TO THE EDITOR:
In a recently published article, “Metabolism of [13N]ammonia in rat lung,” Arthur Cooper and I briefly discussed the issue of units for reporting tracer concentration in tissue. Because I thought this would be of interest to JNM readers, I quote that discussion below (1).
2.6. Expression of Tracer Concentration
Expression of tracer concentration in tissue as “percent dose per g” (or per kg), which seems a natural choice to many workers, has an inherent disability: the artifact of dilution by body mass. If a tracer is similarly distributed in two individuals, one with twice the body mass of the other, all “percent dose per gram” values found in the larger individual will be half those found in the smaller one. This difficulty was recognized by the pioneer medical physicist, G. Failla, and, following his advice, Woodard and coworkers introduced a body mass-normalized unit (they called it the “differential absorption ratio”) for expressing 32P concentrations in excised tissues of cancer patients given a tracer dose before surgery (Kenney et al., 1941). This mode of expression failed to gain a significant following and, 30 years later, other investigators rediscovered the need to express tracer concentrations in mass-normalized units. Thus, Oldendorf et al. (1971) introduced “percent mean body concentration” and Oldendorf (1974) advocated its general use in a letter to the Journal of Nuclear Medicine. Rakusan and Rajhathy (1972) introduced “a new index… percentage of 86Rb uptake/relative organ weight (percentage of body weight)”. Blau (1975), in a letter to the Journal of Nuclear Medicine, pointed out that the improper use of tracer concentrations measured in dogs (which had been expressed in the artifact-prone “percent dose per g”), had caused investigators to greatly overestimate the tracer’s radiation dose to humans. In their letter to the Journal of Nuclear Medicine, Woodard et al. (1975), recalled Failla’s 1941 contribution, supported Oldendorf’s and Blau’s observations and proposed the name “relative concentration” for the mass-normalized unit. (As a coauthor (BRF) of that letter and this report remembers, the term was a compromise and not Woodard’s first choice.) Subsequently, the field of quantitative in vivo nuclear medicine experienced a similar “reinvention” of mass-normalized concentration units (“standardized uptake value,” “differential uptake ratio,” “dose absorption ratio” and the like), with little, if any, acknowledgment of their antecedents.
Our experience has been that the term “relative concentration” masks the unit’s universality and makes the concept appear vague, thereby limiting its use. In retrospect, we think that Oldendorf’s “percent mean body concentration,” modified to “ratio to mean body concentration” (or RMBC, which gives less cumbersome numerical values), has the advantages of clarity and specificity. We propose to use this nomenclature for reporting our 13N concentration data and recommend it for general use. Defined most simply, for any specimen of tissue (including whole organs), RMBC is the decay-corrected fraction of injected tracer recovered in a specimen divided by the fraction of body weight contained in that specimen. (This is the same as “tracer found per g of specimen divided by tracer injected per g of body weight,” where it is understood that for many radiotracers, quantitation requires correction for physical decay.) It is important to recognize that this unit is dimensionless. Also, it must be emphasized that this formulation is not limited to radiotracers. The tissue concentrations of any measurable substance introduced into the body (e.g., drugs) may be expressed in this form.