Radiation Dose to Technologists per Nuclear Medicine Examination and Estimation of Annual Dose

MATERIALS AND METHODS

Previous studies have demonstrated that the external ionizing radiation exposure of nuclear medicine technologists arises primarily from radioactive patients rather than from the preparation and injection of radiopharmaceuticals (4). Therefore, we focused on the deep-dose equivalent to technologists. Eight technologists with more than 5 y of experience in general nuclear medicine were instructed to spend as little time near the patient as possible. In Cerrahpasa Nuclear Medicine Laboratory, there were 2 technologists responsible for performing the ^99mTc-MDP whole-body bone scans, ^99mTc-DMSA renal scans, and ^99mTc-pertechnetate thyroid scans considered in this work. Also, 2 technologists were responsible for both ^99mTc-MIBI and ²⁰¹Tl myocardial perfusion scans throughout the year. Each of the 2 technologists responsible for a particular procedure performed the same number of diagnostic procedures in a day and did not participate in any other diagnostic procedures. Each of the 2 technologists performed the same procedure. The patients who participated in this study were selected randomly.

Radiation doses to technologists were estimated by measuring the time interval and dose rate at various distances (0.25, 0.50, 1.0, and 2 m) from the patient, with and without a 2-mm lead shield. During data acquisition, the lead shield was 1 m from the patient and technologist for all measurements. In these measurements, the distance of the technologist from the patient was about 25 cm during patient positioning on the scanner bed. In some cases, patients positioned themselves after receiving instructions. The technologists stood on a mark (0.5 or 1 m from the scanner bed) while they were speaking with patients about the procedure. Operators carefully measured the mean time spent by technologists at these distances and, using a portable Geiger-Müller detector (Eberline Smart Portable 2) in rate meter operating mode after patient positioning, measured the dose rates at these distances. By regulation, the Turkish Atomic Energy Authority mandates annual calibration of the exposure rate of portable radiation monitoring instruments in Turkey. Therefore, this detector was calibrated by the Turkish Atomic Energy Authority. Moreover, the detector was tested with a point source of ^99mTc and ²⁰¹Tl. In this test, the relative SD on the measurements was 9.65% and 9.94% for ^99mTc and ²⁰¹Tl, respectively. These relative SDs were considered in calculations.

In all, 71 patients were measured, for a total of forty-eight ^99mTc-MIBI and ²⁰¹Tl myocardial perfusion scans (stress + rest), twenty ^99mTc-MDP whole-body bone scans, twelve ^99mTc-pertechnetate thyroid scans, and fifteen ^99mTc-DMSA renal scans.

The measurements were obtained as mR/h using the Geiger-Müller detector and were converted to μSv/h using the equation mR/h = 10 μSv/h. Because the selected unit of time was seconds, the equation mR/h = (10/3,600) μSv/s was used. Time (seconds) and relative dose rates were multiplied by each other. Finally, mean external doses to technologists and their SDs for 3 scenarios were calculated. These were the doses to technologists arising from positioning patients and the total doses to technicians arising from diagnostic procedures both with and without lead shields.

For ^99mTc-pertechnetate thyroid scans, the patients were imaged using a γ-camera after the intravenous injection of 75–370 MBq (2–10 mCi) of ^99mTc-pertechnate. Imaging began 15–30 min after the injection.

For ^99mTc-MDP whole-body bone scans, the patients were imaged 2–3 h after the intravenous injection of 740 MBq (20 mCi) of ^99mTc-MDP. Standard anterior and posterior whole-body images were obtained on a dual-head planar γ-camera.

The procedure for myocardial perfusion scanning with ^99mTc-MIBI had 2 stages. The first involved stress imaging after injection of 555 MBq–1.11 GBq (15–30 mCi) of ^99mTc-MIBI at peak exercise. Gated SPECT was performed from 15 min to 2 h after the injection, preferably within 15–30 min. The second stage involved rest imaging after administration of 555 MBq–1.11 GBq (15–30 mCi) of ^99mTc-MIBI. Gated SPECT was performed within 45–60 min after the injection.

Likewise, the procedure for myocardial perfusion scanning with ²⁰¹Tl-chloride had 2 stages. The first involved stress imaging after injection of 148 MBq (4 mCi) of ²⁰¹Tl at peak exercise. Imaging began within 5–7 min after the injection. The second stage, redistribution imaging, involved administering 37–55.5 MBq (1.0–1.5 mCi) of ²⁰¹Tl and imaging 3–4 h after the first injection.

For ^99mTc-DMSA renal scanning, the administered activity was 185 MBq (5 mCi) for adults. In most cases, imaging could be performed 1–3 h after the injection.