Linear attenuation coefficients of tissue-equivalent materials of differing compositions for low energy photons

Total linear attenuation coefficients of three tissue equivalent materials differing in their fat contents were experimentally determined for low energy photons in the range 13-51 keV. It is found that their variations with photon energy for each of these materials are describable by two distinct power functions with the validity ranges being (13-23 keV) and (32-51 keV), respectively. Therefore, to adequately represent the variation of total linear attenuation coefficients over the full photon energy range i.e., 13-51 keV, a sum of two power functions is needed. Least squares fitted equations to the entire experimental data are thus included. For muscle, the experimental data show a reasonably good agreement with the theoretically computed values that are available in literature.     

Mass attenuation coefficients of clear-Pb for photons from 125I, 103Pd, 99mTc, 192Ir, 137Cs and 60Co.

The mass attenuation coefficients, mu/rho, for Clear-Pb for photon energies ranging from 10 keV to 10 MeV were determined using Monte Carlo methods and simple equations used to manipulate elemental mass attenuation coefficients. It was determined that the effectiveness of Clear-Pb as a radiation shielding material was greater than plain acrylic for all photon energies, especially those less than 150 keV, and for deep penetration problems where the differences in mu/rho between Clear-Pb and acrylic became more significant. Finally, the usefulness of Clear-Pb as a shielding material when compared with acrylic was determined for the following commonly used radionuclides: 125I, 103Pd, 99mTc, 192Ir, 137Cs, and 60Co.     

Depth scaling of solid phantom for intensity modulated radiotherapy beams

To reduce the uncertainty of absorbed dose for high energy photon beams, water has been chosen as a reference material by the dosimetry protocols. However, solid phantoms are used as media for absolute dose verification of intensity modulated radiotherapy (IMRT). For the absorbed dose measurement, the fluence scaling factor is used for converting an ionization chamber reading in a solid phantom to absorbed dose to water. Furthermore the depth scaling factor is indispensable in determining the fluence scaling factor. For IMRT beams, a photon energy spectrum is varied by transmitting through a multileaf collimator and attenuating in media. However, the effects of spectral variations on depth scaling have not been clarified yet. In this study, variations of photon energy spectra were determined using the EGS Monte Carlo simulation. The depth scaling factors for commercially available solid phantoms were determined from effective mass attenuation coefficients using photon energy spectra. The results clarified the effect of spectral variation on the depth scaling and produced an accurate scaling method for IMRT beams.     

Photon interaction parameters of dosimetric interest in bone

The effective atomic numbers (Z(eff)) and electron densities (N(el)) of cortical and compact bone have been computed for total and partial photon interactions (photoelectric absorption, coherent scattering, incoherent scattering, pair production in a nuclear field, pair production in an electronic field, and sum of non-coherent scattering) by computing the molecular, atomic, and electronic cross section in the wide energy range of 1 keV-100 GeV using WinXCom. The variations of effective atomic number and electron density with energy are shown graphically for all photon interactions. The effective atomic numbers (ZPEA(eff)) for photon energy absorption are also calculated from mass energy absorption coefficients in the energy range 1 keV-20 MeV. The kerma values of bone relative to air are also computed. Additionally, computed tomography (CT) numbers of bone for photon interaction and energy absorption are also computed. The computed Z(eff) and N(el) may be useful in choosing a substitute composite material in place of bone. The estimated mass energy absorption coefficient may be used to evaluate dose and determine the surviving fraction (S) for bone. The usefulness of computed data in the simulation of tissue substitutes is also discussed.     

The effective atomic number of dosimetric gels

Radiological properties of gel dosimeters and phantom materials are often compared against each other and against water or tissue by consideration parameters including their effective atomic number, Zeff. Effective atomic numbers have been calculated for a range of ferrous-sulphate and polymeric gel dosimeters using mass attenuation coefficient data over the energy range 10 keV to 10 MeV. Data is presented relative to water to allow direct comparison over a range of energies. These data provide energy specific values of Zeff which improves on the practice of applying a power-law based formula to estimate an energy independent value. For applications that require a single value of Zeff, the data presented here allows the choice of a value appropriate to the energy of the photon source or a spectrum-weighted average. Studying the variation of Zeff, which is equivalent to taking into account the variation of mass attenuation coefficients with photon energy, it is found that gels typically match water better than water matches human tissues. As such, the subtle differences in effective atomic number between water and gels are small and may be considered negligible. Consideration of the mean disparity over a large energy range shows, broadly, BANG-1 to be the most water equivalent gel.     

