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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.
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):
_{i} is an observed value and E_{i} 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. |