effect by phenylalanine and tyrosine. Analysis by real-time RT-PCR suggests that the attenuator regulates the transcription of aroF in dependence of the cellular amount of tRNA loaded with phenylalanine when comparing a phenylalanine-auxotrophic C. glutamicum mutant fed with limiting and excess amounts of a phenylalanine-containing dipeptide. Additionally, the very interesting finding was made that all analyzed attenuators are leaderless transcripts.

Ultrasound tomography imaging of radiation dose distributions in polymer gel dosimeters: preliminary study.

A novel imaging system for investigation of absorbed dose distributions in radiotherapy polymer gel dosimeters using ultrasound is introduced. A prototype transmission ultrasound computed tomography (UCT) imaging system is developed and evaluated. The imaging capabilities of the system are assessed through investigation of an irradiated polyacrylamide gel test phantom. Images based on transmitted signal amplitude and time of flight (TOF) of the ultrasonic signal through the phantom are reconstructed using a filtered backprojection technique. In general, the reconstruction of the square field in the TOF image was superior to the transmission image, however, transmission images displayed superior contrast to TOF images. The image quality achieved with this prototype system is promising and could be significantly enhanced through improvements, in particular through the development of more sophisticated experimental equipment. It is concluded that UCT is a viable technique for imaging absorbed dose distributions in polymer gel dosimeters and investigations are continuing to further improve the system.

Comparison of Monte Carlo simulations of photon/electron dosimetry in microscale applications

It is important to establish reliable calculational tools to plan and analyse representative microdosimetry experiments in the context of microbeam radiation therapy development. In this paper, an attempt has been made to investigate the suitability of the MCNP4C Monte Carlo code to adequately model photon/electron transport over micron distances. The case of a single cylindrical microbeam of 25-micron diameter incident on a water phantom has been simulated in detail with both MCNP4C and the code PSI-GEANT, for different incident photon energies, to get absorbed dose distributions at various depths, with and without electron transport being considered. In addition, dose distributions calculated for a single microbeam with a photon spectrum representative of the European Synchrotron Radiation Facility (ESRF) have been compared. Finally, a large number of cylindrical microbeams (a total of 2601 beams, placed on a 200-micron square pitch, covering an area of 1 cm2) incident on a water phantom have been considered to study cumulative radial dose distributions at different depths. From these distributions, ratios of peak (within the microbeam) to valley (mid-point along the diagonal connecting two microbeams) dose values have been determined. The various comparisons with PSI-GEANT results have shown that MCNP4C, with its high flexibility in terms of its numerous source and geometry description options, variance reduction methods, detailed error analysis, statistical checks and different tally types, can be a valuable tool for the analysis of microbeam experiments.

A Monte-Carlo Method for Interface Dosimetry of Beta Emitters

Biologically targeted radiotherapy optimization requires accurate dose estimation, from macroscopic to cellular/subcellular dimensions. In particular, dose perturbations produced at interfaces between dissimilar media could affect therapy outcome. The magnitude of these perturbations depends on a complex set of parameters. This study investigates perturbations on electron dose for materials with atomic numbers (Z) up to 79 (79Au) at their interface with water as a function of Z, energy, distance from interface and geometry. A Monte-Carlo method that produces absorbed dose distributions in a voxel geometry has been developed using EGSnrc transport routines. Heterogeneous media and activity distributions can be input into this code. The backscatter dose factor (BSDF), which quantifies interface dose perturbations, was estimated using this code. The BSDF magnitude ranged from approximately 3% to approximately 50%, depending on source energy and Z. The BSDF decreased with increasing energy and showed a logarithmic dependence on Z. Empirical functions were fit to the results, that could be used to correct dose calculations performed using dose-point-kernels estimated in water, to cases involving different scattering materials. The BSDF was found to be highly dependent on interface geometry and scoring volume; thus it is vital that BSDFs are used only in geometry conditions that are similar to those in which they were originally produced.

Review of surface dose detectors in radiotherapy

Several instruments have been used to measure absorbed radiation dose under non-electronic equilibrium conditions, such as in the build-up region or near the interface between two different media, including the surface. Many of these detectors are discussed in this paper. A common method of measuring the absorbed dose distribution and electron contamination in the build-up region of high-energy beams for radiation therapy is by means of parallel-plate ionisation chambers. Thermoluminescent dosimeters (TLDs), diodes and radiographic film have also been used to obtain surface dose measurements. The diamond detector was used recently by the author in an investigation on the effects of beam-modifying devices on skin dose and it is also described in this report.