Investigation of mass attenuation coefficient of almond gum bonded <Emphasis Type="Italic">Rhizophora</Emphasis> spp. particleboard as equivalent human tissue using XRF technique in the 16.6–25.3 keV photon energy

This paper reports the novel use of almond gum as a binder in manufacturing Rhizophora spp. particleboard. X-ray fluorescence spectroscopy was employed for analysis under photon energy range of 16.6–25.3 keV. Results showed that almond gum-bonded Rhizophora spp. particleboard can be used as tissue-equivalent phantom in diagnostic radiation. The calculated mass attenuation coefficients of the particleboards were consistent with the values of water calculated using XCOM program for the same photon energies, with p values of 0.056, 0.069, and 0.077 for samples A8, C0, and C8, respectively. However, no direct relationship was found between the percentage of adhesive and the mass attenuation coefficient. The results positively supported the use of almond gum as a binding agent in the fabrication of particleboards, which can be used as a phantom material in dosimetric and quality control applications.     

Dependence of acoustic attenuation of hearing protectors on incident sound level.

The relationship between incident sound level and acoustic attenuation for four types of earplug and four types of earmuff have been investigated using freshly prepared and instrumented cadaver ears. Pure tones and 1/3-octave bands of random noise in the frequency range 125-8000 Hz were employed as steady-state stimuli with sound pressure levels between 75 and 125 dB. Impulses with peak sound levels in the range 135-175 dB(P) were also presented. For the steady-state signals employed, the eight hearing protectors have been shown to have constant attenuation characteristics over the range of incident sound levels investigated. This was also the case for the six conventional protectors (with intentionally linear characteristics) for the impulse stimuli. The two intentionally amplitude-sensitive protectors provided attenuation which increased with incident sound level for impulse noises. Comparison of the protector attenuation-frequency characteristics determined for steady-state sounds shows good agreement with those obtained from subjective (threshold shift) national standard measurement procedures. It may be concluded, therefore, that the six conventional hearing protectors studied here have attenuation characteristics that are equal for incident sound levels at about 40 and 75 dB, and that they are constant for levels between 75 and 175 dB. Consequently, the results of national standard threshold-shift procedures, although measured at low sound levels, may be applied with confidence to occupations where hazardous high-level noises are present.     

Linear attenuation coefficients for compensator based imrt

With rapid technological improvements in computer driven 3-D radiotherapy treatment planning systems (RTPS) the use of compensating filters for intensity modulated radiation therapy (IMRT) will dramatically increase the ease of treatment. The procedure for commissioning .decimal (Sanford, Florida) compensators involved the measurement of the effective linear attenuation coefficients for aluminium and brass. Field sizes to be measured vary from small square field size of 5 cm to the larger square field size of 25 cm with additional measurements at each 5 cm2 increments. The energies commissioned where 6 MV and 18 MV photons. The depth of measurements varied from 5 cm to 10 cm within phantom material and the source surface distance varied from 100 cm to 90 cm. The beam quality was measured by obtaining percentage depth dose (PDD) curves for the various field sizes with and without the compensating material. Results of the series of measurements showed no significant differences in the effective linear attenuation coefficients with respect to chamber depth and source surface distance with constant energy and field size. The main factor that was shown to influence the effective linear attenuation coefficient was field size variation. A correlation was shown between the effective linear attenuation coefficient and field size, up to a field size of 15 cm x 15 cm. Our results showed that for optimal patient treatments using IMRT compensating filters, there is a need for establishing two field size dependent linear attenuation coefficients.     

Photon attenuation for samples in Marinelli beaker geometry: an analytical computation.

