Editorial

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This space is used, of course, to comment on each new report at the time of its release, and at the same time it provides an opportunity to highlight topical issues and news from the Commission. After some words on the present report, I will come back to some aspects of the Commission’s views on tritium below.

Medical exposure IS unique. This report is one of the ‘Foundation Documents’ underpinning the Commission’s 2007 Recommendations (ICRP, 2007). However, while Foundation Documents comprising detailed explanations of the biological and physical considerations underlying the Recommendations were published as Annexes A and B of the actual Recommendations, we thought that this summary would fare better as a stand-alone document, particularly for those readers who are directly concerned with medical uses of radiation and want detailed information about medical exposure protection policy.

The main message in the present report is that medical exposure of patients has unique considerations that affect how the fundamental principles are applied. Dose limits are not at all relevant, since ionising radiation, used at the appropriate level of dose for the particular medical purpose, is an essential tool that will cause more good than harm.

Justification in radiological protection of patients is different from justification of other radiation applications, in that generally the very same person enjoys the benefits and suffers the risks associated with a procedure. (There may be other considerations: attendant occupational exposures could be correlated with patient doses or sometimes there can be a trade-off; screening programmes may benefit the population rather than every screened person. But usually, risks and benefits accrue to the same person). And, a very important aspect in daily medical practice: the fact that a method or procedure can be regarded as justified as such does not necessarily mean that its application to the particular patient being considered is justified.

Optimisation of protection for patients is also unique. In the first place, radiation therapy is entirely different from anything else in that the dose to a human being is intentional and its potentially cell-killing properties the very purpose of the treatment. In such cases, optimisation becomes an exercise in minimising doses (and/or their deleterious effects) to surrounding tissues without compromising the pre-determined and intentionally lethal dose and effect to the target volume.

In optimisation of protection of the patient in diagnostic procedures, again the same person gets the benefit and suffers the risk, and again individual restrictions on patient dose could be counterproductive to the medical purpose of the procedure. Therefore, source-related individual dose constraints are not relevant. Instead, Diagnostic Reference Levels (DRLs) for a particular procedure, which apply to groups of similar patients rather than individuals, are used to ensure that doses do not deviate significantly from those achieved at peer departments for that procedure unless there is a known, relevant, and acceptable reason for the deviation. This is in contrast to the Commission’s usual balancing of utilitarian protection policies based on collective doses against deontological safeguards using dose constraints for the individual. The policy for radiological protection in medicine is that the radiation exposure be commensurate with the medical purpose.

These various considerations are discussed in some detail in the present report, and in addition, the report includes an overview of the advice provided in the Commission’s recent series of topical reports on current issues in medical radiological protection.

The current w_R for tritium IS sufficient. It is implicit in the above that calculation and application of protection quantities and radiation and tissue weighting factors need careful thought in the medical context. They are, of course, fundamental considerations in all exposure situations, and sometimes the source of heated debate. Recently the relative biological effectiveness (RBE) of beta radiation from tritium and the radiation weighting factor (w_R) to be applied for tritium in routine radiological protection have been the subject of much discussion. To cut a long story short, there is some evidence that at least under some circumstances, an RBE value of 2 for tritium, relative to gamma rays, might be appropriate for cancer induction at low doses, and this in turn has prompted questions as to why ICRP continues to recommend a w_R of 1 for tritium.

Basically, the answer is that for planned exposures, appropriate levels of protection are determined by constrained optimisation, resulting in doses that will be typically a small fraction of the relevant dose limit. There are many uncertainties in the estimation of, e.g., the RBE of tritium radiation, and many intentional simplifications in the assumptions underpinning effective doses which are defined for reference persons and evaluated using reference phantoms. Increased complexity in the calculation of equivalent and effective dose would not improve protection and might suggest a degree of precision in the calculations that is unwarranted.

For a more thorough understanding of the topic, a review of the methodology used in the estimation of doses and risks from internal emitters is provided by Harrison and Day (2008) in the Journal of Radiological Protection. The Commission’s position is explained in detail in an invited editorial by Cox (Commission Vice-Chair), Menzel (Committee 2 Chair), and Preston (Committee 1 Chair) in the same issue of that journal. These papers, both of which can be downloaded cost-free from www.iop.org/EJ/journal/JRP, constitute highly recommended reading for those concerned with radiological protection policy issues in relation to internal emitters, particularly tritium.

Jack Valentin

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References 

1. Cox R, Menzel H-G, Preston J. Internal dosimetry and tritium – the ICRP position. J. Radiol. Prot. 2008;28:131–135
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2. Harrison JD, Day P. Radiation doses and risks from internal emitters. J. Radiol. Prot. 2008;28:137–159
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3. ICRP, 2007. The 2007 Recommendations of the International Commission on Radiological Protection. ICRP Publication 103. Ann. ICRP 37 (2–4).
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