Release criteria and other radiation safety considerations for radionuclide therapy

Abstract

Historically, nuclear medicine has been largely a diagnostic specialty, utilizing relatively low administered activities to obtain important diagnostic information whose benefits far outweigh the small potential risk associated with the attendant low normal-tissue radiation doses to the patient. Doses and risks to members of the patient's household and other individuals encountering the patient are, of course, far lower-to the point that medical confinement of and other regulatory restrictions on diagnostic nuclear medicine patients are entirely unnecessary. However, by incorporation of appropriate radionuclides in appropriately large amounts into target tissue-avid radiopharmaceuticals, a sufficiently high radiation dose may be delivered to produce a therapeutic response in tumor or other target tissues. And radionuclide therapy-most notably, radioiodine treatment of thyroid diseases such as hyperthyroidism and differentiated thyroid cancer-has long proven to be effective and safe for patients and for individuals around the patient. With the approval of the Texas State Department of Health, for example, Allen and Zelenski prospectively treated 430 home-bound outpatients over 30 years with 30-400 mCi of iodine-131 and reported that there was no demonstrable health hazard to family members or the general public [1]. Nonetheless, concerns persist regarding stochastic radiogenic risks (i.e., carcinogenesis and germ cell mutagenesis) to individuals incidentally irradiated by radionuclide-treated patients. Such concerns have led governmental authorities worldwide to establish regulatory criteria for the release of radionuclide therapy patients from medical confinement, until 1997 1,110 MBq (30 mCi) of iodine-131 ( 131 I) in the United States but as low as 74 MBq (2 mCi) in some European countries [2-7]. To optimize clinical efficacy, cost-effectiveness, and accessibility to 131 I and other radionuclide therapies and their benefits, such regulations must be based on sound dosimetric and radiobiologic principles and available relevant data. In the 1990s, major regulatory changes were implemented in the United States by the Nuclear Regulatory Commission (NRC) regarding release from medical confinement of patients who have received therapeutic amounts of radioactivity [6, 7]. Most notably, release may now be based on the projected effective dose equivalent to individuals exposed to radioactive patients rather than retained activity, thus allowing consideration of patient-specific kinetic and dosimetric data and other patient-specific factors.