Neutron, proton, and photonuclear cross-sections for radiation therapy and radiation protection

I review recent work at Los Alamos undertaken to evaluate neutron, proton, and photonuclear cross-sections up to 150 MeV (to 250 MeV for protons), based on experimental data and nuclear model calculations. These data are represented in the ENDF format and can be used in computer codes to simulate radiation transport. They permit calculations of absorbed dose in the body from therapy beams, and through use of kerma coefficients allow absorbed dose to be estimated for a given neutron energy distribution. In radiation protection, these data can be used to determine shielding requirements in accelerator environments and to calculate neutron, proton, gamma-ray, and radionuclide production. Illustrative comparisons of the evaluated cross-section and kerma coefficient data with measurements are given.     

Thermoluminescence characteristics of calcite with a Gaussian process regression model of machine learning

Thermoluminescence (TL) is defined as a luminescence phenomenon that can be detected when an insulator or semiconductor is thermally stimulated. Defective crystals store radiation until they are stimulated. Thermoluminescence is a method of monitoring the absorbed dose of dosimeters. The irradiation crystal is heated to 500°C to display the absorbed dose as a luminescent light. The TL dosimetric properties of calcite obtained from nature were investigated in this study. Machine learning was also examined using Gaussian process regression (GPR) for stimulated TL characteristics. According to the experimental output, the TL glow curve had two main peaks located at 90°C and 240°C with good dosimetric properties. In the four regression models of GPR, the data for the heating rate of 3°C s⁻¹ have the lowest residual.     

Estimation of background radiation doses for the Peninsular Malaysia’s population by ESR dosimetry of tooth enamel

Background radiation dose is used in dosimetry for estimating occupational doses of radiation workers or determining radiation dose of an individual following accidental exposure. In the present study, the absorbed dose and the background radiation level are determined using the electron spin resonance (ESR) method on tooth samples. The effect of using different tooth surfaces and teeth exposed with single medical X-rays on the absorbed dose are also evaluated. A total of 48 molars of position 6–8 were collected from 13 district hospitals in Peninsular Malaysia. Thirty-six teeth had not been exposed to any excessive radiation, and 12 teeth had been directly exposed to a single X-ray dose during medical treatment prior to extraction. There was no significant effect of tooth surfaces and exposure with single X-rays on the measured absorbed dose of an individual. The mean measured absorbed dose of the population is 34 ± 6.2 mGy, with an average tooth enamel age of 39 years. From the slope of a regression line, the estimated annual background dose for Peninsular Malaysia is 0.6 ± 0.3 mGy y⁻¹. This value is slightly lower than the yearly background dose for Malaysia, and the radiation background dose is established by ESR tooth measurements on samples from India and Russia.     

Variation in lunar neutron dose estimates

The radiation environment on the Moon includes albedo neutrons produced by primary particles interacting with the lunar surface. In this work, HZETRN2010 is used to calculate the albedo neutron contribution to effective dose as a function of shielding thickness for four different space radiation environments and to determine to what extent various factors affect such estimates. First, albedo neutron spectra computed with HZETRN2010 are compared to Monte Carlo results in various radiation environments. Next, the impact of lunar regolith composition on the albedo neutron spectrum is examined, and the variation on effective dose caused by neutron fluence-to-effective dose conversion coefficients is studied. A methodology for computing effective dose in detailed human phantoms using HZETRN2010 is also discussed and compared. Finally, the combined variation caused by environmental models, shielding materials, shielding thickness, regolith composition and conversion coefficients on the albedo neutron contribution to effective dose is determined. It is shown that a single percentage number for characterizing the albedo neutron contribution to effective dose can be misleading. In general, the albedo neutron contribution to effective dose is found to vary between 1-32%, with the environmental model, shielding material and shielding thickness being the driving factors that determine the exact contribution. It is also shown that polyethylene or other hydrogen-rich materials may be used to mitigate the albedo neutron exposure.     

