339. Radiometabolic treatment of hyperthyroidism after a patient specific dosimetric study: The radiation protection impact

Purpose Hyperthyroidism is a diffuse disease mainly treated with 131I-NaI radiometabolic therapy. Recently published data report that in Europe at least 15.000/y treatments are performed. As hyperthyroidism is a benign condition, the optimization principle stated in art.56 of 2013/59/Euratom Council Directive should be strongly applied and radiometabolic treatment should be performed with the minimum activity necessary to reach the clinical purpose minimizing the exposure of non-target tissues. Aim of the work was to evaluate the radioprotection impact of 131I therapies for hyperthyroidism after patient-specific dosimetric-studies, with respect to a standard administration of 600 MBq (max activity allowed in Italy for outpatient treatments). Methods 503 patients who solved hyperthyroidism within 1 year were considered (183 Graves, 383F, median age = 64y 17–89y]). Results Median 131I administered activity was 385 MBq [66–629 MBq] and only 29% of the patients had the maximum allowed activity. Total administered activity was 209 GBq; if the standard activity of 600 MBq had been administered to all patients total would have been 302 GBq, 45% greater than the really used one. This acts on the patient mean effective dose and on the mean absorbed dose to the stomach wall (critical organ for 131I-NaI/ICRP53) that were 5.8 Sv instead of 9.0 Sv and 177 mGy instead of 276 mGy respectively, with a maximum dose saving (9 times) for the patient who was given 66 MBq. The reduction of 131I administered activity also acts on the exposure of the nuclear medicine staff and of population: the mean dose rate at 1 m at the patient discharge time is 12 μSv/h instead of 20 μSv/h. This approach involves also the environment, as in the first 24 h 60% of administered activity is excreted. Conclusions The application of a patient specific pretreatment-dosimetric study can reduce of 45% the radioprotection impact of 131I treatments of hyperthyroidism with clinical results guaranteed and activity administered to the single patient can be decreased down to 9 times.