Biomedical subjects

Wesley E Bolch

Publications and source records attributed to Wesley E Bolch.

At least 19 recordsLinked to original sources

PubMed2006

Consideration of the ICRP 2006 revised tissue weighting factors on age-dependent values of the effective dose for external photons.

The effective dose recommended by the International Commission on Radiological Protection (ICRP) is the sum of organ equivalent doses weighted by corresponding tissue weighting factors, w(T). ICRP is in the process of revising its 1990 recommendations on the effective dose where new values of organs and tissue weighting factors have been proposed and published in draft form for consultation by the radiological protection community. In its 5 June 2006 draft recommendations, new organs and tissues have been introduced in the effective dose which do not exist within the 1987 Oak Ridge National Laboratory (ORNL) phantom series (e.g., salivary glands). Recently, the investigators at University of Florida have updated the series of ORNL phantoms by implementing new organ models and adopting organ-specific elemental composition and densities. In this study, the effective dose changes caused by the transition from the current recommendation of ICRP Publication 60 to the 2006 draft recommendations were investigated for external photon irradiation across the range of ICRP reference ages (newborn, 1-year, 5-year, 10-year, 15-year and adult) and for six idealized irradiation geometries: anterior-posterior (AP), posterior-anterior (PA), left-lateral (LLAT), right-lateral (RLAT), rotational (ROT) and isotropic (ISO). Organ-absorbed doses were calculated by implementing the revised ORNL phantoms in the Monte Carlo radiation transport code, MCNPX2.5, after which effective doses were calculated under the 1990 and draft 2006 evaluation schemes of the ICRP. Effective doses calculated under the 2006 draft scheme were slightly higher than estimated under ICRP Publication 60 methods for all irradiation geometries exclusive of the AP geometry where an opposite trend was observed. The effective doses of the adult phantom were more greatly affected by the change in tissue weighting factors than that seen within the paediatric members of the phantom series. Additionally, dose conversion coefficients for newly identified radiosensitive organs--salivary glands, gall bladder, heart and prostate--were reported, as well as the brain, which was originally considered in ICRP Publication 60 as a member of the remainder category of the effective dose.

Adolescent↗

PubMed2006

An assessment of bone marrow and bone endosteum dosimetry methods for photon sources.

The rather complex and microscopic histological structure of the skeletal system generally limits one's ability to accurately model this tissue during dosimetric evaluations. Consequently, various assumptions must be made to evaluate the absorbed dose from external and internal photons to the radiosensitive tissues of the red (or haematopoietically active) bone marrow and the osteogenic tissues of the skeletal endosteum. These various methods for photon skeletal dosimetry have not been inter-compared, partly due to the lack of a realistic reference model that can provide a high-resolution three-dimensional geometry for secondary electron particle transport. In the present study, the paired-image radiation transport (PIRT) model developed by Shah et al (2005 J. Nucl. Med. 45 344) was utilized to evaluate the absorbed dose per incident photon fluence to these skeletal regions from idealized parallel beams of monoenergetic photons. The PIRT model results were then used as a local reference against which absorbed doses via other methods were compared. For red bone marrow dosimetry, four approximate techniques were considered: (1) the dose response function method (DRF method) presented in ORNL/TM-8381, (2) the mass-energy absorption coefficient ratio method (two-parameter MEAC method), (3) the MEAC method with the additional use of energy-dependent dose enhancement factors from King and Spiers (1985 Br. J. Radiol. 58 345) (three-parameter MEAC method), and (4) the three-parameter MEAC method applied at the voxel level through the use image-specific CT numbers (CTN method). For the bone endosteum (i.e., bone surfaces), two approximate techniques were compared: (1) the DRF method for bone surfaces and (2) the homogeneous bone approximation (HBA) method. In each case, the local reference standard was assumed to be that of the PIRT model. Four different ex vivo bone specimens with distinctively different internal structures were used in the study: the cranium, the lumbar vertebra, the os coxae and the left middle rib, each excised from a 66 year male cadaver (body mass index, 22.7 kg m(-2)). High-resolution CT images of these skeletal sites were used to construct computational voxel models for Monte Carlo radiation transport. Study results indicated that skeletal sites with thick cortical regions and thick trabeculae such as in the cranium provide considerable beam attenuation at low photon energies, which is not properly accounted for in methods based on a homogeneous skeletal tissue structure (DRF, MEAC, HBA). For bone marrow dose assessment, the CTN method showed the best agreement with PIRT model results over a broad range of photon energies, while the HBA method showed better agreement with the PIRT model in assessing bone endosteum dose at energies above 100 keV. Bone surface doses were better approximately by the DRF method at energies below 50 keV. Considerable secondary electron escape at photon energies over 1-3 MeV were accounted for in RBM dose assessment only in the PIRT model, as the other methods presume either an infinite expanse of spongiosa (DRF) or the existence of charge-particle equilibrium (MEAC, CTN).