Iodine metabolism and pathologic processes in the thyroid gland under radioiodine lesions in the regions of goitrous endemia

An existing discrepancy between the prognostic estimations and real thyroid gland sickness rate due to radiation exposure from Chernobyl is an evidence of inaccuracy of radiation doses determination. The estimation of the thyroid cancer risks is based on the assumption that the absorbed dose is uniformly distributed in the organ. But functional asynchronicity specific for iodine metabolism in thyroid may modify a space distribution of the dose. The biochemical features of the contaminated areas including iodine deficit and goitrous endemia usually are not taken into account may influence the second phase of thyroid carcinogenesis that is the promotion due to increased accumulation of some carcinogenic microelements in goitrous thyroid. In the work we consider these problems which can make significant changes in radiation risks estimation.     

Factors affecting total dissolved solids concentration of γ-ray-irradiated aqueous hexamethylenetetramine solution: A dosimetric study

A new γ-ray-radiation dosimetric system (TDS-HMTA), comprising a 'total dissolved solids (TDS)' meter and 0.02 M aqueous hexamethylenetetramine (HMTA) solution, is introduced for medical and biological applications. Gamma-ray radiolysis of aqueous HTMA solutions increases the concentrations (ppm) of TDS, which is measured by the TDS meter. The effects of HMTA concentration, absorbed radiation dose, absorbed dose rate, and storage time on the TDS concentration of irradiated HMTA solutions were studied. It was found that 0.02 M aqueous HMTA solution yields the highest sensitivity to γ-ray-radiation according to TDS concentration measurements. The effect of absorbed radiation dose was studied in the range 1.64–435.5 kGy. The TDS concentration increases linearly up to the maximum of the studied absorbed radiation dose range (R² = 0.9965). The overall coefficient of variation (CV %) associated with TDS concentration measurements of 0.02 M HMTA solution as a function of absorbed dose was found to be 0.732%. The effect of dose rate on the TDS concentration was studied in the range 0.33–3.31 kGy/h. It was found, also, that the TDS concentration is relatively stable over a storage period of 144 h after irradiation with different doses. The tissue equivalency of 0.02 M aqueous HMTA solutions allow it to be used for radiation dose measurement during sterilization in human tissue banks. Therefore, this system (TDS–HMTA) could be considered as a promising candidate for γ-ray radiation dosimetry in technical, medical and research fields.     

Using matrix summation method for three dimensional dose calculation in brachytherapy

AimThe purpose of this study is to calculate radiation dose around a brachytherapy source in a water phantom for different seed locations or rotation the sources by the matrix summation method.BackgroundMonte Carlo based codes like MCNP are widely used for performing radiation transport calculations and dose evaluation in brachytherapy. But for complicated situations, like using more than one source, moving or rotating the source, the routine Monte Carlo method for dose calculation needs a long time running.Materials and methodsThe MCNPX code has been used to calculate radiation dose around a ¹⁹²Ir brachytherapy source and saved in a 3D matrix. Then, we used this matrix to evaluate the absorbed dose in any point due to some sources or a source which shifted or rotated in some places by the matrix summation method.ResultsThree dimensional (3D) dose results and isodose curves were presented for ¹⁹²Ir source in a water cube phantom shifted for 10 steps and rotated for 45 and 90° based on the matrix summation method. Also, we applied this method for some arrays of sources.ConclusionThe matrix summation method can be used for 3D dose calculations for any brachytherapy source which has moved or rotated. This simple method is very fast compared to routine Monte Carlo based methods. In addition, it can be applied for dose optimization study.     

Effects of nonuniform distribution of absorbed radiation energy among elements of a population

The reaction of a particular element of the population, when this element has absorbed some portion of ionizing radiation energy, and the response of the whole population to a given radiation dose are discussed. Functions of the element reaction are supposed to be increasing or decreasing. The formulae have been obtained describing a correlation between the element reaction as a function of the radiation energy absorbed and the response of the whole population as a function of the radiation dose absorbed.     

Space radiation absorbed dose distribution in a human phantom.

