RADIATION-DOSE RATES FROM ADULT PATIENTS RECEIVING I-131 THERAPY FOR THYROTOXICOSIS

Recommendations for restricting the exposure to radiation of members of the public coming into contact with thyrotoxic patients treated with I-131 are currently based on the activity retained by the patient, and not on the doses likely to be received by such individuals. In order to examine whether these current recommendations restrict these doses to less than the current annual limit of 5 mSv, and to identify the implications of a reduction in this limit to 1 mSv, measurements were made of the dose rates at distances of 0.1, 0.5 and 1.0 m from 60 patients just before they left the nuclear medicine department. These measurements were repeated 1, 3, 6, 8 and 10 days after administration for 30 patients, and the radioactivity in samples of saliva taken on each of these days and secreted in sweat over the first 24 h were also measured. Doses were estimated for administered activities of approximately 200-600 MBq, assuming appropriate values for the times and distances spent near other individuals while travelling, at work, at home and near to young children considered in three age groups (<2, 2-5 and 5-11 years). Periods of restriction were derived which would reduce these doses to 5 or 1 mSv. For a dose limit of 5 mSv, there is no need to restrict private travel, public transport journeys can last up to 7 h, a patient can return to work immediately, but sleeping with a partner will have to be restricted even for the minimum activity of 200 MBq. For a dose limit of 1 mSv, public transport journeys should not exceed 1 h for an administered activity of 600 MBq or 3.5 h for 200 MBq, and return to work should be delayed if the administered activity exceeds 300 MBq. The periods to restrict close contact with young children in order to limit their dose to 1 mSv were verv similar to the current recommendations based on retained activity.