Aged↗

PubMed2006

Organ and effective doses in newborn patients during helical multislice computed tomography examination.

In this study, two computational phantoms of the newborn patient were used to assess individual organ doses and effective doses delivered during head, chest, abdomen, pelvis, and torso examinations using the Siemens SOMATOM Sensation 16 helical multi-slice computed tomography (MSCT) scanner. The stylized phantom used to model the patient anatomy was the revised ORNL newborn phantom by Han et al (2006 Health Phys.90 337). The tomographic phantom used in the study was that developed by Nipper et al (2002 Phys. Med. Biol. 47 3143) as recently revised by Staton et al (2006 Med. Phys. 33 3283). The stylized model was implemented within the MCNP5 radiation transport code, while the tomographic phantom was incorporated within the EGSnrc code. In both codes, the x-ray source was modelled as a fan beam originating from the focal spot at a fan angle of 52 degrees and a focal-spot-to-axis distance of 57 cm. The helical path of the source was explicitly modelled based on variations in collimator setting (12 mm or 24 mm), detector pitch and scan length. Tube potentials of 80, 100 and 120 kVp were considered in this study. Beam profile data were acquired using radiological film measurements on a 16 cm PMMA phantom, which yielded effective beam widths of 14.7 mm and 26.8 mm for collimator settings of 12 mm and 24 mm, respectively. Values of absolute organ absorbed dose were determined via the use of normalization factors defined as the ratio of the CTDI(100) measured in-phantom and that determined by Monte Carlo simulation of the PMMA phantom and ion chamber. Across various technique factors, effective dose differences between the stylized and tomographic phantoms ranged from +2% to +9% for head exams, -4% to -2% for chest exams, +8% to +24% for abdominal exams, -16% to -12% for pelvic exams and -7% to 0% for chest-abdomen-pelvis (CAP) exams. In many cases, however, relatively close agreement in effective dose was accomplished at the expense of compensating errors in individual organ dose. Per cent differences in organ dose between the stylized and tomographic phantoms at 120 kVp and 12 mm collimator setting ranged from -25% (skin) to +164% (muscle) for head exams, -92% (thyroid) to +98% (ovaries) for chest exams, -144% (uterus) to +112% (ovaries) for abdominal exams, -98% (SI wall) to +20% (thymus) for pelvic exams and -60% (extrathoracic airways) to +13% (ovaries) for CAP exams. Better agreement was seen between the two phantom types for organs entirely within the scan field. In these cases, corresponding per cent differences in organ absorbed dose did not vary more than 17%. For all scans, the effective dose was found to range approximately 1-13 mSv across the scan parameters and scan regions. The largest effective dose occurred for CAP scans at 120 kVp.

Body Burden↗

PubMed2006

TENORM aerosols in the Florida phosphate industry--assessment of lung fluid solubility and annual effective dose to workers.

Inhalation exposure to workers in the Florida phosphate industry due to TENORM aerosols has not been adequately addressed owing to lack of aerosol information. One of the more critical factors is the absorption rate of inhaled radionuclides into blood. In this study, this parameter was characterised using an in vitro dissolution test. The solubility data and other aerosol information were then used for individualised dose assessments at six different Florida phosphate facilities. The solubility data support the selections of ICRP Publication 66 Type M for uranium and lead isotopes and Type S for thorium isotopes. Total annual effective doses are 0.34 +/- 0.12 mSv at granulator areas, 0.30 +/- 0.10 mSv at storage areas and 0.23 +/- 0.02 mSv at shipping areas. These findings are considerably lower than originally postulated in previous studies where no site-specific information on particle size and lung fluid solubility had been available.

Aerosols↗

PubMed2006

Whole-body voxel phantoms of paediatric patients--UF Series B.