The radiation risk to astronauts has always been based on measurements using passive thermoluminescent dosimeters (TLDs). The skin dose is converted to dose equivalent using an average radiation quality factor based on model calculations. The radiological risk estimates, however, are based on organ and tissue doses. This paper describes results from the first space flight (STS-91, 51.65 degrees inclination and approximately 380 km altitude) of a fully instrumented Alderson Rando phantom torso (with head) to relate the skin dose to organ doses. Spatial distributions of absorbed dose in 34 1-inch-thick sections measured using TLDs are described. There is about a 30% change in dose as one moves from the front to the back of the phantom body. Small active dosimeters were developed specifically to provide time-resolved measurements of absorbed dose rates and quality factors at five organ locations (brain, thyroid, heart/lung, stomach and colon) inside the phantom. Using these dosimeters, it was possible to separate the trapped-proton and the galactic cosmic radiation components of the doses. A tissue-equivalent proportional counter (TEPC) and a charged-particle directional spectrometer (CPDS) were flown next to the phantom torso to provide data on the incident internal radiation environment. Accurate models of the shielding distributions at the site of the TEPC, the CPDS and a scalable Computerized Anatomical Male (CAM) model of the phantom torso were developed. These measurements provided a comprehensive data set to map the dose distribution inside a human phantom, and to assess the accuracy and validity of radiation transport models throughout the human body. The results show that for the conditions in the International Space Station (ISS) orbit during periods near the solar minimum, the ratio of the blood-forming organ dose rate to the skin absorbed dose rate is about 80%, and the ratio of the dose equivalents is almost one. The results show that the GCR model dose-rate predictions are 20% lower than the observations. Assuming that the trapped-belt models lead to a correct orbit-averaged energy spectrum, the measurements of dose rates inside the phantom cannot be fully understood. Passive measurements using 6Li- and 7Li-based detectors on the astronauts and inside the brain and thyroid of the phantom show the presence of a significant contribution due to thermal neutrons, an area requiring additional study.     

Derivation of radiation quality average parameters in neutron-gamma radiation fields with the high-pressure ionization chamber: theory and practice

The established radiation quality parameters in mixed neutron-gamma radiation fields may be measured by applying the initial (columnar) recombination of ions in tissue-equivalent (TE) high-pressure ionization chambers (recombination chambers). The mean quality factor can be determined to within 10-15% for mixed fields with neutrons ranging from thermal to 10 MeV, and the dose mean LET of the proton component can be determined to within 10-15% if the gamma-ray absorbed dose fraction is known. These average parameters are derived by measuring the ratio of the ionization currents collected at two high-field strengths and constant gas pressure applied to the ionization chamber. By utilizing approximate correlations between physical parameters in the neutron energy region from thermal to 10 MeV, the dose mean LET of the heavy ion component, the overall dose mean LET, and the microdosimetric parameter y0,D of the mixed field can also be derived. Experimental verification of the method is presented for various neutron-gamma radiation spectra in air and in water by comparison to theoretical calculations and results from low-pressure proportional counter measurements. Good agreement is shown. The TE high-pressure ionization chamber appears to have wide potential for use as a dose-equivalent meter in radiation protection or as a beam characterization device in radiobiology.     

Current status of biodosimetry based on standard cytogenetic methods

Knowledge about dose levels in radiation protection is an important step for risk assessment. However, in most cases of real or suspected accidental exposures to ionizing radiation (IR), physical dosimetry cannot be performed for retrospective estimates. In such situations, biological dosimetry has been proposed as an alternative for investigation. Briefly, biodosimetry can be defined as individual dose evaluation based on biological endpoints induced by IR (so-called biomarkers). The relationship between biological endpoints and absorbed dose is not always straightforward: nausea, vomiting and diarrhoea, for example, are the most well-known biological effects of individual irradiation, but a precise correlation between those symptoms and absorbed dose is hardly achieved. The scoring of unstable chromosomal-type aberrations (such as dicentrics and rings) and micronuclei in mitogen-stimulated peripheral blood, up till today, has been the most extensively biodosimetry assay employed for such purposes. Dicentric assay is the gold standard in biodosimetry, since its presence is generally considered to be specific to radiation exposure; scoring of micronuclei (a kind of by-product of chromosomal damages) is easier and faster than that of dicentrics for dose assessment. In this context, the aim of this work is to present an overview on biodosimetry based on standard cytogenetic methods, highlighting its advantages and limitations as tool in monitoring of radiation workers’ doses or investigation into accidental exposures. Recent advances and perspectives are also briefly presented.     

