Levels of 131I Activity in Patients to Enable Hospital Discharge, Based on External Exposure of Family Members of the Patient in Japan

The activity levels for discharge from hospital of patients treated with 131I in Japan are described. The level was determined by considering the public annual dose limit in Japan of 1 mSv and the possible exposure of family members with whom the patients were in daily contact. All analyses are based on a partner, where both the partner and the children will be subject to exposure. The external exposure was calculated by using the data on the distance between the patient and family members, the length of the time spent at each distance, and each member's age. It was assumed that the mother must continue to take responsibility for care of children during the period that she is radioactive. The conclusions of this work are that a maximum residual 131I radioactivity of <97 MBq, with a distance from the patient in bed greater than 50 cm and the ages of patient's children all over 1 year, will be required to ensure that the maximum dose to any contact will not exceed 1 mSv. More than 50 cm distance suggests either an unmarried housewife or the partner sleeping in an adjacent room or twin beds. No restriction is needed in respect of external exposure for patients with residual radioactivity of 42 MBq, although levels lower than this may be required where there is a risk of contamination, e.g. due to urinary incontinence. This conflicts with recommended activity levels for discharge (ICRP 25) of 560 MBq (for a dose limit of 5 mSv). The findings of this work are only directly relevant for the age range under 55 years. No account was taken in this paper of internal exposure from internal pathways. (author)