Following the previous development of the head and torso voxel phantoms of paediatric patients for use in medical radiation protection (UF Series A), a set of whole-body voxel phantoms of paediatric patients (9-month male, 4-year female, 8-year female, 11-year male and 14-year male) has been developed through the attachment of arms and legs from segmented CT images of a healthy Korean adult (UF Series B). Even though partial-body phantoms (head-torso) may be used in a variety of medical dose reconstruction studies where the extremities are out-of-field or receive only very low levels of scatter radiation, whole-body phantoms play important roles in general radiation protection and in nuclear medicine dosimetry. Inclusion of the arms and legs is critical for dosimetry studies of paediatric patients due to the presence of active bone marrow within the extremities of children. While the UF Series A phantoms preserved the body dimensions and organ masses as seen in the original patients who were scanned, comprehensive adjustments were made for the Series B phantoms to better match International Commission on Radiological Protection (ICRP) age-interpolated reference body masses, body heights, sitting heights and internal organ masses. The CT images of arms and legs of a Korean adult were digitally rescaled and attached to each phantom of the UF series. After completion, the resolutions of the phantoms for the 9-month, 4-year, 8-year, 11-year and 14-year were set at 0.86 mm x 0.86 mm x 3.0 mm, 0.90 mm x 0.90 mm x 5.0 mm, 1.16 mm x 1.16 mm x 6.0 mm, 0.94 mm x 0.94 mm x 6.00 mm and 1.18 mm x 1.18 mm x 6.72 mm, respectively.

Adolescent↗

PubMed2006

Age-dependent organ and effective dose coefficients for external photons: a comparison of stylized and voxel-based paediatric phantoms.

This present study investigates the anatomical realism of conventional stylized models of children by comparing organ dose conversion coefficients for the ORNL paediatric phantom series with those determined in the UF (University of Florida) voxel paediatric phantoms. The latter includes whole-body models of a 9 month male, 4 year female, 8 year female, 11 year male and a 14 year male. Of these phantoms, the 1 year, 5 year and 10 year ORNL phantoms, and 9 month male, 4 year female and 11 year male UF voxel phantoms were selected for side-by-side comparisons under idealized external photon irradiation. Organ absorbed dose per unit air kerma (Gy/Gy) for various radiosensitive organs and tissues were calculated for monoenergetic photons over the energy range of 15 keV to 10 MeV and for six irradiation geometries: anterior-posterior (AP), posterior-anterior (PA), right lateral (RLAT), left lateral (LLAT), rotational (ROT) and isotropic (ISO). Differences in organ dose conversion coefficients for the gonads, bone marrow, colon, lung and stomach, to which prominent tissue weighting factors are assigned, were depicted and analysed. Two major causes of observed differences were suggested: differences in organ shape and position and the differences in tissue shielding by overlying tissue regions within the phantoms. Significant discrepancies caused by anatomical differences between the two types of phantoms are also reported for several organs, and in particular, the thyroid and urinary bladder. The results of this study suggest that the paediatric series of ORNL phantoms also have less realistic internal organ and body anatomy and that dose conversion coefficients from these stylized phantoms should be re-evaluated using paediatric voxel phantoms.

Adolescent↗

PubMed2006

Correlations of total pelvic spongiosa volume with both anthropometric parameters and computed tomography-based skeletal size measurements.

Patient-specific dosimetry within the field of molecular radiotherapy continues to pose a challenge owing to the difficulty in predicting marrow toxicity. This study examined the correlation between total pelvic spongiosa volume (TPSV) and independent variables, which include both readily measured or calculated anthropometric parameters (AP), and image-based skeletal measurements requiring computed tomography (CT) images or skeletal radiographs. Fourteen (14) patients (5 male and 9 female) undergoing total hip arthroplasty (THA) were subjected to modified pelvic CT scans. These scans were utilized to estimate TPSV, which was comprised of the volumes of spongiosa within the L5 vertebra, os coxae, sacrum, and both proximal femurs. The APs investigated included total body height (TBH), total body mass (TBM), body mass index (BMI), body surface area (BSA), maximum effective mass (MEM), lean body mass (LBM), and fat-free mass (FFM). Skeletal measurements were also obtained from the CT images of the pelvic region. Correlation coefficients (r) were obtained for TPSV and each set of APs as well as each set of skeletal measurements. Total body height (r = 0. 80) and os coxae height (r = 0.83) had the highest correlation coefficients of all the APS and skeletal measurements, respectively. FFM (r = 0.50), LBM (r = 0.42), TBM (r = 0.11), and BSA (r = 0.11) did not correlate well with TPSV, which accounts for approximately 45% of total spongiosa seen throughout the skeleton at sites associated with active bone marrow. Skeletal height measurements appear to have a much higher correlation with TPSV than either their corresponding skeletal width measurements or parameters that are a function of an individual's TBM.

