Dr. Wolfgang Weiss wolfgang.weiss1@googlemail.com
Member of the German Emmendingen 27 July 2018
Radiological Protection Commission (SSK)
Re: ‘ICRP, 20YY. The Use of Effective Dose as a Radiological Protection Quantity.
The foundation of E – a recapitulation of previous recommendations
Effective dose (E) has been and continues to be a central element of the international system of radiological protection (RP). It is used for the practical implementation of the principles of limitation, optimization and justification in operational radiological protection decisions. E is providing a risk-adjusted measure of total body dose from external and internal sources in relation to risks of cancer and hereditary effects. Although E is generally used at doses below 100 mSv, its use exceptionally in emergency exposure situations at acute doses in the range up to around 1 Sv is reasonable. In medical applications, estimates of E to Reference Persons are used for comparing doses from different diagnostic and interventional imaging modalities (e.g. CT and nuclear medicine) and exposure techniques that result in different spatial distributions of radiation within the body tissues. The application of E provides a generic indicator for classifying different types of medical procedures into broad risk categories for the purpose of communicating risks to clinicians and patients. Bearing in mind the uncertainties associated with risk projection to low doses or dose rates, E may be considered as an approximate indicator of possible risk, with the additional consideration of variation in risk with age, sex and population group. Collective effective dose is a valuable tool in the optimization process, particularly for occupational exposures. It is not intended for use in risk projection.
Many of the characteristic properties of effective dose described in the paper are
not new
but I think it is important that the scientific criteria and the rationale behind the definition of E are regularly updated in response to the available scientific evidence. This is what the paper does.
I. General comments
I highly appreciate the update of the scientific foundation of the rationale of the foundation and application of the effective dose concept. Professional editing of the document could substantially reduce the volume of the document and increase the readability without reducing the understanding of the key features of E.
The new recommendation to “consider that the use of equivalent dose to set limits on organ/tissue doses to prevent tissue reactions should be discontinued” is of particular importance for the future development of the application of the system of RP. It may take some time to implement this recommendation in the operational world but the recommendation is a good start both from a conceptual and a communication point of view.
When applying the system of protection including the concept of effective dose in medical applications and emergency situations a number of questions (which are not new) have to be addressed. The most important one is the question how to answer frequently asked questions related to individual and lifetime risks. It is obvious that the assessment of radiation risks for individuals or groups of individuals is not an objective of the ICRP system of radiation protection but it would be more than helpful to provide scientific arguments (beyond risk communication, see II.3), which could be used for this purpose.
II. Specific comments
The document covers a wide spectrum of relevant thematic aspects including scientific, operational and communication topics. A reduction of the thematic spectrum could result in a better understanding of the meaning of the details of the new recommendations and the requirements for practical application.
II.1 Addressing individual risk – a key element of optimisation and justification
Dealing with individual risk requires more than just optimisation (as stated in line 2011 of the document). In the medical field as well as in emergency situations it is obvious that justification is the key attribute to answer the question: does a medical application of ionizing radiation for diagnostic and/or therapeutic purposes or the implementation of a disruptive protective measure like evacuation do more good than harm for the affected individuals?
Further guidance on the use of E during the application of the justification principle in these challenging situations would be very welcome.
It would beneficial for members of the operational world if the document (or another document specifically dealing with medical or emergency matters) would provide more specific guidance to deal with questions frequently asked in these situations. Such a guidance document should include considerations that best assessments of individual radiation related risks need to take account of all available specific exposure data, relevant information on the individuals concerned and all available relevant scientific information:
- best estimates of absorbed dose to organs / tissues;
- best scientific information on RBE for the radiation concerned and for the specific cancer types;
- age and sex specific cancer incidence and mortality data for the different organs instead of nominal risk coefficients and tissue weighting factors used in the definition of E
and how this could be done in practical situations taking into account the uncertainties of these parameters.
It should be recognised, however, that ‘balancing’ good and harm is not confined to issues associated with radiation exposure. Other non-radiation-related benefits and detriments arising from medical applications and/or the introduction of disruptive protective action in emergencies must also be considered during decision making. The selection of the best available option is a task beyond the responsibility of radiological protection professionals; their remit should be confined to ensuring a positive net radiation-related benefit. The remaining question is who in the operational world would provide input to this ‘balancing’ process to achieve “best practice”.
II.2 The practical application of the concepts of the representative person and age- and gender-specific risks
The paper presents very useful information about the available scientific evidence about gender and age-specific variations of the radiation risk of different population groups by factors of two to three (tables 2.4, 2.5, 5.3 and 5.4; figure 5.1).
While exposures may relate to individuals or population groups, E is calculated for Reference Persons exposed in the same way as before. The commentary in lines 1997 to 1999 (correctly) states: “It has been argued that this approach does not adequately protect women and younger children and that differences between males and females and greater risks at younger ages should be reflected more explicitly in the ICRP system, including the use of different detriment values and wT values.”
The conclusion of ICRP in lines 2000 to 2008: it is notable that estimated differences in lifetime risk of cancer incidence between males and females and between age groups, as illustrated in Tables 2.4 and 2.5, are not large in the context of the practical application of the system of protection at low doses and uncertainties associated with estimates of risk at low doses. Protection would not be improved by introducing separate considerations for males and females and for children of different ages, with different nominal risk coefficients and associated sets of tissue weighting factors.
Without further quantitative discussion of the uncertainties involved these statements are more confusing than convincing. Clarification is needed how the statement about optimisation in line 2011 should be implemented.
II.3 Risk communication
There is no question that risk communication is an important element of the communication of the system of protection to non specialists. I highly welcome and support the recommendation (lines 1802/1803) that „the concept of effective dose and a knowledge of typical effective doses from common procedures should therefore be included in the education and training of medical practitioners“. Available experience clearly shows that a successful implementation of this recommendation would require major efforts well beyond the ones on terminology, which are described in table 5.2.
The limitation of the education/training efforts to the use of simple language/ terminology as discussed in the draft document could be misunderstood by RP professionals and members of the public and it could result in misunderstanding and mistrust which is certainly not what is intended. A more detailed description of the minimum requirements to implement the recommendation in practical education/ training programmes would be needed. Guidance documents describing how to do this are available in the published literature.