A simple but precise mathematical method is used to estimate self-attenuation corrections for Marinelli beaker samples; the method may be useful for environmental monitoring and complements experimental calibration data. Self-attenuation corrections for a given sample depend not only on the sample geometry and linear attenuation coefficient, but also on detector parameters and photon energy. In the present analytical computation, the dependence on detector parameters and photon energy is neglected. The resulting errors are smaller than 5%.     

Assessment of the sensitivity of the relation between current exposure to carbon black and lung function parameters when using different grouping schemes.

BACKGROUND: Equations using variance components in exposure data to predict attenuation and standard error of exposure--response slopes have been published recently. However, to date, no comparisons have been made between results of applying these equations to a real data set with the exposure-response relations estimated directly. METHODS: Data on lung function parameters and personal inhalable and respirable dust exposure levels from the European carbon black respiratory health study were used. The predicted attenuation and standard error of the relation between current inhalable and respirable dust levels and lung function parameters (FVC and FEV(1)) were calculated for various exposure grouping schemes. These results were compared with the observed exposure-response relations. Workers were grouped by Job Category, the combination of factory and Job Category and five a posteriori created Exposure Groups. In addition, the individual approach was also used, as exposure data were available for each worker. RESULTS: The rank orders of the coefficients from the regression analyses using the different grouping schemes were similar to those predicted by the equations, although the differences were larger than predicted. When using inhalable dust exposure, the predicted standard errors of the exposure-response slopes were slightly lower than those estimated directly; for respirable dust the predicted standard errors were about a factor two to three smaller than those from the regression analyses. When considering FVC, the predicted exposure-response relations were all statistically significant, whilst the observed relation was only significant when using the five a posteriori Exposure Groups. When reviewing the relations between dust exposure and level of FEV(1), all relations were statistically significant, with the exception of the (observed) relation between respirable dust and FEV(1), when the individual approach was used. CONCLUSIONS: Using different grouping schemes for estimating exposure can have large effects on the slope and standard error of the exposure-response relation. It is, therefore, important that the effect of the different grouping schemes on the level and precision of the exposure-response slope be estimated. Despite violation of most of the assumptions when applying the equations to predict attenuation and the standard error of the exposure-response slope, the similarities in predicted and observed exposure-response relations and standard errors are indicative of the robustness of these equations. Therefore, the equations appear to be a useful tool in establishing the most efficient way of utilizing exposure measurements.     

Calculation of effective dose-equivalent responses for external exposure from residual photon emitters in soil

Effective dose-equivalent responses have been calculated for external exposure from residual photon emitters in soil. The calculations are based on the assumption that the receptor is located 1 m above the contaminated ground. A Monte Carlo algorithm was developed to perform the photon transport calculation for the soil/air configuration, in which the soil constituents were assumed to be similar to those of the earth's crust. Photon cross-sections for soil were based on the latest cross-sectional information generated by the U.S. National Institute of Standards and Technology. For every incident photon at the receptor, an estimate of the air-absorbed dose was calculated first and then converted into the effective dose equivalent. The effective dose equivalent is based on the concept of weighted organ doses, as recommended by the ICRP. The ICRP's latest conversion coefficients were used to transform point air-absorbed doses into effective dose equivalents. Baseline effective dose responses were obtained for monoenergetic photon sources assumed to be distributed uniformly in soil for energies ranging from 0.01 to 10 MeV, soil thicknesses from 0 to 5 mean-free-path (mfp) lengths, and soil densities from 1 to 2 g cm-3. On the basis of the calculated dose responses, empirical relationships were obtained for correlating the dose responses with the soil depths and densities for each source energy. The derived correlations contain a convenient exponential form that describes the depth-dose relationship for an estimated accuracy within about 15%; soil densities show an inverse relationship with dose responses. Results of the effective dose-equivalent response can be used to calculate effective dose-equivalent responses for gamma-emitting radionuclides that are commonly identified as residual radioactive materials in soil.     

Comparison of the 60- and 100-item NCI-block questionnaires with validation data.

