Low-energy Photon Emitters Research Articles

A whole body counter for an emergency and occupational monitoring of an internal contamination with low energy photon emitters

Abstract A whole-body counter in SURO (NRPI) Prague, Czech Republic has been upgraded recently with the goal to enhance its capability of a safe, smooth, accurate and reproducible positioning of detectors for whole- and partial-body counting. The counter is intended especially for counting of low energy gamma emitters in various organs and tissues of the human body. Counting efficiency calibration of a four-detector system installed in the shielded room has been performed by means of physical and voxel phantoms. The consistency of in vivo bioassay data of three internal contamination cases long-term monitored in the Institute is shown.

Assessment of uncertainties in the lung activity measurement of low-energy photon emitters using Monte Carlo simulation of ICRP male thorax voxel phantom.

Assessment of intake due to long-lived actinides by inhalation pathway is carried out by lung monitoring of the radiation workers inside totally shielded steel room using sensitive detection systems such as Phoswich and an array of HPGe detectors. In this paper, uncertainties in the lung activity estimation due to positional errors, chest wall thickness (CWT) and detector background variation are evaluated. First, calibration factors (CFs) of Phoswich and an array of three HPGe detectors are estimated by incorporating ICRP male thorax voxel phantom and detectors in Monte Carlo code 'FLUKA'. CFs are estimated for the uniform source distribution in lungs of the phantom for various photon energies. The variation in the CFs for positional errors of ±0.5, 1 and 1.5 cm in horizontal and vertical direction along the chest are studied. The positional errors are also evaluated by resizing the voxel phantom. Combined uncertainties are estimated at different energies using the uncertainties due to CWT, detector positioning, detector background variation of an uncontaminated adult person and counting statistics in the form of scattering factors (SFs). SFs are found to decrease with increase in energy. With HPGe array, highest SF of 1.84 is found at 18 keV. It reduces to 1.36 at 238 keV.

Historical review of lung counting efficiencies for low energy photon emitters.

This publication reviews the measured efficiency and variability over time of a high purity planar germanium in vivo lung count system for multiple photon energies using increasingly thick overlays with the Lawrence Livermore Torso Phantom. The measured variations in efficiency are compared with the current requirement for in vivo bioassay performance as defined by the American National Standards Institute Standard in ANSI Standard N13.30.

64: DNA damage, protein expression and migration of melanoma cells irradiated with proton beam

64: DNA damage, protein expression and migration of melanoma cells irradiated with proton beam

Monte Carlo simulation of the BEGe detector response function for in vivo measurements of 241Am in the skull

This paper reports on the procedure of the BEGe detector characterization for the Monte Carlo calibrations. A project is under way to improve the counting and operating capabilities of the Whole Body Counter (WBC) installed in SÚRO, v.v.i. (NRPI) Prague, Czech Republic. Possible emergency monitoring should mainly benefit from the rapid, safe and flexible operation of the WBC. The system of the WBC for the detection of low energy X and gamma radiation comprises four HPGe detectors intended for the routine, emergency, and research measurements of persons internally contaminated with low-energy photon emitters, mainly actinides. Among them, 241 Am is the main subject of interest. A precise detection efficiency calibration of the detector is required for the measurement of activity in individual organs and tissues. The use of physical phantoms in the calibrations is often supplemented with the application of voxel phantoms and a Monte Carlo technique that are used for the calculation of the detector response function and the full energy peak efficiency. Both experimental and computational approaches have been used for the calibration of the BEGe (Broad Energy Germanium) detector. In this paper, the process of the Monte Carlo simulation of the detector response function and the peak efficiency calculation is described. Results of the simulations are provided in the paper and discussed. • MC calibration of the BEGe detector for internal contamination measurement. • Description of the BEGe detector parameters for MC simulations. • Three voxel skull phantoms have been used for MC simulations. • Comparison of the experimental and simulated gamma ray spectra is provided. • Efficiency calibration of WBC for measurements of 241 Am in the human skull.