3D absorbed dose calculations based on SPECT: evaluation for 111-In/90-Y therapy using Monte Carlo simulations.

A general method is presented for patient-specific three-dimensional (3D) absorbed dose calculations based on quantitative SPECT activity measurements. The computational scheme includes a method for registration of the CT study to the SPECT image, and compensation for attenuation, scatter, and collimator-detector response including septal penetration, performed as part of an iterative reconstruction method. From SPECT images, the absorbed dose rate is calculated using an EGS4 Monte Carlo code, which converts the activity distribution to an absorbed dose rate distribution. Evaluation of the accuracy in the activity quantification and the absorbed dose calculation is based on realistic Monte Carlo simulated SPECT data of a voxel-computer phantom and (111)In and (90)Y. Septal penetration was not included in this study. The SPECT-based activity concentrations and absorbed dose distributions are compared to the actual values; the results imply that the corrections for attenuation and scatter yield results of high accuracy. The presented method includes compensation for most parameters deteriorating the quantitative image information. Inaccuracies are, however, introduced by the limited spatial resolution of the SPECT system, which are not fully compensated by the collimator-response correction. The proposed evaluation methodology may be used as a basis for future inter-comparison of different dosimetry calculation schemes.

Synthesis and Characterization of a Soft-tissue Substitute for Neutron Dosimetry

To develop a physical phantom for neutron dosimetry, a solid soft-tissue substitute was synthesized. The synthesized tissue substitute, NAN-JAERI, is improved in both hydrogen and oxygen elemental composition in comparison with existing tissue substitutes. To examine the radiation characteristics of the new soft-tissue substitute, absorbed dose distributions in NAN-JAERI were measured using a 252 Cf neutron source. The measured absorbed dose distributions of neutrons and photons agree with those calculated by a Monte Carlo simulation code MCNP. The agreement between the experiment and the simulation verifies this method of evaluating the soft-tissue equivalence of NAN-JAERI for 252 Cf neutrons. Similar simulations for some mono-energetic neutron sources showed that the newly developed tissue substitute has soft-tissue equivalent characteristics in the neutron energy range from 1 MeV up to 14 MeV, in terms of the absorbed dose distributions in a slab phantom. Neutron dosimetry has become more important for radi- ation protection than it has been in the past, since neutron sources, obtained from nuclear reactors or accelerators, have been widely used in solid state physics, material science, nuclear physics and neutron therapy. Experimental tech- niques of neutron diffraction or neutron scattering are so useful that they are applicable to other research fields men- tioned above. To perform experiments safely using the neu- tron sources, neutron dosimetry of high precision is required. Under the present circumstances, however, neutron dosimetry in radiation protection depends highly on numerical methods. Dose conversion coefficients relating neutron fluence to or- gan doses or to the effective dose have been used for dose assessments of workers and the public. The dose conversion coefficients have been calculated using mathematical anthro- pomorphic human models and radiation transport simulation techniques. Although it is almost impossible to construct re- alistic human models, experimental verification of the dose assessment technique is needed. To perform accurate neutron dose assessments based on ex- periments, elements such as appropriate neutron sources, de- tectors and phantoms are indispensable. Among these three elements, the appropriate phantom is one of the most impor- tant elements for dose assessments and for the calibration of detectors as an alternative to the human body. For photon dosimetry, various phantoms have been developed. The tis- sue substitutes of phantoms for photons are made of materi- als whose electron densities are close to those of the body tis- sues, because the photon deposits its energy through interac- tions with atomic electrons. In the case of neutrons, it is ideal that phantoms are made of materials with elemental compo- sitions that are identical to those of the body tissues, because the neutron interacts with the atomic nuclei. In ICRU Report

Neutron Source for Boron-Neutron-Capture Therapy Based on the Reaction 7Li(p, n)7Be Near Threshold

The results are presented of calculations of the absorbed dose distribution produced in a tissue-equivalent phantom by a neutron source based on the reaction 7Li(p, n)7Be near threshold. The investigation was conducted to study the possibility of using the KG-2,5 accelerator at the Physics and Power-Engineering Institute to treat cancer patients by boron-neutron-capture therapy.

