Answer to Question #4166 Submitted to "Ask the Experts"

Following a thyroid uptake and scan, using 16 microcuries of ¹³¹I and 10 millicuries of ^99mTc pertechnetate: (1) How long should a nursing mother stop breast-feeding? (2) Is there any value to taking potassium iodide after the scan to prevent concentration of ¹³¹I in the thyroid? (3) Is there any value to pumping and discarding breast milk to rid the body of the radionuclides sooner, whether or not breast-feeding is resumed? (4) Is it true that one should not hold the baby close to the chest/throat area or kiss the baby after such tests? (5) Are nuclear-imaging professionals required by law to provide information pertaining to the effects of such tests on breast-feeding, prior to the test? If so, is the information to be given verbally or in writing?

Before I answer your questions, I would like to state my opinion that there is some disconnection between realistic, scientifically based radiation risks and the regulatory requirements and recommendations set forth by the Nuclear Regulatory Commission (NRC) or Agreement States.  These regulatory requirements are listed in Table U.3, "Activities of Radiopharmaceuticals that Require Instructions and Records When Administered to Patients Who Are Breast-Feeding an Infant or Child." This table and adjunct regulatory requirements can be found in NUREG-166, Volume 9, "Program-Specific Guidance About Medical Use Licenses."  The radionuclide activity limits listed in this table and the corresponding calculated doses are based (1) on the highest infant uptake ever measured, (2) on the highest secretion of radionuclide into milk ever measured, and (3) on the maximum quantity of breast milk ingested per day per infant.  No allowance is made for infant formula for the feedings.  In essence, these are not dosimetry calculations, they are default calculations for the highest intakes possible.  In a realistic setting, the administration of less than the calculated activity could not possibly result in an infant dose above the 100 millirem regulatory limit.

With the above explanation in mind, your specific questions are considered below.

1. After ¹³¹I administration the NRC recommends stopping breast-feeding permanently, as it takes about three months for all ¹³¹I with a half-life of eight days to disappear from breast milk. After administration of 12 millicuries of ^99mTc pertechnetate, about 12 hours of interruption of breast-feeding is recommended by NRC. Alternatively, one may use Na¹²³I instead of the ¹³¹I and ^99mTc pertechnetate, and breast-feeding needs to be stopped for a maximum of approximately three days.  In my practice, I switched to Na¹²³I, with a half-life of 13.1 hours, for all patients in 1982.
    
2. There is no value in taking potassium iodide. Maximum thyroid uptake has occurred by about 24 hours, and potassium iodide has no significant effect after that time.
    
3. Pumping and discarding breast milk might get tiny amounts of ¹³¹I out of your body a bit faster.  However, I am unaware of any data showing the magnitude of such a possible effect, and I wouldn't think it would be significant enough to consider as a means to reduce ¹³¹I levels in the breast milk.
    
4. Large quantities of administered ¹³¹I usually make holding the baby close to you unadvisable. However, your doses of ¹³¹I and of ^99mTc are insignificant here. As far as kissing the baby, your saliva will contain ¹³¹I and ^99mTc pertechnetate. If you kiss the baby "dry," it's okay.
    
5. Your physician's radioactive material use license probably dictates that you should have been given written instructions. This is not in the law, but are regulation and license conditions.  It is the standard of medical practice for the physician or technologist to ask about pregnancy and breast-feeding before any radiopharmaceutical is administered, especially ¹³¹I. With ¹³¹I, the physician should take extra care that the questions are asked.

If we put aside the regulatory aspects of your scan and ask scientifically how long you should hold off breast-feeding to avoid imparting a significant risk of thyroid or other problems to your baby after the administration of 16 microcuries of Na¹²³I, which is probably what you really want to know, assuming you have normal renal function, I would recommend four days should probably be ample if you pump your breasts and discard your milk. However, I do not know you personally and am not knowledgeable about your particular case, so do not construe this as medical advice.  Ask your physician.  If your physician doesn't know, tell him or her to obtain a calculation from a competent medical physicist.

Carol S. Marcus, PhD, MD