Role of Reference Levels in Nuclear Medicine: A Report of the SNMMI Dose Optimization Task Force.

The concept of reference levels has had a long history in diagnostic imaging (1). In general, reference levels provide guidance regarding appropriate or conventional levels of radiation dose to be delivered to patients. In Europe, the concept has origins in the 1950s with x-ray examination surveys in the United Kingdom (2). In the United States, the use of reference levels began with “Nationwide Evaluation of X-Ray Trends,” a 1974–1981 survey (3). Since then, concepts have been more formalized in the international community by publications from the International Commission on Radiological Protection (ICRP) (4–7) and in the United States by the National Council on Radiation Protection and Measurements (NCRP) (8). With increased awareness of the potential risks from ionizing radiation, numerous recent publications have been addressing and suggesting reference levels. These publications offer some differing definitions of reference levels and raise some philosophic questions about their origin, purpose, and appropriate use (9–14). In this article, we discuss some considerations for application of reference levels to nuclear medicine in response to the ICRP Committee 3 recommendations encouraging “authorized bodies to set diagnostic reference levels that best meet their specific needs and that are consistent for the regional, national, or local area to which they apply” (7). We will focus on two widely accepted concepts regarding reference levels: diagnostic reference levels (DRLs) and achievable doses (ADs). DRLs are set at the 75th percentile of radiation doses for examinations (5) and provide an investigational level to help identify unusually high doses. ADs are set at the median (50th percentile) of radiation doses for examinations (8) and are intended to identify common practice. When DRLs and ADs are established, the distribution of radiation doses is typically determined by a survey of clinical sites based on protocol reviews, actual patient data, or phantom experiments. Here, we will refer to both DRLs and ADs as reference levels. This terminology is not to be confused with the term reference level as used by some publications to describe interventional radiologic examinations, or diagnostic reference level as used to describe diagnostic examinations (8). Along with a growing consensus on the definition of DRL and AD, there is also general agreement that these should not be interpreted as absolute measures of appropriate use of medical radiation (5,9). Reference levels should be used only to supplement, not replace, professional judgment and do not provide a dividing line between good and bad medicine. They are not intended for regulatory or commercial purposes or to establish legal standards of care. Even with these agreed constraints on the role of reference levels in diagnostic imaging, several philosophic and practical questions remain open about them. Should all clinics follow the same national reference levels? Is there some leeway within which a clinic would be considered compliant with the reference levels? What is the appropriate source for setting reference levels? Are reference levels set solely for radiation protection, or should they also guide appropriate image quality? How can clinics use reference levels for protocol optimization and ultimately improved patient care?