Why "Measurand" Is the First Scientific Word We Should Teach Health Physicists.

ABSTRACT: The word "measurand" means "the quantity intended to be measured." The authors argue that health physicists should distinguish between measurands and measurement results because the former exist in the domain of theory, while the latter exist in the domain of reality in which we make measurements and observations. The authors demonstrate the importance of separating the quantities used in theory and those used in experiment, clearing up conceptual confusions in three examples of problems routinely encountered in health physics: (1) detection and quantification of radioactive material; (2) multiple definitions of "activity," and (3) the relationship between radiation and health effects. The first example looks into probabilities of various measurement results (mi) given the measurand (µ) in comparison with the inverse problem: determining probable values of the measurand (µ) based on observed measurement results (mi). The second example addresses the distinction between measurands and measurement results given two definitions of activity A provided by the International Commission on Radiation Units and Measurements. Additional consideration is given to use of N + 1 counts in (activity) calculations when we have observed N counts, which results from correctly stating and solving the inverse problem. This makes our measurement uncertainties more accurate and our detection decisions more reliable. The last example emphasizes how the observational results of epidemiology, animal experiments, and other radiation biology studies are used to estimate the probability of a particular cancer in an individual-a measurand that is not otherwise accessible to direct observation. Our measurement results, and our use of those results, are more easily understood when we understand the difference between a measurand and a measurement result and can choose the best calculational approach.

Validation of a system of models for plutonium decorporation therapy.

A recently proposed system of models for plutonium decorporation (SPD) was developed using data from an individual occupationally exposed to plutonium via a wound [from United States Transuranium and Uranium Registries (USTUR) Case 0212]. The present study evaluated the SPD using chelation treatment data, urine measurements, and post-mortem plutonium activities in the skeleton and liver from USTUR Case 0269. This individual was occupationally exposed to moderately soluble plutonium via inhalation and extensively treated with chelating agents. The SPD was linked to the International Commission on Radiological Protection (ICRP) Publication 66 Human Respiratory Tract Model (HRTM) and the ICRP Publication 30 Gastrointestinal Tract model to evaluate the goodness-of-fit to the urinary excretion data and the predictions of post-mortem plutonium retention in the skeleton and liver. The goodness-of-fit was also evaluated when the SPD was linked to the ICRP Publication 130 HRTM and the ICRP Publication 100 Human Alimentary Tract Model. The present study showed that the proposed SPD was useful for fitting the entire, chelation-affected and non-affected, urine bioassay data, and for predicting the post-mortem plutonium retention in the skeleton and liver at time of death, 38.5 years after the accident. The results of this work are consistent with the conclusion that Ca-EDTA is less effective than Ca-DTPA for enhancing urinary excretion of plutonium.

Mapping 241Am Spatial Distribution Within Anatomical Bone Structures Using Digital Autoradiography.

Digital autoradiography with the ionizing radiation quantum imaging detector is used at the US Transuranium and Uranium Registries for visualizing the microdistribution of alpha particles from Am and quantifying the activity. The radionuclide spatial distribution was investigated within cortical and trabecular regions of bone samples from US Transuranium and Uranium Registries case 0846. Multiple specimens from the humerus proximal end, humerus proximal shaft, and clavicle acromial end were embedded in plastic, and 100-µm-thick sections were taken and imaged using the ionizing radiation quantum imaging detector. The detector images were superimposed on the anatomical structure images to visualize Am distribution in cortical bone, trabecular bone, and trabecular spongiosa. Activity concentration ratios were used to characterize Am distribution within different bone regions. The trabecular-to-cortical bone and trabecular-spongiosa-to-cortical bone activity concentration ratios were quantified in both humerus and clavicle. The ionizing radiation quantum imaging detector results were in agreement with those obtained from radiochemical analysis of the remaining bone specimens. The results were compared with International Commission on Radiological Protection default biokinetic model predictions. Digital autoradiography was proven to be an effective method for microscale heterogeneous distribution studies where traditional counting methods are impractical.

Improved Modeling of Plutonium-DTPA Decorporation.

