Answer to Question #11974 Submitted to "Ask the Experts"

Category: Nuclear Medicine Patient Issues

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

Q

Do the dose coefficients for iodine-131 (131I) in International Commission on Radiological Protection (ICRP) Publication 95 Doses to Infants From Ingestion of Radionuclides in Mother's Milk give the dose to the thyroid or the whole-body dose? If it is whole body, can I divide by the tissue-weighting factor to get the dose for the thyroid?

A

Your question raises important issues regarding the radiation risk to nursing infants after ingestion of radioiodine (131I) by their mothers. Ingestion of 131I could be a consequence of a medical procedure, an accident at a nuclear power plant, or fallout after the use of a nuclear weapon.

The dose coefficients in ICRP Report 95 Doses to Infants From Ingestion of Radionuclides in Mothers' Milk are not for individual organs or for the whole body. They are expressed as effective dose per unit of radioactivity that is ingested by the mother (sieverts per becquerel [Sv Bq-1] or microsieverts per becquerel [µSv Bq-1]). Effective dose is the sum of individual organ doses multiplied by tissue-weighting factors. The tissue-weighting factors that are published by the ICRP take into consideration the risks of fatal and nonfatal cancers, hereditary diseases, and life shortening in a population of both genders and all ages. In producing the dose coefficient tables in ICRP Report 95, the ICRP task group used organ dose values for three-month-old infants that were published in ICRP Report 56 Age-dependent Doses to Members of the Public: Part 1. After estimating the amount of 131I ingested by the mother that would end up in her breast milk and then in the baby, the task group could calculate the effective dose to the infant using those organ dose values and the tissue-weighting factors.

Ingested radionuclides are taken up by many body tissues, and they contribute to the radiation doses for those tissues. For that reason, dividing the effective dose by the tissue-weighting factor for an organ does not generally give a valid value for the dose to that organ. 131I is taken up not only by the thyroid gland, but also by the stomach, small intestine, salivary glands, and liver. All those organs contribute to the effective dose. However, if you compare the thyroid gland dose to the effective dose for the three-month-old infant in Table 7.3 of ICRP Report 56 (3.7 µSv Bq-1 versus 0.11 µSv Bq-1), you'll see that almost all of the effective dose for 131I is contributed by the thyroid gland. The other organs contribute only a small fraction of the total. 131I is the exception to the rule: a reasonable estimate of the dose to the thyroid gland can be obtained by dividing the effective dose by its tissue-weighting factor.

As an example, let's suppose a mother swallows a capsule containing 370,000 Bq of 131I to test her thyroid function. We can estimate the dose to her baby's thyroid gland if she nurses after the test as follows. First, looking at Table C.1 of ICRP Report 95 tells us that the effective dose to the baby is 54 nanosieverts (nSv) from 1 Bq ingested by the mom. After multiplying the dose coefficient by the radioactivity in the capsule, we see that the effective dose to the baby would be 20 millisievert (mSv). Next, dividing the effective dose by the tissue-weighting factor for the thyroid gland (0.05) gives a thyroid dose of about 400 mSv. Considering that the recommended annual limit on effective dose to members of the public is only 1 mSv, performing the medical test during nursing would not be appropriate. Instead, the recommendation is that the mother stop nursing her baby two weeks before the medical test, switch the baby to formula, and not start nursing again after the test. Stopping nursing two weeks before the test may annoy the baby, but it suppresses milk production and reduces the radiation dose to the mom's breasts.

Robert E. Reiman, MSPH, MD
Associate Professor of Radiology
Faculty, Medical Physics Graduate Program

References

International Commission on Radiological Protection. Age-dependent doses to members of the public: Part 1. Oxford: Pergamon Press; ICRP Report 56; 1990.

International Commission on Radiological Protection. Doses to infants from ingestion of radionuclides in mothers’ milk. Amsterdam: Elsevier; ICRP Report 95; 2004.

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