Influence of initial electron beam characteristics on Monte Carlo calculated absorbed dose distributions for linear accelerator electron beams

The least known parameters in a Monte Carlo simulation of a linear accelerator treatment head are often the properties of the initial electron beam directed onto the exit vacuum window. Several initial beams with different spatial fluence distributions, angular divergences and energy spectra have been transported through the geometry of a scattering foil accelerator. The electron beam characteristics (energy spectrum and angular distribution) at the phantom surface and the subsequent relative absorbed dose distribution in a water phantom were calculated. The dose distribution was found to be insensitive to the geometrical properties of the initial beam. Furthermore, the lateral dose profiles are unaffected by the energy spectrum of the initial beam. The effect on the depth–dose curve is negligible if the initial energy spectrum is symmetric (e.g., Gaussian shaped) and its full width at half maximum (FWHM) is less than approximately 10% of the most probable energy. A larger FWHM will decrease the normalized dose gradient, but will not affect the dose in the build-up region. An asymmetric wedge shaped spectrum with a low-energy extension simultaneously increases the dose in the build-up region and decreases the dose gradient. The relationship between the energy spectral width and the normalized dose gradient is, however, smaller than published analytical expressions indicate. Some well-established energy-range relationships were shown to be accurate for most of the initial beams studied. The energy spectrum at the phantom surface was also derived from a measured depth–dose curve through different methods. The extracted spectrum depends on the beam model and the spectral reconstruction algorithm. Even though the depth–dose curve is fairly independent of initial beam characteristics, a correct description of the low-energy tail of the energy spectrum is important to obtain good agreement between measured and Monte Carlo calculated doses in the build-up region.

A CT image based deterministic approach to dosimetry and radiography simulations

An integral transport equation based deterministic approach has been developed to compute individual specific absorbed dose distributions and photon flux distributions on a detector for image formation studies. An algorithm has been designed to take advantage of parallel processing platforms capable of processing large amounts of data imposed by the small pixel sizes of CT images. CT images have been used to form voxels that would represent both tissue variations and body geometry. Absorbed dose distributions calculated using the Visible Human dataset are provided.

162 Oral Influence of initial energy spectra on Monte Carlo calculated electron beam absorbed dose distributions

162 Oral Influence of initial energy spectra on Monte Carlo calculated electron beam absorbed dose distributions

Spectral emissions and dosimetry of metal tritide particulates.

Inference of intakes and doses from inhalation of metal tritide particles has come under scrutiny because of decommissioning and decontamination of US Department of Energy facilities. Since self-absorption of radiation is very significant for larger particles, interpretation of counting results of metal tritide particles by liquid scintillation requires information about emission spectra. Similarly, inference of dose requires knowledge of charged particle and photon spectra. The PENELOPE Monte Carlo radiation transport computer code was used to compute spectral emissions and other dosimetric quantities for tritide particulates of Sc, Ti, Zr, Er, and Hf. Emission fractions, radial absorbed dose distributions, specific energy distributions and related frequency-mean specific energies and lineal energies, and the emitted spectra of electrons and bremsstrahlung photons are presented for selected particulates with diameters ranging from about 0.01 microm to 25 microm. Results characterising the effects of uncertainties associated with the composition and density of the tritides are also presented. Emission spectra are used to illustrate trends in the relationship between apparent and observed activity as a function of particle type and size. Emissions from metal tritide particles are weakly penetrating, and electron emission spectra tend to 'harden' as particle size increases. Microdosimetric considerations suggest that the radiation emitted by metal tritides can be classified as a low linear energy transfer radiation source. For cells less than about 7 microm away from the surface of a metal tritide, the primary dose component is due to electrons. However, bremsstrahlung radiation may deposit some energy tens, hundreds or even thousands of micrometres away from the surface of a tritide particle. The data and analyses presented in this report will help improve the accuracy of dose determinations for particulates of five metal tritides. Future work on the spectral emissions and dosimetry of metal tritide particulates needs to consider the contributions of so-called internal bremsstrahlung, an additional form of bremsstrahlung radiation emitted during beta decay.

