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Internal Sources - Occupational Exposures

In occupational exposures, one needs to use a knowledge of the input and the pathway with the appropriate dose conversion factors (DCFs) to calculate a cumulative organ dose and effective whole body dose for the exposed person(s). Often, the input is not well known, and must be determined by bioassay measurements (in vivo counting or analysis of excreta). In nuclear medicine, generally the input term and pathway is well known, and one may apply DCFs easily, or even more easily, simply look up dose values, derived from standardized models, to estimate the doses. This is generally more appropriate in diagnostic applications than in therapy, where patient-specific parameters should be applied as much as possible.

In this section of the RADAR site, we will offer some basic tools and data that may be of use in these situations. As noted on the RADAR Services page, RADAR members are also available for specific analyses of data, on a consultant basis, if desired.

Dose Conversion Factors

Dose Conversion Factors (DCFs) provided by the International Atomic Energy Agency (IAEA) in its Safety Series 115 document, are available from this site for viewing or download. These factors give the dose per unit intake by inhalation or ingestion for a large number of radionuclides, for adults and children of various ages. They are thus useful for calculating committed doses for workers, who might experience intakes in the workplace, as well as individuals or populations near nuclear sites, where intakes might occur due to offsite releases of radionuclides. You can choose any or all of the following tables (the rest, and the accompanying textual material can be purchaed from the IAEA):

TABLE II-III. Committed Effective Dose Per Unit Intake via Inhalation and Ingestion (Sv/Bq) for Workers

TABLE II-VI. Committed Effective Dose Per Unit Intake via Ingestion (Sv/Bq) for Members of the Public

TABLE II-VII. Committed Effective Dose Per Unit Intake via Inhalation (Sv/Bq) for Members of the Public

TABLE II-IX. Committed Effective Dose Per Unit Intake via Inhalation (Sv/Bq) for Soluble or Reactive Gases and Vapours

TABLE II-X. Effective Dose Rate for Exposure to Inert Gases for Adults(a)

(That last table might belong better on the External Dose Page, but we'll leave it here, to keep all the tables together). Other such DCFs have been developed by the International Commission on Radiological Protection (ICRP), but are not available from this site. The ICRP provides them on a CD ROM that may be purchased (contact the ICRP for details)

Estimating the Intake - Analysis of Bioassay Data

If an estimate of the intake can be made based on inherent knowledge of the situation (e.g. how much activity was being handled, how much activity is missing, air sampling or other data), perhaps one can estimate the intake easily. However, this type of estimate usually has much uncertainty, and the use of bioassay data from the exposed individual is preferred to provide an estimate of intake. The analysis of bioassay data to determine intakes is not usually a straightforward task. To determine the type and timing of samples, one must consider (1) the route of entry, (2) the radionuclide metabolism in the body, (3) the radionuclide emission spectrum, (4) detection capabilities of available in vivo or in vitro analytical techniques, and perhaps other factors. When one has one or more bioassay measurements, one then needs a model that predicts what fraction of the initial intake would have been in a sample of that type at that time (e.g. a whole body count 10 days after intake, a 24 hour urine sample 2 days after intake, etc.).  Then, one divides the measured value by the expected fraction of intake expected to get an estimate of the actual intake.  If one has multiple values, one can calculate multiple estimates of intake, or a least squares single estimate, using the formula:

where I is the predicted intake, Oi is an observed value and Ei is an expected (model predicted) value. This formula is surprisingly simple, but it is derived from a least squares analysis, and supposes that the relative variance of the measurements is proportional to the absolute value (Skrable et al., proc. 1994 HP Summer School, Internal Radiation Dosimetry).  One should always plot out the observed and model predicted values to be sure that a reasonable estimate of intake - this formula will always yield a result, but if (for example) the effective half-time of activity in your subject was greatly different than that of the model (from which the expected fractions were obtained), the estimate of intake may not be reasonable.

However one gets the best estimate of intake, one then may apply DCFs, as given above, which are usually given as cumulative dose per unit intake, to obtain doses.