Answer to Question #7143 Submitted to "Ask the Experts"

Category: Nuclear Medicine Patient Issues — Therapeutic Nuclear Medicine

The following question was answered by an expert in the appropriate field:

The mathematical model in the National Council on Radiation Protection and Measurements (NCRP) Report 155 (Appendix B) for thyroid cancer patients is different than the model used in the Nuclear Regulatory Commission Regulatory Guide 8.39. One has two compartments, the other three; and the effective half-lives are not the same (5.8 days vs 7.3 days respectively). Since the NCRP 155 publication is the more recent of the two, is that the "preferred" model to use?

The guidance provided in Reg. Guide 8.39 was reiterated more or less unchanged in NUREG-1556, Vol. 9, which was issued in 2005. The method described in these guides is a simple, generic method, intended to provide reasonable assurance that, when applied to any release situation, it will ensure that the primary requirement for patient release will be met. This primary requirement, stated in 10 CFR 35.75, is that the release is not likely to result in a dose to any individual in excess of 5 mSv.

The advantage of using the methods described in the guides is that the licensee will not have to justify them, since NRC has already stated that it considers them acceptable. However, the guides are not requirements, and licensees may use methods other than those described in the guides to show compliance with the primary dose requirement.

The disadvantage with this latter approach is that the licensee must justify the method chosen, showing that it does in fact provide a reasonably reliable estimate of the dose and that it is applicable to the licensee's facility.

The advantage, on the other hand, is that it may provide a much better estimate of the dose, and usually one that is lower than the results produced by the methods described in the guides, because it will be tailored to the specific conditions at the licensee's facility. Such a tailored application of a method will usually remove many of the conservatisms that are necessary in a more generic approach. This is because the generic method is intended to be used at any facility, and it must therefore be sufficiently conservative to ensure that it will be adequate for any reasonably expected set of conditions, leading to the built-in conservatisms.

The answer to the question is, therefore, that the three-compartment model may be used provided the licensee is able to show that it is a method that applies to the licensee's situation and that it is endorsed by a reputable organization such as the NCRP or the International Commission on Radiological Protection.

Sami Sherbini, PhD
Cynthia Jones, PhD

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