Adult↗

PubMed2006

Revisions to the ORNL series of adult and pediatric computational phantoms for use with the MIRD schema.

The age-dependent series of stylized computational phantoms developed at the Oak Ridge National Laboratory in the late 1970's to early 1980's has found wide applicability in dosimetry studies ranging from dose coefficient compilations for external and internal photon emitters, simulations of patient radiological exams, and dose reconstruction activities. In the present study, we report on a series of revisions to the Oak Ridge National Laboratory series for their intended use within the MIRD schema of medical internal dosimetry. These revisions were made to (1) incorporate recent developments in stylized models of the head, brain, kidneys, rectosigmoid colon, and extra-pulmonary airways; (2) incorporate new models of the salivary glands and the mucosa layer of the urinary bladder, alimentary tract organs, and respiratory airways; (3) adopt reference values of elemental tissue compositions and mass densities from ICRP Publication 89 and ICRU Report 46; (4) provide for explicit treatment of left and right organs within organ pairs; (5) provide for a systematic tabulation of electron absorbed fractions as a function of energy and subject age for all internal organs; and (6) provide for methods of deriving patient-specific values of the specific absorbed fraction for both electrons and photons through interpolation/extrapolation of their phantom-derived values. While tomographic computational phantoms provide improved anatomic realism given the CT or MR image sets used in their construction, there does not yet exist a comprehensive series of reference pediatric tomographic phantoms, nor the ability to simulate very fine anatomic structures as can be modeled via mathematical approximation. Consequently, stylized pediatric phantoms will continue to fill this data need in medical dosimetry.

Adult↗

PubMed2006

Influence of particle size distribution on inhalation doses to workers in the Florida phosphate industry.

Previous studies have indicated that inhalation exposures to TENORM aerosols are potentially a major contributor to the annual total effective dose to workers in the Florida phosphate industry. Further research was deemed necessary to characterize the particle size distribution of these aerosols containing various radionuclides of the U decay series. In the present study, individualized assessments of worker committed effective doses are reported in which detailed information is used on the particle size distribution, particle density, particle shape, and radioactivity concentrations from sampled aerosols at 6 different phosphate facilities and at various worker areas within these facilities. Inhalation dose assessments are calculated using the ICRP 66 human respiratory tract model as implemented within the LUDEP and IMBA computer codes. Under the least conservative assumptions of radionuclide-specific lung solubility, the annual total effective doses are shown to be 0.31+/-0.12, 0.27+/-0.07, and 0.22+/-0.02 mSv at granulator, storage, and shipping areas, respectively, and thus all annual doses are below the annual limits to the members of the general public (1 mSv y). In contrast, the most conservative assumptions of lung solubility by radionuclide yield annual total effective doses of 2.24+/-2.53 mSv at granulator areas, 1.26+/-1.19 mSv at storage areas, and 0.56+/-0.36 mSv at shipping areas. In this later case, some 44%, 31%, and 15% of individual dose assessments yield worker doses above the annual dose limit. The study thus demonstrates the importance of facility- and area-specific particle solubility data in dose assessments for regulatory compliance and for making decisions regarding worker respiratory protection.

Aerosols↗

PubMed2006

Effective dose scaling factors for use with uranium series cascade impactor data: a reassessment using the IMBA code.

