Assessment of Internal Dose Due to Occupational Exposure to Tc-99m by Using AIDE Software and Applying IDEAS Guidelines

Doses from intakes of radionuclides must be estimated from internal monitoring, such as thyroid counting or urinary excretion measurements. Those methods require application of appropriate biokinetic models and parameters, the identification of relevant measurement data and estimation of the uncertainties within that data, estimation of the time of intake, properties of the radioactive material inside the body. Because of the variety of parameters involved, the results of assessed internal doses depend on the skill and experience of the assessor. To check the quality of internal dose assessment methods, the methodology of European project IDEA (Internal Dosimetry – Enhancements in Application) was applied by using AIDE software (Activity and Internal Dose). AIDE meets the IDEAS approach for dealing with uncertainty on data, and contains all mathematical methods proposed by IDEAS for evaluating intakes For example, in vivo monitoring of 99mTc in thyroid was not recommended in ICRP Publication 78, thus, technetium deposited in thyroid cannot be interpreted by using the bioassay table from the ICRP 78. On the other side, the software AIDE allows performing calculation of internal doses and interpretation of intakes for any isotope, and gives the user flexibility to choose a non-default bioassay method that enables user to choose any bioassay method and interpretations of measurements. A special monitoring was carried out for workers in a nuclear medicine center to estimate the intake from inhalation of 99mTc. The software AIDE was used to calculate the intake and committed effective dose for those workers from measurements of 99mTc in thyroid. The results indicated that some workers were classified in Level 1 and Level 2 according to IDEAS guideline; consequently, a routine monitoring should be performed for those workers in order to estimate their internal contamination. (author)