Answer to Question #453 Submitted to "Ask the Experts"

Q

There is increasing excitement about positron emission tomography (PET) scanning using ¹⁸F-FDG (fluorodeoxyglucose) to detect early stages of cancer. Assume that there are two qualified nuclear medicine techs. One is called the "5mCi tech" and the other is called the "10 mCi tech," and they can only inject their own patients. Question:

1. How much radiation exposure does the "5 mCi tech" personally receive when he injects a single 5 mCi dose of ¹⁸F-FDG? What is the maximum number of 5 mCi dose patients that the "5 mCi tech" can inject in a single day, single year, and lifetime (assuming that he has an unlimited number of patients) without exceeding his total occupational radiation dose exposure limits?
    
2. How much radiation exposure does the "10 mCi tech" personally receive when he injects a single 10 mCi dose of ¹⁸F-FDG? What is the maximum number of 10 mCi dose patients that the "10 mCi tech" can inject in a single day, single year, and lifetime (assuming that he has an unlimited number of patients) without exceeding his total occupational radiation dose exposure limits?
    
3. What is the most effective way to reduce this potential excess exposure including, but not limited to the customary approaches (i.e., time, distance, shielding, hire more techs and rotate them through the center)?

A

In May 2000, the European Journal of Nuclear Medicine published an article, "Radiation dose rates from patients undergoing PET: implications for technologists and waiting areas," by N.A. Benatar, B.F. Cronin, and M.J. O'Doherty. The study includes data based upon measurements of 564 patients, including 551 1¹⁸F FDG (fluorodeoxyglucose) injections, 21 ¹³N NH₃ injections, 35 ¹¹C injections, and ¹⁸F fluoride injections. (The routinely injected activities were 250 MBq for FDG limited views, 350 MBq for FDG whole/half body scans, 550 MBq of ¹³N ammonia, 370 MBq of ¹¹C methionine for brain scans, 740 MBq of ¹¹C methionine for other than brain scans, 370 MBq of ¹¹C flumazenil for brain scans, and 222 MBq of ¹⁸F fluoride.)

According to this article, on average, each technologist (four were included in the study) administered 831 MBq per day. The mean whole-body dose per MBq injected was 0.02 microsieverts/MBq.

Furthermore, according to this article, the average time of close contact (<2.0 m) with a radioactive source per day was 32 minutes. The average effective dose per minute close contact was 0.5 microsieverts/min which resulted in a mean daily effective dose of 14.4 microsieverts.

At the hospital where I work, we are providing coincidence FDG scans. Five out of 12 technologists are currently trained to do these studies. After completing 80 positron emission tomography (PET) patients (and over 5,000 conventional nuclear medicine patients), the average whole-body doses of these techs working with FDG are no higher than the other techs; their average extremity doses are even a bit lower.

In conventional nuclear medicine, the highest percentage of dose to the staff is from imaging (rather than preparation and assay of the radiopharmaceuticals or injection). Our ALARA program for the PET studies includes (1) purchasing unit doses (rather than multidose vials) from our distributor, (2) drawing the dose in a fume hood rated for >¹³¹I (with additional lead bricks), (3) using a spinal needle to draw the dose from the vial (rather than removing the vial from the transport pig), (4) using tungsten syringe shields, (5) using a 1/4-inch lead carrier to transport the dose from the Hot Lab to the patient, and (6) using a one-inch lead brachytherapy bedside shield between the technologist and the patient.

Carmine Plott, CHP, Ph.D.