Monte Carlo simulation of skull and knee voxel phantoms for the assessment of skeletal burden of low-energy photon emitters

In case of internal contamination due to long-lived actinides by inhalation or injection pathway, a major portion of activity will be deposited in the skeleton and liver over a period of time. In this study, calibration factors (CFs) of Phoswich and an array of HPGe detectors are estimated using skull and knee voxel phantoms. These phantoms are generated from International Commission of Radiation Protection reference male voxel phantom. The phantoms as well as 20 cm diameter phoswich, having 1.2 cm thick NaI (Tl) primary and 5cm thick CsI (Tl) secondary detector and an array of three HPGe detectors (each of diameter of 7 cm and thickness of 2.5 cm) are incorporated in Monte Carlo code 'FLUKA'. Biokinetic models of Pu, Am, U and Th are solved using default parameters to identify different parts of the skeleton where activity will accumulate after an inhalation intake of 1 Bq. Accordingly, CFs are evaluated for the uniform source distribution in trabecular bone and bone marrow (TBBM), cortical bone (CB) as well as in both TBBM and CB regions for photon energies of 18, 60, 63, 74, 93, 185 and 238 keV describing sources of (239)Pu, (241)Am, (238)U, (235)U and (232)Th. The CFs are also evaluated for non-uniform distribution of activity in TBBM and CB regions. The variation in the CFs for source distributed in different regions of the bones is studied. The assessment of skeletal activity of actinides from skull and knee activity measurements is discussed along with the errors.

Determination of long-lived radionuclides in radioactive wastes from the IEA-R1 nuclear research reactor

Ion-exchange resins and activated charcoal beds are employed for purification of the cooling water that is pumped through the core of pool type nuclear research reactors. Once expended, these media are replaced and become radioactive wastes that contain low concentrations of long-lived fission and activation products, uranium isotopes and transuranium elements. Determination of the radioactive inventory is of paramount importance in the management of such radioactive wastes, which, besides high-energy photon emitters that can be identified and quantified directly by gamma-ray spectrometry, also contain pure alpha, pure beta and low-energy photon emitters whose quantitative determination require radiochemical separation. These later are collectively known as difficult to measure (DTM) radionuclides. A characterization program embracing the DTM radionuclides is currently in progress for spent ion-exchange resins and activated charcoal beds that were definitively withdrawn from the water cleanup system of the IEA-R1 nuclear research reactor. Radiochemical methods used in the characterization program include separations with specific anionic resins, chromatographic extractions and co-precipitation, which enabled the measurement of the activity concentrations of 90Sr, 234U, 235U, 238U, 238Pu, 239+240Pu, 241Pu, 241Am and 244Cm. An enhanced retention of uranium and transuranium elements was observed in the activated charcoal compared to the ion-exchange resins as a result of the tendency of actinides to undergo hydrolysis in aqueous solutions.

A new position recording system for the partial-body counter at KIT

The position of detectors used for partial-body counters relative to a measured person has considerable influence on the counting efficiency of low-energy photon emitters. To minimise reading errors and enhance reproducibility of position data, an electronic position recording system was installed in the partial-body counter chamber at KIT. Phoswich detectors have been equipped with electronic sensors to determine the position and axes of the detectors. Additionally, a laser measurement system determines the position of arbitrary landmarks, e.g. point sources, anatomical features of phantoms and test persons. A network interface enables the reading of the position data on any connected computer. The system was successfully used to validate Monte Carlo simulations of partial-body counting scenarios for the numerical determination of counting efficiencies.

A FIELD DEPLOYABLE HIGH-RESOLUTION URINE GAMMA ANALYZER

The Human Monitoring Laboratory has extended the use of its portable whole body counters to portable gamma spectrometers for urinalysis. The protocol tested measured a 120-mL sample in a polypropylene sample container for 5 min. Minimum detectable activities were estimated for 241Am, 57Co, 137Cs, and 60Co. The former is 113 Bq per sample, and the latter three are between 27-29 Bq per sample. Assuming an intake 5 d before the measurement, and all other parameters as default, the committed effective doses are 517 Sv, 76 muSv, 402 muSv, and 1.5 mSv, respectively. Clearly, this instrument can be used as a field deployable gamma spectrometer for urinalysis for activation and fission products, but actinides (and other low energy photon emitters) remain problematic.

