Fetal dose estimates and the ICRP abdominal dose limit for occupational exposure of pregnant staff to technetium-99m and iodine-131 patients.

The International Commission on Radiological Protection has recently recommended a supplementary dose limit of 2 mSv to the abdominal surface of a pregnant member of staff in order to provide protection to her fetus comparable to that in members of the public, whose annual limit is recommended to be 1 mSv. In order to determine whether this apparent attenuation factor of 50% is appropriate for nursing and imaging staff exposed to nuclear medicine patients, estimates were made of the ratios of the maternal abdominal surface to fetal dose appropriately weighted for time, distance and dose rate. Thermoluminescent dosimeter (TLD) measurements were made at various depths in an anthropomorphic phantom irradiated at different distances by a distributed source of either technetium-99m or iodine-131 in order to determine the corresponding attenuation factors at the average fetal midline depth. Dose estimates were based on these factors and on published values of dose rate and exposure times for nursing and imaging staff at these distances from the patient. Fetal doses to nursing staff caring for an adult 99mTc patient were estimated to vary from 86 microSv to 1.6 microSv, with the corresponding ratio of the abdominal surface to fetal dose varying from about 1.8:1 to 1.5:1 as the patient became less dependent on nursing care and the mean distance from the patient increased. Fetal doses to imaging staff varied from 1.12 microSv to 0.17 microSv for three types of 99mTc scan, but the ratio only varied from 1.4:1 to 1.3:1. Fetal doses to imaging staff were estimated to be 6.7 microSv and 9.0 microSv for a whole-body scan of a thyroid cancer patient after 131I ablation and therapy respectively, and the ratio was 1.3:1 for both types of scan. It was concluded that for a pregnant ward nurse or imaging technologist exposed to an adult or paediatric patient administered 99mTc or 131I, a dose limit of 1.3 mSv to the maternal abdominal surface will restrict their fetal dose to 1 mSv. A pregnant imaging technologist should perform no more than six adult 99mTc studies or one 131I whole-body scan per day, and may have to wear a more sensitive personal dosimeter than a film badge.