Dose estimation to the public and biota resulting from the use of 99mTc and 131I in nuclear medicine in the city of Rio de Janeiro

In Brazil there are around 340 Nuclear Medicine Services (NMS), 30 of them, are located in the city of Rio de Janeiro. The most widely used radionuclides in the country are 131I and 99mTc, but among them, 99mTc is the most concern, because it has a short half-life (6 hours) and decays to 99Tc, which is a beta emitter with a long half-life (214,000 years). It is estimated that 80% of radiopharmaceuticals administered to the patients are excreted still on the NMS, and then discharged to the city sewer system. In Rio de Janeiro the Alegria Station receives most of the sewage from the NMS, and after passing through all stages of treatment, the effluent is discharged into the Canal do Cunha and flows into the Guanabara Bay. The aim of this study is to perform a survey to estimate the 131I and 99Tc discharged levels at the Canal do Cunha and evaluate the radiological impact to non-human biota and to the populations exposed to these radionuclides. It was estimated based on data supplied by the national regulatory body that the treatment plant receives approximately 1014 Bq of 99Tc and 1012 Bq of 131I per year. Considering a conservative approach, in which all the radionuclides received by the station are released from it, the activity concentration of 131I at the discharge point (DP) is around 63 Bq/L in river water and 3 Bq/kg in the bottom sediments. The estimated activity concentration for 99Tc is 32 Bq/L in water and 6 Bq/kg in the sediment. For the community located downstream at 1200 meters from the DP, the activity concentration values in water and sediments are 18 Bq/L and 1 Bq/kg for 131I and 9 Bq/L and 2 Bq/kg for 99Tc, respectively. The annual effective dose for adult bathers near the DP is estimated to be 10-3 mSv/y for 131I and 10-7 mSv/y for 99Tc and, for local swimmers and fish consumers living along the Canal do Cunha, the annual effective dose is estimated to be 10-2 mSv/y for 131I and 10-3 mSv/y for 99Tc. For biota, the zooplankton showed the highest dose, 6μGy/h. Therefore, according to the results, the radiopharmaceuticals discharge at the Canal do Cunha offer no significant risk to human as well as to non-human biota. (author)