Calibration of a phoswich type partial body counter by Monte Carlo simulation of low-energy photon transport

The Forschungszentrum Karlsruhe operates a partial body counter, which is designed for the in vivo measurement of low-energy photon emitters in the human body. Recently, a numerical procedure has been developed which allows for the calculation of individual calibration factors for this partial body counter. The procedure is based on a Monte Carlo simulation of the radiation transport from the contaminated organ or tissue within the body to the detectors using the MCNP5 code. For simulation of the human body, the MEET Man dataset of the Institute of Biomedical Techniques of the University Karlsruhe has been applied. The derived calibration factors were compared with the respective values measured using some physical phantoms such as the Lawrence Livermore National Laboratory torso phantom and the bone phantoms of the New York University Medical Center and the US Transuranium and Uranium Registry.

Dosimetric study of a new polymer encapsulated palladium-103 seed

The use of low-energy photon emitters for brachytherapy applications, as in the treatment of prostate or ocular tumours, has increased significantly over the last few years. Several new seed models utilizing 103Pd and 125I have recently been introduced. Following the TG43U1 recommendations of the AAPM (American Association of Physicists in Medicine) (Rivard et al 2004 Med. Phys. 31 633), dose distributions around these low-energy photon emitters are characterized by the dose rate constant, the radial dose function and the anisotropy function in water. These functions and constants can be measured for each new seed in a solid phantom (i.e. solid water such as WT1) using high spatial resolution detectors such as very small thermoluminescent detectors. These experimental results in solid water must then be converted into liquid water by using Monte Carlo simulations. This paper presents the dosimetric parameters of a new palladium seed, OptiSeed™ (produced by International Brachytherapy (IBt), Seneffe, Belgium), made with a biocompatible polymeric shell and with a design that differs from the hollow titanium encapsulated seed, InterSource103, produced by the same company. A polymer encapsulation was chosen by the company IBt in order to reduce the quantity of radioactive material needed for a given dose rate, and to improve the symmetry of the radiation field around the seed. The necessary experimental data were obtained by measurements with LiF thermoluminescent dosimeters (1 mm3) in a solid water phantom (WT1) and then converted to values in liquid water using Monte Carlo calculations (MCNP-4C). Comparison of the results with a previous study by Reniers et al (2002 Appl. Radiat. Isot. 57 805) shows very good agreement for the dose rate constant and for the radial dose function. In addition, the results also indicate an improvement in isotropy compared to a conventional titanium encapsulated seed. The relative dose (anisotropy value relative to 90°) from the seed at a distance of 3 cm is close to 70% at 0° whereas that for the titanium encapsulated InterSource103seed is close to 40%. This paper also presents some new Monte Carlo calculations relating to shadowing produced by the seeds in an array implanted for a prostate cancer treatment. Recently, Mobit and Badragan (2004 Phys. Med. Biol. 49 3171) reported shadowing resulting in a 10% decrease in dose from titanium encapsulated 125I seed. We used Monte Carlo simulations (MCNP-4C) to evaluate shadowing for the InterSource103 titanium encapsulated seed and the OptiSeed™ polymer encapsulated seed. For a specific geometry specified, dose decreases of 13% and 7% were found for the InterSource103 titanium encapsulated and the OptiSeed™ polymer encapsulated seed, respectively.

Effects of off-centering on dose uniformity along and around blood vessels undergoing catheter-based intravascular brachytherapy

