[Radiometabolic therapy: dose equivalents from external exposure and internal contamination]

We describe our method for evaluating the dose received by external irradiation and internal contamination during radiometabolic therapy considering the daily activity at Ente Ospedaliero Ospedali Galliera, Genova, Italy.

We used environmental and personal thermoluminescence dosimeters (LiF-100) and a whole-body counter system (WBC 6000 Harshaw) for direct activity measurements within the body after radionuclides administration.

Measurements were carried out both on workers allowed to access the restricted area and on people volunteering to assist non-self-sufficient patients. In the latter, we estimated the doses from activity measurements and using ICRP's radiometabolic models: more definite access to the treatment room and higher doses were the main reasons for a more accurate evaluation of volunteers than of workers. The combined study of 1-meter exposure immediately before the patient's discharge and both environmental and personal dosimetry on volunteers showed about the same value (50 microSv/hr) as mean exposure rate at 1 meter. Internal intake in workers showed similar contamination dose values.

These results, combined with the average working time of individual workers, permitted an a priori evaluation for different professional categories; the highest estimated value (< 75 microSv/month) for both irradiation and internal intake, is in agreement with the results of direct controls on workers. Therefore, we concluded that WBC controls should be carried out to verify the estimated doses. Routine monitoring is not useful because, I-131 half-life being very short, frequent periodic evaluations would turn out too expensive. Finally, these measurements would be affected by major errors because atypical ways of contamination do not permit to apply the strict protocols used in radiometabolic models.