Individuals with significant intakes of plutonium (Pu) are typically treated with chelating agents, such as the trisodium salt form of calcium diethylenetriaminepentaacetate (CaNa3-DTPA, referred to hereafter as Ca-DTPA). Currently, there is no recommended approach for simultaneously modeling plutonium biokinetics during and after chelation therapy. In this study, an improved modeling system for plutonium decorporation was developed. The system comprises three individual model structures describing, separately, the distinct biokinetic behaviors of systemic plutonium, intravenously injected Ca-DTPA and in vivo-formed Pu-DTPA chelate. The system was linked to ICRP Publication 100, "Human Alimentary Tract Model for Radiological Protection" and NCRP Report 156, Development of a Biokinetic Model for Radionuclide-Contaminated Wounds and Procedures for Their Assessment, Dosimetry and Treatment." Urine bioassay and chelation treatment data from an occupationally-exposed individual were used for model development. Chelation was assumed to occur in the blood, soft tissues, liver and skeleton. The coordinated network for radiation dosimetry approach to decorporation modeling was applied using a chelation constant describing the secondorder, time-dependent kinetics of the in vivo chelation reaction. When using the proposed system of models for plutonium decorporation, a significant improvement of the goodness-of-fit to the urinary excretion data was observed and more accurate predictions of postmortem plutonium retention in the skeleton, liver and wound site were achieved.

Monte Carlo simulation of in vivo measurement of the most suitable knee position for the optimal measurement of activity.

A new computational model has been developed using the Monte Carlo (MC) technique to simulate in vivo measurements with the objective of understanding the most precise measurement location with respect to quantifying the activity of Am in the bones. To benchmark the model, in vivo measurements were performed on the U.S. Transuranium and Uranium Registries (USTUR) case 0846 leg. Front and lateral measurements of the knee of the USTUR case 0846 leg in a bent position and the same measurements with the leg in a straight position using a HP(Ge) detector were completed. Experimental results concluded that the front measurement of the knee in a bent leg position gave the highest count rate, which is an indication of optimal detection efficiency. Therefore, this geometry and knee-detector position were considered as the most appropriate position for knee monitoring. A computational model using MCNPX version 2.6.0 was used to simulate the experimental measurements by using a leg voxel phantom. The mean value and standard deviation (SD) of peak efficiency due to an isotropic 59.5-keV photon from Am were calculated in four different counting geometries. An extra sum of squares F-test was performed on the mean values of the simulated detection efficiencies. The p-value obtained from this statistical test indicates that the differences among the mean values for different counting geometries were significant. These results suggest that the front measurement of a knee in a bent leg position is the optimal counting geometry for in vivo measurement of Am deposited in the bones. The computational model was validated through comparison of the measured and simulated detection efficiencies. It was observed that there is no difference at the 0.1 significant levels between the simulated and measured detection efficiencies in assessment of Am within the bones.

Reevaluation of (241)Am content in the USTUR case 0102 leg phantom.

The (241)Am contents in the United States Transuranium and Uranium Registries' (USTUR) case 0102 leg phantom were previously estimated to be 1,243 ± 11 Bq. Recent analysis of the computed tomography images of the phantom revealed multiple bone structures missing from various regions of the phantom skeleton including: posterior ilium, anterior ilium, ischium, femur proximal end, femur middle shaft, femur distal end, patella, tibia distal shaft, fibula distal shaft, and fibula distal end. Additionally, the fifth metatarsal and all of the fifth-digit phalanges were found to be completely missing from the foot. A three-dimensional (3D) model of the leg phantom was created using 3D-Doctor software. Volumes of missing bone structures were outlined separately based on the anatomical assessment of those structures. Weights of the missing bone samples were calculated. Consequently, the value of total( 241)Am activity in the USTUR leg phantom is 1,218 ± 11 Bq. This activity is about 2.0% less than the previously published value of 1,243 ± 11 Bq. External gamma detector response was simulated considering both activity values (1,243 and 1,218 Bq) across the five different locations along the USTUR leg phantom: foot, middle leg, knee, middle thigh, and hip. Each counting position was chosen such that it was above the missing bone structure locations. The highest difference observed between the two counting efficiencies (each corresponding to the two different quantities of estimated activity) was 8.2% and 9.4% for locations above the foot and middle thigh, respectively. Other counting locations (middle leg, knee, and hip) showed efficiency variations of about 1%.