Purpose In intravascular brachytherapy, either photon or beta emitters are often used in a linear arrangement so that blood vessels of lengths in the range of several centimeters can be treated. With a line source, the dose uniformity and the range of doses that various components of the blood vessels receive depend not only on the type of radionuclides used in the treatment but also on the geometric position of radioactive source relative to the blood vessel walls. The aim of this study is to investigate the dose uniformity around the blood vessel and the effects on the uniformity due to the changes of the off-centering of different photon and beta emitters within the lumen. Materials and methods Dose distributions were calculated on a cylindrical blood vessel of various radii. The radioactive sources of 192Ir, 125I, 103Pd, 188Re, 32P, and 90Y/Sr were studied. All the sources were assumed to be in the form of a line and had a length of 2 cm. The dose rate at a point in space produced by a radioactive source was computed by integrating the point dose rate kernel of the corresponding radionuclide over the 2-cm-long radioactive line. The point dose rate kernel was computed with Monte Carlo simulation of radiation transport. Dosimetric calculations were performed for both concentric and nonconcentric radioactive line source locations. Off-centering effects on the dosimetry were characterized with two newly defined quantities LDU and ADU: LDU describes the longitudinal dose uniformity along blood vessels and ADU describes the azimuthal dose uniformity, i.e., the dose deviation from the expected delivery dose around blood vessels. Results The longitudinal dose uniformity did not change significantly with the off-center distance. The azimuthal dose uniformity around the blood vessel deteriorated as the off-center distance increased. The ADU was worse for nonconcentric beta emitters than the photon emitters. For example, if the off-center distance was 1 mm and the radial distance was 1.5 mm, the range of dose around the blood vessel on the central transverse plane (normalized to the corresponding dose under the concentric condition) was from 0.55 to 3.3, 0.56 to 3.3, 0.53 to 3.4, 0.43 to 6.0, 0.38 to 4.3, and 0.31 to 4.7 for 192Ir, 125I, 103Pd, 90Y/Sr, 188Re, and 32P sources, respectively. However, it appeared that there existed a lower limit of underdosing (about 40% of desired delivery dose) caused by the off-centering for the photon emitters. It was also found that both ADU and LDU became almost independent of source length when the length was longer than or equal to 20 mm. Conclusions A generalized formalism for expressing the dose uniformity along and around blood vessels generated with a linear source was developed and used to study the longitudinal and azimuthal dose uniformity for different types of radionuclides. Although concentric beta emitters provide uniform dose coverage along blood vessels, nonconcentric beta emitters produced larger dose deviations and worse dose uniformity around the blood vessels than photon emitters. The off-centering introduced significantly higher dose on proximal vessel walls for both beta and photon emitters; however, the underdosing at distal points due to off-centering was somewhat limited for the high-energy photon emitters. The magnitude of off-centering effects for the low-energy photon emitters ( 103Pd) was less than that for beta emitters but more than that for higher energy photon emitters ( 125I and 192Ir).

The radial dose function of low-energy brachytherapy seeds in different solid phantoms: comparison between calculations with the EGSnrc and MCNP4C Monte Carlo codes and measurements

The use of low-energy photon emitters for brachytherapy applications, as in the treatment of the prostate or of eye tumours, has drastically increased in the last few years. New seed models for 103Pd and 125I have recently been introduced. The American Association of Physicists in Medicine recommends that measurements are made to obtain the dose rate constant, the radial dose function and the anisotropy function. These results must then be compared with Monte Carlo calculations to finally obtain the dosimetric parameters in liquid water. We have used the results obtained during the characterization of the new InterSource (furnished by IBt, Seneffe, Belgium) palladium and iodine sources to compare two Monte Carlo codes against experiment for these low energies. The measurements have been performed in three different media: two solid water plastics, WT1 and RW1, and polymethylmetacrylate. The Monte Carlo calculations were made using two different codes: MCNP4C and EGSnrc. These codes use photon cross-section data of a different origin. Differences were observed between both sets of input data below 100 keV, especially for the photoelectric effect. We obtained differences in the radial dose functions calculated with each code, which can be explained by the difference between the input data. New cross-section data were then tested for both codes. The agreement between the calculations using these new libraries is excellent. The differences are within the statistical uncertainties of the calculations. These results were compared with the experimental data. A good agreement is reached for both isotopes and in the three phantoms when the measured values are corrected for the presence of the TLDs in the phantom.

Response of a BGO detector to photon and neutron sources: simulations and measurements

In this paper Monte Carlo simulations (FLUKA) and measurements of the response of a BGO detector are reported. For the measurements three low-energy photon emitters ( 60Co, 54Mn, 137Cs) were used to irradiate the BGO from various distances and angles. The neutron response was measured with an Am–Be neutron source. Simulations of the experimental irradiations were carried out. Our study can also be considered as a benchmark for FLUKA in terms of its reliability to predict the detector response of a BGO scintillator.

