Reports in Thyroid Research

Aim: Aim of this study was to evaluate five years of incorporation monitoring of the nursing staff of an ¹³¹I therapy ward. The annual effective dose caused by incorporation of ¹³¹I of this profession was compared to the dose limits defined in the German incorporation guideline. Furthermore, the ¹³¹I incorporation factor stated in the incorporation guideline was verified using the obtained data.
Methods: The mandatory calculation of the potential possible annual effective dose due to incorporated ¹³¹I provides values considerably exceeding the dose limit of 1 mSv/a to the staff of the observed ¹³¹I therapy ward. To determine the actual work related incorporation, the ¹³¹I activity concentration in 143 urine samples (collection over 24 h) of 14 employees of the nursing staff (subdivided into 1^st shift: 07:00–15:00; 2^nd shift: 15:00–21:00 and 3^rd shift: 21:00–07:00) were examined over a period of five years. The median time of exposure prior to sampling was 5.1 days. ¹³¹I was administered to patients mainly as capsules. Preparation was done in a fume hood.
Results: The measured activity concentration in urine samples was related to the individual time of exposure. A constant activity supply for at least three days was assumed. The mean annual effective doses were 0.16 mSv/a for the 1^st shift (n=55), 0.12 mSv/a for the 2^nd shift (n=48) and 0.02 mSv/a for the 3^rd shift (n=40). Therefore all determined annual effective doses were well below the dose limits of the German incorporation guideline. In line with a theoretic “worst case” calculation based on the highest measured individual value of each subgroup a maximum annual effective dose was calculated (1.14 mSv/a for the 1^st shift, 1.10 mSv/a for the 2^nd shift and 0.23 mSv/a for the 3^rd shift). The calculated mean incorporation factors ranged from 1.9 × 10^-8 for the 2^nd shift to 2.5 × 10^-9 for the 3^rd shift related to the whole activity administered to the patients during the observation time, and was therefore well below the ¹³¹I incorporation factor assumed in the guideline.
Conclusions: The ¹³¹I incorporation factor stated in the German incorporation guideline of 10^-6 is disproportionally high. Following the evaluated urine samples, the ¹³¹I incorporation factor can be reduced by a factor of 10 to 10^-7 for employees working on an ¹³¹I therapy ward independent of their profession. This will in most cases lead to more accurate calculations and may avoid time and cost intensive incorporation monitoring by the radiation protection authorities.