Interplanetary crew exposure estimates for the August 1972 and October 1989 solar particle events

Using the coupled neutron-proton space radiation transport computer code (BRYNTRN), estimates of human exposure in interplanetary space, behind various thicknesses of aluminum shielding, are made for the large solar proton events of August 1972 and October 1989. A comparison of risk assessment in terms of total absorbed dose for each event is made for the skin, ocular lens, and bone marrow. Overall, the doses associated with the August 1972 event were higher than those with the October 1989 event and appear to be more limiting when compared with current guidelines for dose limits for missions in low Earth orbit and more hazardous with regard to potential acute effects on these organs. Both events could be life-threatening if adequate shielding is not provided.     

A Novel Method of Estimating Dose Responses for Polymer Gels Using Texture Analysis of Scanning Electron Microscopy Images

Polymer gels are regarded as a potential dosimeter for independent validation of absorbed doses in clinical radiotherapy. Several imaging modalities have been used to convert radiation-induced polymerization to absorbed doses from a macro-scale viewpoint. This study developed a novel dose conversion mechanism by texture analysis of scanning electron microscopy (SEM) images. The modified N-isopropyl-acrylamide (NIPAM) gels were prepared under normoxic conditions, and were administered radiation doses from 5 to 20 Gy. After freeze drying, the gel samples were sliced for SEM scanning with 50×, 500×, and 3500× magnifications. Four texture indices were calculated based on the gray level co-occurrence matrix (GLCM). The results showed that entropy and homogeneity were more suitable than contrast and energy as dose indices for higher linearity and sensitivity of the dose response curves. After parameter optimization, an R ² value of 0.993 can be achieved for homogeneity using 500× magnified SEM images with 27 pixel offsets and no outlier exclusion. For dose verification, the percentage errors between the prescribed dose and the measured dose for 5, 10, 15, and 20 Gy were −7.60%, 5.80%, 2.53%, and −0.95%, respectively. We conclude that texture analysis can be applied to the SEM images of gel dosimeters to accurately convert micro-scale structural features to absorbed doses. The proposed method may extend the feasibility of applying gel dosimeters in the fields of diagnostic radiology and radiation protection.     

Estimated human absorbed dose of a new 153Sm bone seeking agent based on biodistribution data in mice: Comparison with 153Sm-EDTMP

PurposeThe main goal in radiotherapy is to deliver the absorbed dose within the target organs in highest possible amount, while the absorbed dose of the other organs, especially the critical organs, should be kept as low as possible. In this work, the absorbed dose to human organs for a new ¹⁵³Sm bone-seeking agent was investigated.Methods¹⁵³Sm-(4-{[(bis(phosphonomethyl))carbamoyl]methyl}-7,10-bis(carboxymethyl)-1,4,7,10-tetraazacyclododec-1-yl) acetic acid (¹⁵³Sm-BPAMD) complex was successfully prepared. The biodistribution of the complex was investigated in male Syrian mice up to 48 h post injection. The human absorbed dose of the complex was estimated based on the biodistribution data of the mice by radiation absorbed dose assessment resource (RADAR) method. The target to non-target absorbed dose ratios for ¹⁵³Sm-BPAMD were compared with these ratios for ¹⁵³Sm-EDTMP.ResultsThe highest absorbed dose for ¹⁵³Sm-BPAMD was observed in bone surface with 5.828 mGy/MBq. The dose ratios of the bone surface to the red marrow and to the total body for ¹⁵³Sm-BPAMD were 5.3 and 20.0, respectively, while these ratios for ¹⁵³Sm-EDTMP were 4.4 and 18.3, respectively. This means, for a given dose to the bone surface as the target organ, the red marrow (as the main critical organ) and the total body would receive lesser absorbed dose in the case of ¹⁵³Sm-BPAMD.ConclusionsGenerally, the human absorbed dose estimation of ¹⁵³Sm-BPAMD indicated that all other tissues approximately received insignificant absorbed dose in comparison with bone surface and therefore can be regarded as a new potential agent for bone pain palliation therapy.     

