Answer to Question #4647 Submitted to "Ask the Experts"

Q

I am a nuclear medicine technologist and I wonder if inpatients, who have been injected with F-18 FDG (fluorodeoxyglucose), should remain in the PET (positron emission tomography) facility for a time after their scan before returning to their ward. Is it safe for portering staff, nursing staff, and other patients on the ward if these patients return at about two hours post injection, or should they stay a while longer? This question has come up in our initial discussions concerning the installation of a PET/CT unit. I work at a hospital in Canada.

A

Great question. The numbers I'm going to provide are based on data in a publication by the International Commission on Radiation Protection, Publication 53.

The approximate unshielded dose rate from ¹⁸F is 0.055 mSv/h at one meter. The ¹⁸F physical half-life is 110 minutes; internally, approximately 30% goes through the renal system with 20% being eliminated in the urine within about 1.5 h.

If we want to be conservative, we could just say that for a 15-minute transport or nurse visit, a person would get just under 0.0015 mSv (0.055 mSv/h divided by 4).

Or, to use the data provided, we could say that, after 2 h, ¹⁸F has gone through one half-life and about 20% has been eliminated in the urine. That means the dose rate at 2 h has gone from 0.055 mSv/h at one meter to about 0.022 mSv/h at one meter assuming the patient is encouraged to void while still in the PET area. Then if the transport or a nurse visit takes 15 minutes, the personal dose would be about 0.005 mSv.

In the United States, the annual radiation exposure limit for the general public is 1 mSv. So then the question is how many patients does any individual transporter or nurse care for? How much time do they spend doing it and how close to the patient are they?

You could keep the patients longer—that would reduce the exposure to others by a factor related to half-life (e.g., 2 h more would decrease exposure by half). Transporters and nursing staff could be encouraged to limit their time around the patient for the first few hours. If you believe that these staff members are exposed quite routinely to many patients, some or all could become radiation workers and have their exposure monitored like your nuclear medical technicians.

Kelly Classic
Certified Medical Health Physicist