Internal dose calculation

Pulmonary retention (net respiratory uptake) of MTBE in volunteers exposed to concentrations ranging from 5 to 75 ppm is around 40%, while pulmonary retention of ETBE over an exposure range of 5 to 50 ppm is about 26% [35]. About 7-9% of the inhaled MTBE is reversibly taken up by the mucous membranes of the upper airways [31]. The internal dose calculated from the area under the inhaled air concentration and alveolar breath concentration curves, after inhalation exposure to 1.7 ppm for 15 min [26], averaged 197 50 pg for all subjects (about 3.86 pg/kg bw). 

Calculation of dermal exposure and internal dose using DDBM data... 

The calculation of potential total dermal exposure of mixer-loaders and re-entry workers using dosimetry data and calculation of the internal dose using biological monitoring data is complex but will be discussed briefly. 

The internal dose (ID) for workers can be calculated by using the analytical data from the urine collection carried out simultaneously with the dosimetry. The calculation of the ID using urine data is complex and will not be dealt with in detail. However, there are several references, that can help guide one through such calculations. Nolan et al have addressed this subject in great detail, as have other researchers. 

The internal dose of propoxur was measured by assessing the total amount of 2-isopropoxyphenol (IPP) excreted in the urine, collected over a period of 24 hr from the start of exposure, and described in detail in previous studies (Brouwer et al., 1993 Meuling et al., 1991). Volunteer kinetics studies revealed a one-to-one relationship of absorbed propoxur and excreted IPP on a mole basis. Based on the results by Machemer et al. (1982), a pulmonary retention of 40% was used to calculate the relative contribution of the respiratory exposure to the internal exposure. To estimate the contribution of the dermal exposure, the calculated respiratory portion was subtracted from the total amount of IPP excreted in urine. 

These results, however, cannot be used to establish a dose-effect curve, because of inadequate dose estimates. For this purpose it is necessary to calculate the individual accumulated external and internal doses of all persons involved into the investigation,... 

A recommended approach for conducting toxicokinetic studies generally involves three steps. Step 1 is a preliminary study, which uses a minimum number of animals to estimate the range of blood/tissue concentrations, the required quantitation limit for the analytical method, and the optimal sampling times for the definitive toxicokinetic studies. Step 2 is the definitive study and generates blood and/or tissue concentration data for calculating the toxicokinetic parameters. Step 3 is the toxicokinetic study conducted in conjunction with the toxicology study to determine the internal dose and the effects of age and continuous exposure on kinetic parameters. 

In analogy with iodine-labeled albumin, absorbed dose calculations are based on the elimination of radioactivity from blood by three half-times, namely 6.8 h (0.40), 1.29 days (0.22), and 19.4 days (0.38, Takeda and Reeve 1963). Uniform distribution of 99mTc-HSA outside the blood pool and rapid renal excretion of the released radionuclide is assumed (International Commission on Radiological Protection 1987). 

INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Task Group on Dose Calculations — Energy and Intensity Data for Emissions Accompanying Radionuclide Transformations, ICRP Publication 38, Pergamon Press, Oxford and New York (1984). 

There are two sources of dose that must be considered when calculating a person s dose external dose and internal dose. External dose is commonly measured with a dosimeter worn on the torso. Section 25.3.1 provides more information regarding calculating external dose. External dose is received for discrete intervals of time, such as when a person enters and exits a radiation area. When the person exits the radiation area, they are no longer receiving any external dose. Three different types of external dose are typically measured deep dose equivalent (DDE), shallow dose equivalent (SDE), and the dose equivalent to the lens of the eye (LDE). The SDE and the LDE are measured or calculated specifically to track the dose to the skin and the lens of the eye. For all other body parts, the DDE is used. 

For members of the public, including licensee employees who are not rad workers, generally no individual monitoring is performed. Compliance with dose limits is determined by conservative generic calculations. Area surveys are performed or area dosimeters are used to determine the external dose rates in locations accessible by members of the public. These dose rates are multiplied by conservative estimates of the length of time a member of the public may be in that area to determine the total potential annual external dose. For internal dose, estimates of the magnitude of intakes of material that might occur from atmospheric or effluent releases of radioactive material are performed to calculate hypothetical conservative internal dose estimates. The summed internal and external doses are used to demonstrate compliance with dose limits for members of the public. 

