A study of a calibration technique for a newly developed thyroid monitor and its uncertainties due to body size for radioiodine measurements

Uncertainties associated with direct thyroid measurements using an NaI(Tl) survey meter: the effect of anatomical differences among individuals

Photon detectors utilised for direct thyroid measurements to determine the radioiodine content in the thyroid are normally calibrated using a phantom that mimics the human neck, including the thyroid. However, such measurements are necessarily subject to uncertainty because of the difference between the phantom and the real human subject and also the morphological differences among individuals. In this study, personal voxel phantoms were created from magnetic resonance images of the necks of 24 adult volunteers (15 males and 9 females), and numerical simulations using these phantoms were performed to examine the variation in a conversion factor (131I kBq in the thyroid per μSv h-1) for a conventional NaI(Tl) survey meter among the individuals and also to confirm the suitability of reference Japanese voxel phantoms as a calibration standard for such measurements. As a result, it was found that the conversion factor obtained from the reference Japanese male (female) phantom was 1.29 (1.18) times larger than the average conversion factor for the male (female) subjects, suggesting that the conversion factors of the reference Japanese adult male and female phantoms would generally overestimate the 131I thyroidal contents and that the thyroid volume would be one of the factors influencing the conversion factor. This study also revealed a wide difference in the thyroid volume among individuals, which would be of concern when performing specific dose assessments for heavily exposed persons.

Thyroid dose assessments due to inhalation of 131I for nuclear medicine workers

Background

In general, medical staff who work in nuclear medicine should be entirely safe in their professional environment. Nevertheless, we already know that the working environment of the nuclear medicine staff is not completely safe due to the handling of high amounts of radionuclides for diagnostic and therapy applications, which is especially relevant for ¹³¹I (as a non-sealed source).

Purpose

The goal of this study was to assess the inhaled ¹³¹I thyroid dose in nuclear medicine workers and to introduce a simple method for internal exposure monitoring.

Methods

Using 2-IN^*2-in NaI (Tl) scintillation spectrometer and its supporting software (InSpector Maintenance Utility and Genie 2000), from 2019 to 2021, internal thyroid irradiation monitoring, an internal thyroid irradiation monitoring investi A NaI (Tl) scintillation spectrometer and its sugation was carried out for 3 consecutive years, between 2019 and 2021, in staff members of nuclear medicine departments engaged with iodine therapy.

Results

¹³¹I activity was found highest in the thyroid of nuclear medicine workers involved with the manual packaging and delivery of the radioisotope, while it was not detected in staff members involved with the automatic packaging and drug delivery. The activity range was found to be 30.00 ± 6.60-6070.00 ± 1335.40 Bq for the exposed personnel, and the estimated dose was 0.05-6.77 mSv. In 2021, three workers had an annual equivalent dose above 5 mSv.

Conclusion

By monitoring the thyroid ¹³¹I in staff members of the nuclear medicine department, it was found that there are ¹³¹I internal occupational exposure risks. The best solution is automatic packaging and drug delivery.