Background: Due to volatility of radioactive iodine solutions, usually internal exposure would occur in personnel that are working in nuclear medicine centers. The activities in the thyroid of individual s who work in nuclear medicine centers of North West provinces of Iran were measured using NaI (Sodium Iodide) detectors. In every center, nearly 40 - 500 mCi of ¹³¹ I and 50 - 600 μ Ci of ¹²⁵ I are used for diagnosis or treating patients, as well as monthly protein labeling.

Materials and Methods: A portable measurement instrument was made for this purpose. A collimator with thickness of 10 mm lead thickness of copper alloy with 2 mm was made to
focus the detector on thyroid gland and to reduce the background. Two NaI (Tl) detectors one with lower thickness for ¹²⁵ I and one with higher thickness for ¹³¹I were used for measurement. The goal of these measurements was to determine the activity of radioiodines in the thyroid gland of individuals in medical centers, and to give some advice such as more care at their working place and increasing the power of their venting system for reducing radioiodines exposure. The detectors had been calibrated for ¹²⁵I and ¹³¹I by a locally made ANSI/IAEA Plexiglass neck phantom (ANSI N44.3 1973).

Results: There were not any contamination and thyroid activities of ¹²⁵I in the thyroid glands of personnel. The activity of ¹³¹ I in the thyroid gland of individuals in all centers was lower than 0.4 kBq, and the activity of ¹³¹I in one center was as high as 3.4 kBq. The background of every place was determined and subtracted from each measurement.

Conclusion: The results indicate that more measurements should have been carried out in all nuclear medicine centers to determine the activity of individuals^' thyroid gland in Iran . In addition, the workers of those centers should try to reduce the intake and exposure to ¹³¹I and ¹²⁵I using more care at their working place and more efficient ventilator. Iran . J. Radiat. Res., 2004; 2 (3): 141-147