Estimated External Doses to Members of the Public from Patients with {sup 131}I Treatment - Paper 143
To protect individuals from the radiation emitted by a patient receiving {sup 131}I treatment a licensee must estimate the maximum likely effective dose to members of the public. Under current guidance by the United States Nuclear Regulatory Commission (NRC), a licensee may authorize the release of the patient if the effective dose is unlikely to exceed 5 mSv. NRC provides guidance to its licensees, in NRC Regulatory Guide 8.39, on acceptable methods that may be used to calculate the dose to a member of the public that may result from releasing the patient. To further assist licensees in implementing the regulation, NRC used conservative assumptions to estimate the activity of radionuclide in a patient, and the dose rate at 1 meter from the patient, that would be indicators of a potential dose to a member of the public of 5 mSv. Licensees may use these tables as operational criteria for releasing a patient if the conservative assumptions are considered acceptable. Licensees may also perform their own calculations, using case-specific assumptions, to show compliance with 5 mSv release criterion. The scope of this work is to provide dose coefficients for a variety of plausible public exposure scenarios that could be used by licensees to to show compliance. In the patient-specific calculations, the licensee is free to consider factors such as occupancy factor, effective half-life of the radionuclide in the patient's body, and tissue shielding. A conservative method of calculating the maximum likely dose to members of the public is provided in NRC Reg. Guide 8.39, which assumes no tissue attenuation of by the patient's body, no biological component that reduces the effective half-life of the radionuclide, and the patient's body burden is a point source. The assumption of no biological removal from the body introduces some conservatism into dose estimates for {sup 131}I. Also, it has been shown that the assumption of a point source with no tissue attenuation substantially overestimates the dose rate to nearby persons. The current method for calculating the maximum dose rate to members of the public from a nuclear medicine patient provided in US-NRC Regulatory Guide 8.39 assumes the patient's body burden is a point-source. Lack of consideration of tissue attenuation by the patient's body and biological removal of the radionuclide leads to a substantial overestimate of the actual external dose rate to a nearby member of the public. However, use of these assumptions in estimating potential dose to a member of the public simplifies the calculations considerably and, although generally conservative, the calculations serve as a useful screening tool. Many, if not most patients, could be released on that basis. If not, then refined calculations may be used, and the coefficients described in this paper provide one of the means for refining the calculations in cases where the screening calculations were not adequate. (authors)