Large epidemiological studies often require short food frequency questionnaires (FFQ) to minimize the respondent burden or to control for confounding from dietary factors. In this analysis, we compared the extensively used National Cancer Institute-Block 60- and 100-item FFQs with one another and with usual intake as estimated from 12 days of validation data. The analysis focused on nutrients from different aspects of the diet, including energy, fat, saturated fat, beta-carotene, dietary fiber, and vitamin C. By use of an errors-in-variables analysis, the correlations of usual intake with the two types of FFQs for these nutrients were not different. Attenuation coefficients, a measure of misclassification error, for both FFQs were of similar magnitude and indicated that substantial attenuation of logistic regression coefficients would result from either FFQ. Our results confirm previous analyses describing the validity and utility of the 60-item FFQ (Epidemiology 1, 58-64, 1990) and indicate that it is essentially equivalent to the 100-item FFQ for epidemiological analyses of major nutrients.     

Short-term natural attenuation of copper in soils: effects of time, temperature, and soil characteristics

Natural attenuation of metals added to soils refers to the processes by which the mobility and bioavailability/toxicity of the added metals decline with time. In this paper, we used isotopic dilution techniques to investigate the short-term (30 d) natural attenuation of Cu added to 19 European soils at two effective concentrations shown to inhibit plant (tomato) growth by 10 and 90%. The results showed that the lability of Cu added to soils rapidly decreased after addition, especially in the soils with pH > 6.0, followed by a slow decrease in Cu lability. The lability of Cu added to soils also decreased with increasing incubation temperature. The activation energies and the apparent diffusion rate coefficients for the attenuation processes were 33 to 36 kJ/mol and 0.66 to 20.9 x 10(-10)/s at 20 degrees C, respectively, and were consistent with Cu diffusion in meso- and micropores. The attenuation of Cu lability was modeled on the basis of three processes: precipitation/nucleation of Cu on soil surfaces, Cu occlusion within organic matter, and diffusion of Cu into micropores. The soil and environmental factors governing attenuation rates were soil pH, organic matter content, incubation time, and temperature. Soil pH is the key factor for natural attenuation of Cu added to soils. The model can be used to scale ecotoxicological data generated from different soils and under different incubation times and temperatures.     

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

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

Conversion coefficients based on the VIP-Man anatomical model and EGS4

A new set of conversion coefficients from kerma free-in-air to absorbed dose and kerma free-in-air to "effective VIP-Man dose" has been calculated for external monoenergetic photon beams from 10 keV to 10 MeV using an image-based whole-body anatomical model. This model, called VIP-Man, was recently developed at Rensselaer from the high-resolution color images of the National Library of Medicine's Visible Human Project. An EGS4-based Monte Carlo user code, named EGS4-VLSI, was developed to efficiently process the extremely large image data in VIP-Man. Irradiation conditions include anterior-posterior, posterior-anterior, right lateral, left lateral, rotational, and isotropic geometries. Conversion coefficients from this study are compared with those obtained from two mathematical models, ADAM and EVA. "Effective VIP-Man doses" differ from the previously reported effective dose results by 10%-50% for photons between 100 keV and 10 MeV. Discrepancies are more significant at lower energies and for individual organ doses. Since VIP-Man is a realistic model that contains several tissues that were not previously defined well (or not available) in other models, the reported results offer an opportunity to improve the existing dosimetric data and the mathematical models.     

应用BED．CEIA检测干血斑样本监测HIV-1发病率的可行性研究

目的 研究干血斑样本应用于现场HIV-1 BED-CEIA新近感染的检测以及监测HIV-1发病率的可行性.方法 收集27个自愿咨询检测(VCT)哨点的10 313名咨询者的血浆和干血斑样本,进行HIV抗体检测,对经免疫印迹方法确认的349例HIV感染者的血浆和干血斑样本同时进行BED-CEIA检测,检验干血斑样本BED-CEIA新近感染检测结果的稳定性和重复性,比较两种样本检测结果的差异及对HIV发病率估计的一致性.结果 应用干血斑样本检测具有较好的稳定性和重复性,R2值分别达到0.9551和0.95.349对样本的检测结果显示,294对样本被同时判定为长期感染,53对样本被同时判定为新近感染,两种样本对HIV-1是否为新近感染判断的一致性达到99.43%.而2对样本得到不同的结果,其An值均处于临界值附近.在人群总体水平上,两种样本计算得到的HIV-1发病率完全一致.结论 在HIV-1 BED-CEIA新近感染检测中,尽管个别样本检测结果存在差异,但在人群水平上千血斑样本的检测结果对HIV-1发病率的估计与血浆样本没有差异,因此在中国可以应用于血斑样本进行HIV-1 BED-CEIA新近感染检测.     

