Answer to Question #11600 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
My husband had a scan with 185 megabecquerels (MBq) of iodine-123 (123I). Are there any precautions for the spouse after such a scan? When is intimacy okay? Does he emit radiation and if so, for how long and how much? Is 30 hours from the injection long enough to wait?
Fortunately, the radiation dose to others from your husband's scan is negligible and no precautions are needed. Intimacy is perfectly okay from the start.
I assume from the quantity of 123I you cited that your husband had a total body scan to look for residual thyroid tissue or thyroid cancer metastases. Assuming this, I performed dose calculations to someone with whom he shares living quarters according to the Radiation Absorbed Dose Assessment Resource (RADAR). That person's dose would be on the order of 0.046 millisievert (mSv), which is about what you get after six days of natural background radiation. Current regulations limit doses to others to 5 mSv, about 100 times higher than the calculated dose.
The physical half-life of 123I is about 13 hours. But your husband excreted all or almost all of it in his urine with a half-life of eight hours if he received thyrogen injections beforehand or with a half-life of 11 or 12 hours if he became naturally hypothyroid before receiving the 123I.
Let's assume he received thyrogen. If we combine the renal half-life of eight hours with the physical half-life of 13 hours, we get an effective half-life of (8)(13)/(8+13) = 5 hours. This means that every five hours the remaining radioactive material is half of what he received. After the first five hours after receiving the 123I, the remaining activity is 93 MBq. After another five hours, it is 46 MBq. And so on. As you can see, it's disappearing fast, and no precautions are necessary.
Carol S. Marcus, PhD, MD
Professor of Molecular and Medical Pharmacology (Nuclear Medicine), of Radiation Oncology, and of Radiological Sciences, David Geffen School of Medicine, UCLA