Application of mathematical modeling-based algorithms to “off-carrier” cobalt-60 irradiation processes

The irradiation of materials and products “off carrier” has historically been performed using a “drop-and-read” methodology whereby the radioisotope source is raised and lowered repeatedly until the desired absorbed dose is achieved. This approach is time consuming from both a manpower and process perspective. Static irradiation-based processes can also be costly because of the need for repeated experimental verification of target dose delivery. In our paper we address the methods used for predicting Ethicon Endo Surgery's (EES's) off-carrier absorbed dose distributions. The scenarios described herein are complex due to the fact that the on-carrier process stream exhibits a wide range of densities and dose rates. The levels of observed complexity are attributed to the “just-in-time” production strategy and its related requirements as they apply to the programming of EES's cobalt-60 irradiators. Validation of off-carrier processing methodologies requires sophisticated parametric-based systems utilizing mathematical algorithms that predict off-carrier absorbed dose rate relative to the on-carrier process stream components. Irradiation process simulation is achieved using a point kernel computer modeling approach, coupled with database generation and maintenance. Dose prediction capabilities are validated via routine and transfer standard dosimetry.

A model-based approach for the optimization of radioimmunotherapy through antibody design and radionuclide selection.

The effectiveness of radioimmunotherapy (RIT) is known to depend on at least six factors: total absorbed dose and pattern of delivery, radiosensitivity, rate of repair of sublethal damage, ongoing proliferation during treatment, tumor heterogeneity, and tumor size. The purpose of this study was to develop a mathematic model that would relate the absorbed dose and its pattern of delivery to tumor response by incorporating information on each factor. This model was used to optimize therapeutic efficacy in mice by matching the antibody and radionuclide characteristics while ensuring recoverable marrow toxicity. Pharmacokinetic data were acquired in mice for a range of antibodies that varied in molecular weight, specificity, affinity, and avidity, and for a range of tumor sizes. This information was combined with the properties of iodine-131, rhenium-86, and yttrium-90 to determine the pattern of dose delivery. Tumor response was characterized in terms of radiosensitivity, rate of repair, and proliferation. Values for these parameters were obtained from in vitro assays and were incorporated into a response model based on the linear-quadratic model. Storage phosphor plate technology was used to acquire images of antibody distribution in tumor sections. These were registered with corresponding images showing tumor morphology, which were subsequently used to delineate regions that were distinct in terms of their response to radiation: oxygenated, radiosensitive areas that contained viable cells and hypoxic areas containing resistant viable cells and necrotic cells. Beta point dose kernels were then used to estimate the absorbed dose distribution in these regions. Therapy in normoxic areas was more effective than in hypoxic areas. The multivalent, tumor-specific antibodies, with intermediate clearance rates, delivered the highest absorbed dose to viable tumor cells. Antibody affinity and avidity facilitated the prolonged retention in radiosensitive areas of tumor, where most of the dose was deposited. The effectiveness of therapy could be enhanced further by matching the radionuclide with the antibody and tumor size. The model presented in this article allows the interaction between important radiobiologic parameters to be assessed and provides a tool for optimizing therapy in animal models and in patients.

Radiation dosimetry for radionuclide therapy in a nonmyeloablative strategy.

Radionuclide therapy extends the usefulness of radiation from localized disease of multifocal disease by combining radionuclides with disease-seeking drugs, such as antibodies or custom-designed synthetic agents. Like conventional radiotherapy, the effectiveness of targeted radionuclides is ultimately limited by the amount of undesired radiation given to a critical, dose-limiting normal tissue, most often the bone marrow. Because radionuclide therapy relies on biological delivery of radiation, its optimization and characterization are necessarily different than for conventional radiation therapy. However, the principals of radiobiology and of absorbed radiation dose remain important for predicting radiation effects. Fortunately, most radionuclides emit gamma rays that allow the measurement of isotope concentrations in both tumor and normal tissues in the body. By administering a small "test dose" of the intended therapeutic drug, the clinician can predict the radiation dose distribution in the patient. This can serve as a basis to predict therapy effectiveness, optimize drug selection, and select the appropriate drug dose, in order to provide the safest, most effective treatment for each patient. Although treatment planning for individual patients based upon tracer radiation dosimetry is an attractive concept and opportunity, practical considerations may dictate simpler solutions under some circumstances. There is agreement that radiation dosimetry (radiation absorbed dose distribution, cGy) should be utilized to establish the safety of a specific radionuclide drug during drug development, but it is less generally accepted that absorbed radiation dose should be used to determine the dose of radionuclide (radioactivity, GBq) to be administered to a specific patient (i.e., radiation dose-based therapy). However, radiation dosimetry can always be utilized as a tool for developing drugs, assessing clinical results, and establishing the safety of a specific radionuclide drug. Bone marrow dosimetry continues to be a "work in progress." Blood-derived and/or body-derived marrow dosimetry may be acceptable under specific conditions but clearly do not account for marrow and skeletal targeting of radionuclide. Marrow dosimetry can be expected to improve significantly but no method for marrow dosimetry seems likely to account for decreased bone marrow reserve.