Once the magnitude and the route of the intake have been determined, the internal dose, the CEDE and CDE for organs of concern can be calculated. 

The internal doses to a critical population group in conditions of chronic (prolonged) exposure due to the ingestion of contaminated food and/or drinking water should be determined on the basis of environmental monitoring data by the use of a simple calculation model in which account is taken of the origin and consumption rate of particular food products as well as seasonal variations in relevant parameters. 

The reactor will be pulsed with reactivity insertions from 1% to 1.4%(or the maximum available) in 0.1% increments. Calculate the transient rod fired position to obtain the desired reactivity insertions. One pulse will be fired for each reactivity step. A second pulse will be fired for one of the steps to provide an indication of the reproducibility of the measurements. The signal from the gamma ion chamber is sampled at one millisecond intervals and stored in the TestLab. The TestLab internal program calculates the information needed for the reactor log and records two channels of power-level data derived from the gamma-ray dose from the reactor core. The first channel, labeled "pulsetrace", records the full pulse. The second, labeled "pulserise", uses a smaller range and thus records the early part of the pulse most useful for reactor period measurements. In both cases the raw data is normalized such that the output is in MW. 

The most accurate assessments of internal dose can be made when the distribution and total body content of an incorporated radionuclide can be determined reliably by direct in vivo counting of emissions from the body. Nevertheless, biokinetic modelling of retention and biophysical modelling of energy deposition may still be needed to calculate the intake and the committed effective dose, so direct methods can also depend on the interpretation of rates of excretion, which often vary markedly over time and between individuals. 

In another study carried out at a different nuclear establishment, the isotopes and were determined (along with calculated concentrations) in a series of human urine samples, using a concentrated aqua regia wet oxidation method to dissolve the uranium and destroy the organic matter. The uranium was selectively separated from the matrix using anion exchange, eluted with dilute nitric acid, and then aspirated into the ICP mass spectrometer. Using this method, a detection limit of 6 ng/L was achieved, with excellent spike recoveries at the 200 ng/L level, which met both plant and industry standard (American National Standards Institute 13.30) internal dose assessments for total uranium. ... 

Bioassay. Bioassay involves taking biological samples (e.g. urine, faeces, nose swabs, breath) and measuring their activity. From the activity of the samples, the internal contamination can be assessed and the intake of the person and the resulting internal dose can be calculated using models. 

The third comment concerns external radiation from direct exposure to the fallout. It was obviously difficult to find a control population. A few subjects were found who could serve as control for this part of the survey, because they had not been exposed to external radiation, living behind a mountain which protected them. The results showed clearly that the prevalence of nodular goiter was 5-fold higher in subjects who had been exposed to external radiation than in the control population. The dose calculated was at a maximum of 20-40 Rads. Unfortunately, it was not possible to assess what the potential role of accompanying internal radiation might have been, since this was not available at that time. The important conclusion is that 40 years after the atomic bombs, a significant increase in thyroid nodular disease was clearly evident. 

Comparing the results of PBPK calculated internal dose after oral 13-cw-retinoic acid found the effective concentrations across species to be of the same order of magnitude. The results of... 

PBPK calculated internal dose after topical application of 0.5% all-fraw5-retinoic acid to the face, arms and chest found the circulating concentrations of retinoic acids remained four orders of magnitude less than those required for increased teratogenic risk [29]. The low absorbed and delivered dose concentrations calculated using the PBPK model are consistent with the empirical data in hamster [30] and rabbit [31] after topical compared to oral administration of this material [28, 32]. 

The contractor has reported that actions required to achieve full compliance in the reactor facilities have been completed including modification of procedures and computer programs for dose calculations. Additionally, WSRC has issued a Technical Basis Manual (Reference 26) intended to provide a technical and philosophical discussion of the radiobioassay and dose assessment aspects of the internal dosimetry program. 

---