As pointed out in the previous section, the intercomparison exercises have shown that there is a wide variety of evaluation procedures, depending on the experience and the skill of the assessor as well as on the hardware and software tools available. However, for a given set of internal monitoring data in terms of body/organ activity and/or urine/faecal activity there should be one standard estimate for the intake and the committed equivalent dose. This standard estimate is defined by the monitoring data, the biokinetic models for the description of the metabolism, dosimetric models, and – if available – some additional information, such as time of intake, route of intake, aerosol size, respiratory tract absorption Type, gastro-intestinal (GI) tract absorption factor (f1 value) and previous internal exposures.

 

So the aim of the IDEAS guidelines [49] is to enable all assessors to derive the best estimate for any given set of incorporation monitoring data taking into account the following principles:

  • Harmonisation: by following the procedures any two assessors should obtain the same estimate of dose from a given data set
  • Accuracy: the “best” estimate of dose should be obtained from the available data

  • Proportionality: the effort applied to the evaluation should be proportionate to the dose – the lower the dose, the simpler the process should be.

Harmonisation

A well-defined procedure is needed and for this reason the process is defined here primarily by means of a series of flow-charts. So far as possible, the process has been made widely applicable, i.e., it does not assume that the assessor has the use of sophisticated bioassay interpretation software. For routine monitoring situations, where typically there is only one measurement relating to each intake, it is reasonably straightforward to define a procedure. However, in special monitoring situations, where typically there is more than one measurement and quite possibly more than one type of measurement (urine, faeces…) different options for data handling can easily lead to different evaluated doses, even when the same model, parameter values and software are used. Another range of options, and opportunities for different evaluated doses, arises in situations where it is appropriate to consider changing parameter values from the ICRP defaults. Proposals are made here for a systematic approach to dose assessment in all these situations.

Accuracy

It is recognised that the uncertainties associated with assessed internal dose can be considerable, especially for actinides which are difficult to detect in the body and have relatively high dose coefficients (Sv Bq-1). If the initial estimate of dose exceeds 1 mSv, it could well be that the possibility of a substantially higher dose (e.g. 6 mSv) cannot easily be excluded. It is then important to make best use of the available information. To do so may well involve changing parameter values from their ICRP default values and guidance is therefore needed on which parameter values might reasonably be varied according to the circumstances.

 

Proportionality

The effort applied to the evaluation of incorporation monitoring data should broadly correspond to the expected level of exposure, and the complexity of the case. On the one hand, if the exposure is likely to be very low with respect to the dose limits, simple evaluation procedures with a relatively high uncertainty may be applied. On the other hand, if the monitoring values indicate the exposure to be close to or even above the dose limits, more sophisticated evaluation procedures will need to be applied. These take account of any case-specific information available, so that the uncertainty and bias on the best estimate are as low as reasonably achievable.