Air sampling with a multi-stage cascade impactor enables one to assess airborne radioactivity as a function of particle size, significantly enhancing the accuracy of the dose assessment. The application of cascade sampling data to inhalation dose assessments can require more computational effort if something other than a mono-sized distribution per impactor stage is to be considered. To overcome this limitation, Kim et al. (Health Phys 89:359-374; 2005) introduced the concept of an effective dose scaling factor SF(E) enabling one to consider more realistic impactor stage radioactivity distributions (uniform, linearly decreasing, or linearly increasing variations with particle size). The SF(E) is the ratio of the effective dose given under a uniform or linearly changing radioactivity distribution across the particle size interval to that given for a mono-sized radioactivity distribution for the same impactor stage. The latter approach can initially be used (which requires less computational effort) followed by a rescaling of the effective dose either upward or downward by the SF(E) value. In this earlier study, the LUDEP code was employed which utilizes the ICRP 66 human respiratory tract model along the radionuclide biokinetic models given in ICRP Publication 30. In the present study, inhalation dose coefficients and effective dose scaling factors were reexamined for several radionuclides of the (238)U series using the IMBA program, which employs more recent and physiologically realistic biokinetic models published by the ICRP. An update of the effective dose scaling factors is thus the primary focus of this study rather than an extensive inter-comparison of the IMBA and LUDEP codes. Inhalation dose coefficients calculated by the two programs differ by up to a factor of 5 for Type F (238)U and (234)U, but are within only 2% of each other for Type S radionuclides. The ICRP 69 biokinetic model of uranium predicts retention in bone and kidneys that is slightly higher than predicted in ICRP Publication 30, but is significantly higher in the liver and other soft tissues by up to 1 or 2 orders of magnitude. Nevertheless, effective dose scaling factors generated using the IMBA program are nearly identical to those calculated by the LUDEP program as their magnitude is primarily dictated by the dependence of particle deposition and lung clearance with particle size, and less by the systemic biodistribution of the radionuclide following absorption to blood. For both codes, greater than 10% re-scaling of the effective dose is required for third-stage (4.5 to 12 microm) and filter-stage (0.03 to 0.35 microm) particles in the approximation of uniform or linearly decreasing radioactivity distributions per particle stage. For linearly increasing distributions, greater than 10% corrections in the effective dose are found irregularly across impactor stage, radionuclide, and solubility class, especially for rather steep (1:5) impactor stage activity ratios.

Air Pollutants, Radioactive↗

PubMed2006

A tomographic physical phantom of the newborn child with real-time dosimetry. II. Scaling factors for calculation of mean organ dose in pediatric radiography.

Following the recent completion of a tomographic physical newborn dosimetry phantom with incorporated metal-oxide-semiconductor field effect transistor (MOSFET) dosimetry system, it was necessary to derive scaling factors in order to calculate organ doses in the physical phantom given point dose measurements via the MOSFET dosimeters (preceding article in this issue). In this study, we present the initial development of scaling factors using projection radiograph data. These point-to-organ dose scaling factors (SF(POD)) were calculated using a computational phantom created from the same data set as the physical phantom, but which also includes numerous segmented internal organs and tissues. The creation of these scaling factors is discussed, as well as the errors associated when using only point dose measurements to calculate mean organ doses and effective doses in physical phantoms. Scaling factors for various organs ranged from as low as 0.70 to as high as 1.71. Also, the ability to incorporate improvements in the computational phantom into the physical phantom using scaling factors is discussed. An comprehensive set of SF(POD) values is presented in this article for application in pediatric radiography of newborn patients.

Body Burden↗

PubMed2006

CT volumetry of the skeletal tissues.

Computed tomography (CT) is an important and widely used modality in the diagnosis and treatment of various cancers. In the field of molecular radiotherapy, the use of spongiosa volume (combined tissues of the bone marrow and bone trabeculae) has been suggested as a means to improve the patient-specificity of bone marrow dose estimates. The noninvasive estimation of an organ volume comes with some degree of error or variation from the true organ volume. The present study explores the ability to obtain estimates of spongiosa volume or its surrogate via manual image segmentation. The variation among different segmentation raters was explored and found not to be statistically significant (p value >0.05). Accuracy was assessed by having several raters manually segment a polyvinyl chloride (PVC) pipe with known volumes. Segmentation of the outer region of the PVC pipe resulted in mean percent errors as great as 15% while segmentation of the pipe's inner region resulted in mean percent errors within approximately 5%. Differences between volumes estimated with the high-resolution CT data set (typical of ex vivo skeletal scans) and the low-resolution CT data set (typical of in vivo skeletal scans) were also explored using both patient CT images and a PVC pipe phantom. While a statistically significant difference (p value <0.002) between the high-resolution and low-resolution data sets was observed with excised femoral heads obtained following total hip arthroplasty, the mean difference between high-resolution and low-resolution data sets was found to be only 1.24 and 2.18 cm3 for spongiosa and cortical bone, respectively. With respect to differences observed with the PVC pipe, the variation between the high-resolution and low-resolution mean percent errors was a high as approximately 20% for the outer region volume estimates and only as high as approximately 6% for the inner region volume estimates. The findings from this study suggest that manual segmentation is a reasonably accurate and reliable means for the in vivo estimation of spongiosa volume. This work also provides a foundation for future studies where spongiosa volumes are estimated by various raters in more comprehensive CT data

Anatomy, Cross-Sectional↗

PubMed2006

Linear regression model for predicting patient-specific total skeletal spongiosa volume for use in molecular radiotherapy dosimetry.