An analysis of MCNP cross-sections and tally methods for low-energy photon emitters

Monte Carlo calculations are frequently used to analyse a variety of radiological science applications using low-energy (10–1000 keV) photon sources. This study seeks to create a low-energy benchmark for the MCNP Monte Carlo code by simulating the absolute dose rate in water and the air-kerma rate for monoenergetic point sources with energies between 10 keV and 1 MeV. The analysis compares four cross-section datasets as well as the tally method for collision kerma versus absorbed dose. The total photon attenuation coefficient cross-section for low atomic number elements has changed significantly as cross-section data have changed between 1967 and 1989. Differences of up to 10% are observed in the photoelectric cross-section for water at 30 keV between the standard MCNP cross-section dataset (DLC-200) and the most recent XCOM/NIST tabulation. At 30 keV, the absolute dose rate in water at 1.0 cm from the source increases by 7.8% after replacing the DLC-200 photoelectric cross-sections for water with those from the XCOM/NIST tabulation. The differences in the absolute dose rate are analysed when calculated with either the MCNP absorbed dose tally or the collision kerma tally. Significant differences between the collision kerma tally and the absorbed dose tally can occur when using the DLC-200 attenuation coefficients in conjunction with a modern tabulation of mass energy-absorption coefficients.

Dosimetric study of the new Intersource125 iodine seed.

The use of low energy photon emitters for brachytherapy applications, as in the treatment of the prostate or of eye tumors, has significantly increased these last few years. New seed models for 125I have been recently introduced. The aim of this study is to determine the dosimetric parameters as recommended by the AAPM in the TG43 formalism for a new iodine seed design: the InterSource125 (Furnished by IBt, Seneffe, Belgium). Measurements are made with LiF thermoluminescent dosimeters (size of 1 mm3) in solid water phantoms to obtain the dose constant, the radial dose function, and the anisotropy function. The TLDs were calibrated at 6 MV and an energy correction factor of 1.41 has been applied. The same dose parameters are also obtained by Monte Carlo calculations (MCNP4B) in solid water and in liquid water. The radial function was measured at 1, 1.5, 2, 3, 4, 5, 6, and 7 cm and calculated between 0.3 and 7 cm. The anisotropy functions were measured at 2, 3, and 5 cm and calculated between 0.3 and 7 cm. The calculated and the measured TG43 functions for solid water are in excellent agreement. We have then calculated these functions in liquid water to obtain the dosimetric information for clinical applications as per TG43 recommendations. In WTI, the calculated dose rate constant is 0.98+/-1% and the measured value is 1.03 +/- 7 %. The calculated value for water is 1.02+/- 1 %. In conclusion, the dosimetric functions for the new iodine seed InterSource125 have been determined. They are quite different from the data of the well-known model 6711 from Amersham due to the absence of silver in the new seed. The characteristics are very similar to those of model 6702.

Shielding effects of metallic encapsulations and radiographic contrast agents for catheter-based intravascular brachytherapy

Purpose/Objective: Both photon- and beta-emitting radionuclides for intravascular brachytherapy (IVB) are under active investigation for prevention of restenosis following conventional angioplasty with or without stents. High atomic number materials are usually present in the coronary vessels undergoing treatment in the form of metallic encapsulations, stents, calcified plaque, or radiographic contrast agent. The high atomic number materials are likely to interfere with the photons and betas and, thus, change the dosimetry in the treatment volume. The purpose of this study is to investigate the shielding effects caused by the presence of high atomic number materials in IVB. Materials and Methods: Dose rates at various distances in water, with and without the presence of various high atomic number materials, were calculated using Monte Carlo simulation techniques for photon and electron transport in extended media. The high atomic number materials investigated included titanium, stainless steel, calcified plaque, Hypaque, and Omnipaque. A wide range of monoenergetic photon and electron sources and several photon- and beta-emitting radionuclides, which have been under consideration for IVB, were used. The energy of the monoenergetic photon sources was in the range from 10 keV to 1 MeV, and that of the monoenergetic electron sources in the range from 0.5 to 2 MeV. Photon-emitting radionuclides 192Ir, 125I, and 103Pd and beta-emitting radionuclides 90Y, 32P, and 188Re were also considered. Results: It was found that the high atomic number materials interfere considerably with the transport of photons of relatively low energies (below 40 keV) and all electron sources. When the energy of photon exceeds 100 keV, the interference becomes minimum for the high atomic number materials that are likely to be present in clinical situations. The shielding correction factors (SCFs) for dose rate at 2 mm from center were essentially 1.00 for photon energies above 100 keV; as the energy decreased below 100 keV, the SCF became smaller reaching a value of almost 0 for the lowest energy studied, 10 keV. For the photon source of 192Ir, the SCF was essentially 1.00; while for the photon sources of 125I and 103Pd, shielding corrections were considerably lower than 1.00 depending on the type and thickness of the high atomic number material. For the beta emitting sources, the shielding effect can be expressed as a loss in effective penetration depth. This loss depends both on the material and its thickness. For titanium and stainless steel, the loss of range was about two and four times the thickness of the metal. Conclusions: The effects of high atomic number materials, such as metallic stents, calcified plaque, and contrast agents are minimal for high energy photon emitters, such as 192Ir. The effects are pronounced for beta emitters and low energy photon emitters, and must be included in dosimetry planning.

