Answer to Question #6975 Submitted to "Ask the Experts"

Q

This evening I had dinner with a friend of mine. She had a PET (positron emission tomography) scan done this morning. She was never told to drink lots of fluids to get rid of the radioactive material that was injected. I saw her about eight hours after she was injected and she was sitting right next to me in the car for about 30 minutes and for an hour and a half across from me at the same dinner table (about two feet away). She did not drink much at all today and I'm scared that she was still very radioactive eight hours after the injection. I also gave her a hug. Do you know how much radiation exposure I received?  Do I need to be concerned about my radiation exposure?

I'm very concerned because I just had three foot x rays done yesterday and feel like I have been exposed to excessive radiation in a short period of time.

Could you please tell me if I should be concerned?

A

Patients who have PET scans do become radioactive for a short period of time. For most PET studies, radioactive material is injected into the patients an hour or two before the procedure. After patients have had the scan, they may still contain some of the radioactive material. The remaining radioactive material does not stay in them very long. Based on some information from a National Council on Radiation Protection and Measurements Report (Report No. 124), if you're about three feet from the patient an hour after the PET scan, you receive about 0.004 rad/hour. This means that if you stood at three feet from the patient for one hour, you would receive 0.004 rad (a rad is a unit of radiation dose). At eight hours after she was injected, the dose rate would be about 0.0005 rad/hour. Patients having a PET scan have to empty their bladder before the scan, so a lot of the PET drug did go down the toilet even before her scan.

If your friend drank water or any fluids during the day, biological elimination would reduce the dose and probably offset you being about two feet from your friend most of the time. An exposure rate of 0.0005 rad/hour is probably a good estimate. For two hours time together, you may have been exposed to 0.001 rad. That is about the same exposure as natural background radiation exposure in one day. Even if you think I am not being conservative in my estimate, double that dose is still a very low exposure.

X rays of your feet were about 0.0005 rad each, using the data in our fact sheet "Radiation Exposure From Medical Diagnostic Imaging Procedures." So that was a total of 0.0015 rad. This is also a very low dose.

You may find it helpful to read the Health Physics Society Position Statement on "Radiation Risk in Perspective," which states that there is no demonstrated health effect for radiation exposures below 5-10 rem. A rem is approximately equal to a rad.

If I had been in your same situation, I would not be concerned about my radiation exposure. I hope that this explanation is helpful.

Marcia Hartman, MS