UNLABELLED: The toxicity of red bone marrow is widely considered to be a key factor in restricting the activity administered in molecular radiotherapy to suboptimal levels. The assessment of marrow toxicity requires an assessment of the dose absorbed by red bone marrow which, in many cases, requires knowledge of the total red bone marrow mass in a given patient. Previous studies demonstrated, however, that a close surrogate-spongiosa volume (combined tissues of trabecular bone and marrow)-can be used to accurately scale reference patient red marrow dose estimates and that these dose estimates are predictive of marrow toxicity. Consequently, a predictive model of the total skeletal spongiosa volume (TSSV) would be a clinically useful tool for improving patient specificity in skeletal dosimetry. METHODS: In this study, 10 male and 10 female cadavers were subjected to whole-body CT scans. Manual image segmentation was used to estimate the TSSV in all 13 active marrow-containing skeletal sites within the adult skeleton. The age, total body height, and 14 CT-based skeletal measurements were obtained for each cadaver. Multiple regression was used with the dependent variables to develop a model to predict the TSSV. RESULTS: Os coxae height and width were the 2 skeletal measurements that proved to be the most important parameters for prediction of the TSSV. The multiple R(2) value for the statistical model with these 2 parameters was 0.87. The analysis revealed that these 2 parameters predicted the estimated the TSSV to within approximately +/-10% for 15 of the 20 cadavers and to within approximately +/-20% for all 20 cadavers in this study. CONCLUSION: Although the utility of spongiosa volume in estimating patient-specific active marrow mass has been shown, estimation of the TSSV in active marrow-containing skeletal sites via patient-specific image segmentation is not a simple endeavor. However, the alternate approach demonstrated in this study is fairly simple to implement in a clinical setting, as the 2 input measurements (os coxae height and width) can be made with either pelvic CT scanning or skeletal radiography.

Bone Marrow↗

PubMed2005

Uncertainties in electron-absorbed fractions and lung doses from inhaled beta-emitters.

The computer code LUDUC (Lung Dose Uncertainty Code), developed at the University of Florida, was originally used to investigate the range of potential doses from the inhalation of either plutonium or uranium oxides. The code employs the ICRP Publication 66 Human Respiratory Tract model; however, rather than using simple point estimates for each of the model parameters associated with particle deposition, clearance, and lung-tissue dosimetry, probability density functions are ascribed to these parameters based upon detailed literature review. These distributions are subsequently sampled within LUDUC using Latin hypercube sampling techniques to generate multiple (e.g., approximately 1,000) sets of input vectors (i.e., trials), each yielding a unique estimate of lung dose. In the present study, the dosimetry component of the ICRP-66 model within LUDUC has been extended to explicitly consider variations in the beta particle absorbed fraction due to corresponding uncertainties and biological variabilities in both source and target tissue depths and thicknesses within the bronchi and bronchioles of the thoracic airways. Example dose distributions are given for the inhalation of absorption Type S compounds of 90Sr (Tmax = 546 keV) and 90Y (Tmax = 2,284 keV) as a function of particle size. Over the particle size range of 0.001 to 1 microm, estimates of total lung dose vary by a factor of 10 for 90Sr particles and by a factor of 4 to 10 for 90Y particles. As the particle size increases to 10 microm, dose uncertainties reach a factor of 100 for both radionuclides. In comparisons to identical exposures scenarios run by the LUDEP 2.0 code, Reference Man doses for inhaled beta-emitters were shown to provide slightly conservative estimates of lung dose compared to those in this study where uncertainties in lung airway histology are considered.

Administration, Inhalation↗

PubMed2005

A comparison of skeletal chord-length distributions in the adult male.