Finding sensitive parameters in internal dose calculations for radiopharmaceuticals commonly used in clinical nuclear medicine

Internal dosimetry after incorporation of radionuclides requires standardized biokinetic and dosimetric models. The aim of the present work was to identify the parameters and the components of the models which contribute most to dosimetric uncertainty. For this a method was developed allowing for the calculation of the uncertainties of the absorbed dose coefficients. More specifically, the sampling-based regression method and the variance-based method were used to develop and apply a global method of sensitivity analysis. This method was then used to quantify the impact of various biokinetic and dosimetric parameters on the uncertainty of internal doses associated with the incorporation of seven common radiopharmaceuticals. It turned out that the correlation between biokinetic parameters and time-integrated activity or calculated absorbed dose is strongest when the source and target organ are identical, in accordance with the ICRP and the MIRD approach. According to the ICRP approach, the parameter F_s which describes the fractional distribution of any incorporated radioactivity to organ S, has the greatest correlation with the time-integrated activity in the corresponding source organ or with the calculated dose in the corresponding target organ. In contrast, the MIRD approach suggested several biokinetic parameters with similar correlation. The dosimetric parameters usually contribute more to uncertainty in the calculated dose coefficients than the biokinetic parameters, in both approaches. The results obtained are helpful for the revision of biokinetic models for radiopharmaceuticals, because the most important parameters in clinical applications can now be identified and investigated in future studies.     

Simulations of the MATROSHKA experiment at the international space station using PHITS

Concerns about the biological effects of space radiation are increasing rapidly due to the perspective of long-duration manned missions, both in relation to the International Space Station (ISS) and to manned interplanetary missions to Moon and Mars in the future. As a preparation for these long-duration space missions, it is important to ensure an excellent capability to evaluate the impact of space radiation on human health, in order to secure the safety of the astronauts/cosmonauts and minimize their risks. It is therefore necessary to measure the radiation load on the personnel both inside and outside the space vehicles and certify that organ- and tissue-equivalent doses can be simulated as accurate as possible. In this paper, simulations are presented using the three-dimensional Monte Carlo Particle and Heavy-Ion Transport code System (PHITS) (Iwase et al. in J Nucl Sci Tech 39(11):1142–1151, 2002) of long-term dose measurements performed with the European Space Agency–supported MATROSHKA (MTR) experiment (Reitz and Berger in Radiat Prot Dosim 120:442–445, 2006). MATROSHKA is an anthropomorphic phantom containing over 6,000 radiation detectors, mimicking a human head and torso. The MTR experiment, led by the German Aerospace Center (DLR), was launched in January 2004 and has measured the absorbed doses from space radiation both inside and outside the ISS. Comparisons of simulations with measurements outside the ISS are presented. The results indicate that PHITS is a suitable tool for estimation of doses received from cosmic radiation and for study of the shielding of spacecraft against cosmic radiation.     

Physical and biological organ dosimetry analysis for international space station astronauts

In this study, we analyzed the biological and physical organ dose equivalents for International Space Station (ISS) astronauts. Individual physical dosimetry is difficult in space due to the complexity of the space radiation environment, which consists of protons, heavy ions and secondary neutrons, and the modification of these radiation types in tissue as well as limitations in dosimeter devices that can be worn for several months in outer space. Astronauts returning from missions to the ISS undergo biodosimetry assessment of chromosomal damage in lymphocyte cells using the multicolor fluorescence in situ hybridization (FISH) technique. Individual-based pre-flight dose responses for lymphocyte exposure in vitro to gamma rays were compared to those exposed to space radiation in vivo to determine an equivalent biological dose. We compared the ISS biodosimetry results, NASA's space radiation transport models of organ dose equivalents, and results from ISS and space shuttle phantom torso experiments. Physical and biological doses for 19 ISS astronauts yielded average effective doses and individual or population-based biological doses for the approximately 6-month missions of 72 mSv and 85 or 81 mGy-Eq, respectively. Analyses showed that 80% or more of organ dose equivalents on the ISS are from galactic cosmic rays and only a small contribution is from trapped protons and that GCR doses were decreased by the high level of solar activity in recent years. Comparisons of models to data showed that space radiation effective doses can be predicted to within about a +/-10% accuracy by space radiation transport models. Finally, effective dose estimates for all previous NASA missions are summarized.     