Dosimetry characteristics of degraded electron beams investigated by Monte Carlo calculations in a setup for intraoperative radiation therapy

Degraded electron beams, as used for intraoperative radiation therapy (IORT) or similar complicated dosimetric situations, have different characteristics compared to conventional electron therapy beams. If international dosimetry protocols are applied in a direct manner to such degraded beams, uncertainties will be introduced in the absorbed dose determination. The Monte Carlo method has been used to verify experimentally determined relative absorbed dose distributions and output factors in an IORT geometry. Monte Carlo generated dose distributions are mostly within ±2% or ±2 mm of measured data. The simulated output variation between the IORT cones (relative output factors) are mostly within 2% of measured values. By comparing IORT and conventional electron beam characteristics (e.g. energy spectra, angular distributions and the contributions of different system components to these quantities) limitations and uncertainties of commonly used dosimetric techniques in IORT electron fields are quantified. The intraoperative treatment field contains a larger amount of scattered electrons, which leads to a broader energy spectrum as well as a wider angular distribution of electrons at the phantom surface. The dose from the scattered electrons can contribute up to 40% of the total dose at a depth of dose maximum, compared to approximately 10% for standard beams. A study of the energy spectra at the reference depth reveals that an uncertainty of the order of 1% can be introduced if ionization chamber based dosimetry is used to determine output factors for the investigated IORT system. We recommend that relative absorbed dose distributions and output factors in IORT electron beams and for similar complicated dosimetric situations should be determined with detectors having a small energy and angular dependence (e.g. diamond detectors or p-Si diodes).

Measurements and calculations of electron dose distributions in circular materials

In this paper, the absorbed dose distributions of 0.6–2.0 MeV electrons in circular compound materials have been calculated by the calculation method of electron energy deposition in multi-layer media based on bipartition model of electron transport. In addition, the blue cellophane film dosimeters have been used to measure the electron absorbed dose distributions in some circular objects. The calculation results are in agreement with some measurement data. The results indicate the usefulness of the calculation and measurement methods for electron dose monitoring and control in radiation processing of wire and cable.

The use of dose and charge distributions in electron beam processing

Absorbed dose distributions in products treated with energetic electrons are influenced by the electron energy, the atomic composition of the material, the size and shape of the product and its orientation to the electron beam. Dose distributions can be measured by placing small dosimeters at various positions within some products, but more detailed data can be calculated with Monte Carlo codes that are readily available. For high-dose treatment processes, the magnitude and internal distribution of electrostatic charges may be significant. The accumulation of high charges could cause discharges through some materials, which could damage the products. This effect can be avoided by using electrons with sufficiently high energy, so that most of them will pass through the material. Charge distributions are difficult to measure, but they can be calculated with the Monte Carlo method. Dose and charge distributions with a wide range of electron energies in typical materials are presented in this paper. These have been obtained with the Integrated Tiger Series of Coupled Electron/Photon Monte Carlo Transport Codes (ITS). The data illustrate the effects of electron energy and material composition. These quantitative results will enable users to determine the electron energy, beam current and beam power requirements for a variety of electron beam treatment processes.

The electron dose monitoring and control system in EB radiation processing for wires and cables

This paper introduces a close-loop microcomputer control system used for EB radiation processing of wires and cables, which is based on the measurements and calculations of the absorbed dose distribution of 0.6–2.0 MeV electrons in circular compound materials. The calculation of electron energy deposition in 4-layer media is carried out by the bipartition model of electron transport. The design ideas, system configuration and implementation of this control system governed by a 586 personal computer under windows 98 OS are described in this paper. The field operation results such as control precision and step response curves of this system are also given. The control system has been used for EB radiation processing of wires.