In radiation protection, skeletal dose estimates are required for the tissues of the hematopoietically active bone marrow and the osteogenic cells of the trabecular and cortical endosteum. Similarly, skeletal radiation dose estimates are required in therapy nuclear medicine in order to develop dose-response functions for myelotoxicity where active bone marrow is generally the dose-limiting organ in cancer radioimmunotherapy. At the present time, skeletal dose models in both radiation protection and medical dosimetry are fundamentally reliant on a single set of chord-length distribution measurements performed at the University of Leeds in the late 1970's for a 44-y-old male subject. These distributions describe the relative frequency at which linear pathlengths are seen across both the marrow cavities and bone trabeculae in seven individual bone sites: vertebrae (cervical and lumbar), proximal femur (head and neck), ribs, cranium (parietal bone), and pelvis (iliac crest). In the present study, we present an alternative set of chord-length distribution data acquired within a total of 14 skeletal sites of a 66-y-old male subject. The University of Florida (UF) distributions are assembled via 3D image processing of microCT scans of physical sections of trabecular spongiosa at each skeletal site. In addition, a tri-linear interpolation Marching Cube algorithm is employed to smooth the digital surfaces of the bone trabeculae while chord-length measurements are performed. A review of mean chord lengths indicate that larger marrow cavities are noted on average in the UF individual for the cervical vertebrae (1,038 vs. 910 microm), lumbar vertebrae (1,479 vs. 1,233 microm), ilium (1,508 vs. 904 microm), and parietal bone (812 vs. 389 microm), while smaller marrow cavities are noted in the UF individual for the femoral head (1,043 microm vs. 1,157 microm), the femoral neck (1,454 microm vs. 1,655 microm), and the ribs (1,630 microm vs. 1,703 microm). The mean chord-lengths for the bone trabeculae show close agreement for both individuals in the ilium (approximately 240 microm) and cervical vertebrae (approximately 280 microm). Thicker trabeculae were seen on average in the UF individual for the femoral head (ratio of 1.50), femoral neck (ratio of 1.10), lumbar vertebrae (ratio of 1.29), and ribs (ratio of 1.14), while thinner trabeculae were seen on average in the UF individual for the parietal bone of the cranium (ratio of 0.92). In two bone sites, prominent discrepancies in chord distribution shape were noted between the Leeds 44-y-old male and the UF 66-y-old male: (1) the bone trabeculae in the ribs, and (2) the marrow cavities and bone trabeculae within the cranium.

Adult↗

PubMed2005

Effective dose scaling factors for use with cascade impactor sampling data in tenorm inhalation exposures.

When assessing the effective dose to workers following radio-aerosol inhalation exposures, significant reductions in dose uncertainty can be achieved through direct measurement of the particle-size distribution. The University of Washington Mark III cascade impactor is one such air sampling device that permits the user to determine aerosol mass and radioactivity concentrations as a function of particle size within eight different size intervals (each corresponding to a different impactor stage or end filter). Traditionally, dose assessments made using the LUDEP code or other internal dosimetry software utilize this air sampling information by assigning the radioactivity measured at each stage as concentrated at a single representative size central to the size interval. In this study, we explore more realistic assumptions that the measured radioactivity distributes uniformly, linearly increases, or linearly decreases across the particle size interval for each impactor stage. The concept of an effective dose scaling factor, SF(E), is thus introduced whereby (1) the former approach can be used (which requires less computational effort using the LUDEP code), and (2) the resulting values of effective dose per stage can then be rescaled to values appropriate to a linear radioactivity distribution per stage. For a majority of (238)U-series radionuclides, particle size ranges, and absorption classes, differences in these two approaches are less than 10%, and thus no corrections in effective dose per particle stage are needed. Significant corrections, however, were noted in select cases. For uniform or linearly decreasing radioactivity distributions, end-filter particles (0.03 to 0.35 microm) of type F, M, or S radionuclides were assigned values of SF(E) ranging from 1.15 to 1.44, while 3(rd) stage particles (4.5 to 12 microm) of type M and S radionuclides were assigned values of SF(E) ranging from 1.11 to 1.53. When the cascade impactor measurements indicate a linear increase of activity across a given impactor-stage size range, values of SF(E) range from a high of 1.11 (6(th) stage particles of type F radionuclides) to lows of 0.85 to 0.91 (4(th) stage and end-filter particles of type M and S radionuclides). In these cases, the inhalation dose coefficient varies non-linearly across the particle size range, and the assumption of a mono-size distribution per impactor stage either underestimates (SF(E) > 1) or overestimates (SF(E) < 1) that stage's contribution to the worker effective dose.