The In Vivo Detection of Low Energy Photon Emitters in Radiation Protection and in Nuclear Medicine. A New Philosophy for New Needs in In Vivo Counting

A new methodology of whole-body counting adapted to the detection of radionuclides emitting low energy photons is described. This modification in the counting philosophy can the useful because the use of new radionuclides in nuclear medicine and the recycling of plutonium in reactor fuel materials require new techniques. They are based on the use of room temperature detectors (silicon and CdZnTe diodes principally), on long duration counting performed out of shielding rooms and on special detector mountings such as a jacket allowing the detectors to be held in a fixed position on the measured organ. The counting can be done without shielding rooms when the detector geometry is adapted to the investigated photon energy so that the control of the intake can be done soon after the suspected contamination accident. The characterisation of the background is of primary importance in the counting of low energy photons and is described by considering the required detection limits and the effect of the counting geometry on the accuracy. The detector position, the bias voltage, the detector thickness and the shielding environment are described on the basis of experimental results. The study of metabolism is a new application for these designs and techniques.

Problems in Defining the Minimum Detectable Activity in Lung Measurement of Low Energy Photon Emitters

An intercalibration campaign has been carried out in Italy aimed at evaluating and comparing the performances of NaI(Tl), phoswich and HPGe based detector systems for measurement of transuranic elements (241Am and 239Pu) in lungs. For the calibration, a Livermore realistic chest phantom was used. On the basis of a proposed procedure for MDA evaluation, repeated time delayed measurements of definitely uncontaminated subjects, having different body masses, were performed for 'blank' count evaluation in order to assess both intra-subject and inter-subject count variability. Results of this intercalibration campaign are presented together with MDA values for all NaI(Tl), phoswich and HPGe detector based systems involved. The necessity of a common procedure to assess system performance in in vivo measurement of transuranic elements is discussed and the proposed methodology for evaluation of minimum detectable activity is also presented.

The effect of lung deposition patterns on the activity estimate obtained from a large area germanium detector lung counter.

Lung counters for in vivo detection of low energy photon emitters are typically calibrated using phantoms containing lung tissue equivalent material with the radioactivity homogeneously distributed throughout the material. If the activity in a measurement subject is heterogeneously distributed, the activity estimate for that subject will be uncertain due to the assumptions of distribution. The magnitude of the uncertainty for a four-detector germanium array, using the Lawrence Livermore National Laboratory torso phantom with a newly designed lung set that allows the activity to be localized in one or more of 16 areas, was estimated. The results show that detector arrays will reduce the uncertainties arising from the geometry of the lung deposition compared to single detectors. The estimated activity of an internal deposition that emits 17.5 keV photons can be overestimated by a factor of three, or underestimated by a factor of infinity (i.e., the activity is missed completely). As the photon energy rises to 59.5 keV the uncertainty in the activity decreases so that the maximum overestimate (underestimate) will be a factor of two (five). As the energy rises to 344.3 keV only the maximum underestimate changes: it becomes a factor of three.