FETAL DOSE ESTIMATES AND THE ICRP ABDOMINAL DOSE LIMIT FOR OCCUPATIONAL EXPOSURE OF PREGNANT STAFF TO TC-99M AND I-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 or der 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 Tc-99m patient were estimated to vary from 86 mu Sv to 1.6 mu Sv, 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 mu Sv to 0.17 mu Sv for three types of Tc-99m 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 mu Sv and 9.0 mu Sv for a whole-body scan of a thyroid cancer patient after I-131 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 Tc-99m or I-131, 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 Tc-99m studies or one I-131 whole-body scan per day, and may have to wear a more sensitive personal dosimeter than a film badge.