Aerosols↗

PubMed2005

Accounting for beta-particle energy loss to cortical bone via paired-image radiation transport (PIRT).

Current methods of skeletal dose assessment in both medical physics (radionuclide therapy) and health physics (dose reconstruction and risk assessment) rely heavily on a single set of bone and marrow cavity chord-length distributions in which particle energy deposition is tracked within an infinite extent of trabecular spongiosa, with no allowance for particle escape to cortical bone. In the present study, we introduce a paired-image radiation transport (PIRT) model which provides a more realistic three-dimensional (3D) geometry for particle transport in the skeletal site at both microscopic and macroscopic levels of its histology. Ex vivo CT scans were acquired of the pelvis, cranial cap, and individual ribs excised from a 66-year male cadaver (BMI of 22.7 kg m(-2)). For the three skeletal sites, regions of trabecular spongiosa and cortical bone were identified and segmented. Physical sections of interior spongiosa were taken and subjected to microCT imaging. Voxels within the resulting microCT images were then segmented and labeled as regions of bone trabeculae, endosteum, active marrow, and inactive marrow through application of image processing algorithms. The PIRT methodology was then implemented within the EGSNRC radiation transport code whereby electrons of various initial energies are simultaneously tracked within both the ex vivo CT macroimage and the CT microimage of the skeletal site. At initial electron energies greater than 50-200 keV, a divergence in absorbed fractions to active marrow are noted between PIRT model simulations and those estimated under existing techniques of infinite spongiosa transport. Calculations of radionuclide S values under both methodologies imply that current chord-based models may overestimate the absorbed dose to active bone marrow in these skeletal sites by 0% to 27% for low-energy beta emitters (33P, 169Er, and 177Lu), by approximately 4% to 49% for intermediate-energy beta emitters (153Sm, 186Re, and 89Sr), and by approximately 14% to 76% for high-energy beta emitters (32p, 188Re, and 90Y). The PIRT methodology allows for detailed modeling of the 3D macrostructure of individual marrow-containing bones within the skeleton thus permitting improved estimates of absorbed fractions and radionuclide S values for intermediate-to-high energy beta emitters.

Aged↗

PubMed2005

Chord-based versus voxel-based methods of electron transport in the skeletal tissues.

Anatomic models needed for internal dose assessment have traditionally been developed using mathematical surface equations to define organ boundaries, shapes, and their positions within the body. Many researchers, however, are now advocating the use of tomographic models created from segmented patient computed tomography (CT) or magnetic resonance (MR) scans. In the skeleton, however, the tissue structures of the bone trabeculae, marrow cavities, and endosteal layer are exceedingly small and of complex shape, and thus do not lend themselves easily to either stylistic representations or in-vivo CT imaging. Historically, the problem of modeling the skeletal tissues has been addressed through the development of chord-based methods of radiation particle transport, as given by studies at the University of Leeds (Leeds, U.K.) using a 44-year male subject. We have proposed an alternative approach to skeletal dosimetry in which excised sections of marrow-intact cadaver spongiosa are imaged directly via microCT scanning. The cadaver selected for initial investigation of this technique was a 66-year male subject of nominal body mass index (22.7 kg m(-2)). The objectives of the present study were to compare chord-based versus voxel-based methods of skeletal dosimetry using data from the UF 66-year male subject. Good agreement between chord-based and voxel-based transport was noted for marrow irradiation by either bone surface or bone volume sources up to 500-1000 keV (depending upon the skeletal site). In contrast, chord-based models of electron transport yielded consistently lower values of the self-absorbed fraction to marrow tissues than seen under voxel-based transport at energies above 100 keV, a feature directly attributed to the inability of chord-based models to account for nonlinear electron trajectories. Significant differences were also noted in the dosimetry of the endosteal layer (for all source tissues), with chord-based transport predicting a higher fraction of energy deposition than given by voxel-based transport (average factor of about 1.6). The study supports future use of voxel-based skeletal models which (1) permit nonlinear electron trajectories across the skeletal tissues, (2) do not rely on mathematical algorithms for treating the endosteal tissue layer, and (3) do not implicitly assume independence of marrow and bone trajectories as is the case for chord-based skeletal models.

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