Comparison of the radiobiological effect of carbon ion beam therapy and conventional radiation therapy on cervical cancer

Little clinical evidence has been provided to show the minimization of radiation resistance of tumors using high linear energy transfer radiation. We therefore investigated the radiobiological and molecular pathological aspects of carbon beam therapy. A total of 27 patients with squamous cell carcinoma (SCC) of the cervix were treated using a carbon beam and 50 control patients with SCC of the cervix using a photon beam. The expression of Ki-67, p53, and p27 proteins before radiotherapy and 5 and 15 days after therapy initiation were investigated using immunohistochemistry. Similar changes were observed in Ki-67 labeling index (LI) and p53 LI during carbon and photon beam therapies. However, for carbon beam therapy, the mean p27 LI significantly decreased from 25.2% before treatment to 18.6% on the 5th day after treatment initiation, followed by a significant increase to 36.1% on the 15th day. In contrast, for photon beam therapy, the p27 LI consistently decreased from the initial 19.9% to 13.7% on the 15th day. Histological effects were observably stronger under carbon than photon beam therapy, though no statistically significant difference was observed (p = 0.07 on the 5th day and p = 0.10 on the 15th day). The changes in p27 LI under carbon beam therapy were significantly different from those under photon beam therapy, which suggests important molecular differences in the radio-biological response between therapies. Further investigation is required to elucidate the clinical relevance of these putative changes and optimize the relative biological effectiveness of carbon beam to X-ray.     

Relationship between comfort and attenuation measurements for two types of earplugs

Noise-induced hearing loss is almost always preventable if properly fitted hearing protectors are worn to reduce exposure. Many individuals choose not to wear hearing protection because it may interfere with effective communication in the workplace or it may be uncomfortable. Hearing protector comfort has not received the same amount of attention as noise reduction capability. The present study was conducted to evaluate the comfort level of two different types of insert earplugs as well as the attenuation levels achieved by the earplugs. Attenuation levels were obtained with a commercially available earplug fit-test system, and the comfort ratings were obtained by questionnaire. The primary research objective was to determine whether hearing protector comfort was related to measured attenuation values. A linear mixed effects model provided evidence for an inverse relationship between comfort and attenuation.     

Variability of real-world hearing protector attenuation measurements

The attenuation provided by a hearing protection device (HPD) in the field is usually estimated by applying a derating factor to the laboratory-determined noise reduction rating (NRR) of the HPD. However, attenuation is highly dependent on individual-specific HPD fit. Prediction of an individual's attenuation depends on the accuracy of the measurement system and the variability of attenuation over time (e.g. after HPD refitting). Variability in attenuation and attenuation test systems has not been adequately characterized to allow for such an assessment. This study compared attenuation measurements made with two systems, Real-Ear-at-Threshold (REAT) and Microphone-in-Real-Ear (MIRE), on 20 workers using two earplugs (foam and custom-molded). Workers' perceptions of the earplugs were also evaluated. Individuals' attenuation results were summarized as personal attenuation ratings (PARs, similar to NRRs). Variability in PARs from between-subject, within-subject and within-day (i.e. repeated tests on a subject without earplug refitting) differences was assessed and used to present the lower confidence limit, or uncertainty factor (UF), of an average individual's attenuation. The custom-molded earplug PARs achieved a higher mean percentage of labeled attenuation than did the foam earplug with both test systems. The custom-molded earplugs also had higher overall acceptance among workers. MIRE PAR levels were lower than REAT levels for both earplugs, but the relationship between the two test systems was highly variable. The MIRE system had lower within-day variability than the REAT system. One individual's MIRE results were highly influential; removal of these results greatly reduced the UF for the custom-molded earplug/MIRE combination. UFs ranged from 8.8 to 13.5 dB. These findings highlight the importance of evaluating variability in individual-specific protection results for personal protective equipment like HPDs, rather than relying on single measurements.