The effect of the β-emitting yttrium-90 citrate on the dose–response of dicentric chromosomes in human lymphocytes: a basis for biological dosimetry after radiosynoviorthesis

The production of dicentric chromosomes in human lymphocytes by β-particles of yttrium-90 (Y-90) was studied in vitro to provide a basis of biological dosimetry after radiosynoviorthesis (RSO) of persistent synovitis by intra-articular administration of yttrium-90 citrate colloid. Since the injected colloid may leak into the lymphatic drainage exposing other parts of the body to radiation, the measurement of biological damage induced by β-particles of Y-90 is important for the assessment of radiation risk to the patients. A linear dose–response relationship (α = 0.0229 ± 0.0028 dicentric chromosomes per cell per gray) was found over the dose range of 0.2176–2.176 Gy. The absorbed doses were calculated for exposure of blood samples to Y-90 activities from 40 to 400 kBq using both Monte Carlo simulation and an analytical model. The maximum low-dose RBE, the RBE_M which is equivalent to the ratio of the α coefficients of the dose–response curves, is well in line with published results obtained earlier for irradiation of blood of the same donor with heavily filtered 220 kV X-rays (3.35 mm copper), but half of the RBE_M relative to weakly filtered 220 kV X-rays. Therefore, it can be concluded that for estimating an absorbed dose during RSO by the technique of biological dosimetry, in vitro and in vivo data for the same radiation quality are necessary.     

Comparison of photon organ and effective dose coefficients for PIMAL stylized phantom in bent positions in standard irradiation geometries

Computational phantoms with articulated arms and legs have been constructed to enable the estimation of radiation dose in different postures. Through a graphical user interface, the Phantom wIth Moving Arms and Legs (PIMAL) version 4.1.0 software can be employed to articulate the posture of a phantom and generate a corresponding input deck for the Monte Carlo N-Particle (MCNP) radiation transport code. In this work, photon fluence-to-dose coefficients were computed using PIMAL to compare organ and effective doses for a stylized phantom in the standard upright position with those for phantoms in realistic work postures. The articulated phantoms represent working positions including fully and half bent torsos with extended arms for both the male and female reference adults. Dose coefficients are compared for both the upright and bent positions across monoenergetic photon energies: 0.05, 0.1, 0.5, 1.0, and 5.0 MeV. Additionally, the organ doses are compared across the International Commission on Radiological Protection’s standard external radiation exposure geometries: antero-posterior, postero-anterior, left and right lateral, and isotropic (AP, PA, LLAT, RLAT, and ISO). For the AP and PA irradiation geometries, differences in organ doses compared to the upright phantom become more profound with increasing bending angles and have doses largely overestimated for all organs except the brain in AP and bladder in PA. In LLAT and RLAT irradiation geometries, energy deposition for organs is more likely to be underestimated compared to the upright phantom, with no overall change despite increased bending angle. The ISO source geometry did not cause a significant difference in absorbed organ dose between the different phantoms, regardless of position. Organ and effective fluence-to-dose coefficients are tabulated. In the AP geometry, the effective dose at the 45° bent position is overestimated compared to the upright phantom below 1 MeV by as much as 27% and 82% in the 90° position. The effective dose in the 45° bent position was comparable to that in the 90° bent position for the LLAT and RLAT irradiation geometries. However, the upright phantom underestimates the effective dose to PIMAL in the LLAT and RLAT geometries by as much as 30% at 50 keV.     

Recovery of Deinococcus radiodurans from radiation damage was enhanced under microgravity.

Effect of microgravity on recovery of bacterial cells from radiation damage was examined on the IML-2 mission in 1994 using extremely radioresistant bacterium Deinococcus radiodurans. The cells were lyophilized and exposed to 60Co gamma-rays with doses 2 to 12 kGy before the space flight. At the end of the mission, the cells were mixed on board with liquid nutrient medium to allow the cells to start recovery process from the radiation damage. Afterwards the cells were stored at 4 degrees C until landing. The influence of cosmic radiation was negligible, because total absorbed dose of space radiation measured during the mission was less than 2 mGy and this bacterium does not decrease its viability after both gamma-rays and high-LET heavy charged particles irradiation with doses up to 5 kGy. The survival of the cells incubated in space increased significantly compared with the ground controls, suggesting that the recovery of this bacterium from radiation damage was enhanced under microgravity.