Preferred use of curie (ci) rather than becquerel (Bq) in nuclear medicine practice To the Editor In 1975, the ‘15 th Conférence Générale des Poids et Mesures’ adapted Becquerel as SI (The International System of Units, in French: Système international d'unités) unit of radioactivity through a resolution which stated - “…. by reason of the need to make as easy as possible the use of the units for non-specialists, taking into consideration also the grave risks of errors in therapeutic work, adopts the following special name for the SI unit of activity: becquerel, symbol Bq, equal to one reciprocal sec- ond” [1]. The authors of this letter with forty odd years of experience working with radioactive materials, isotope production and nuclear medicine feel that the purpose of adapting Bq unit for radioactivity did not achieve the objectives highlighted in the 1975 resolution as ratio- nalized below and hence suggesting the need for a review. 1. Did the use of Bq make it easy for non-specialists? A 70 kg human body contains approximately 0.0187 g of 4 °K emanating ~4.9 kBq or 0.132 μCi of radioactivity. Non-specialists would assume that a value expressed in “kilo” is large and “micro” is small. Indeed the activity present in human body is insignificant and giving its value in kilo is not rational. A radiation processing facility use high activity levels of 60 Co, typically a few megacuries for steri- lization of medical products. Comprehending a megacurie is easier than 37 petabecqurels (PBq) or 37,000 terabecqurels (TBq) to non- specialists. Nuclear waste is another area where the use of Bq has result- ed in practical difficulties. A recent NEA document assigns a value of 5 × 10 4 -5 × 10 5 TBq/m 3 for high level waste (HLW) [2]. TBq is an exponen- tial quantity (10 12 Bq) and the use of double exponential confuses even specialists. It is easy to comprehend 1.35–13.5 megacuries rather than 5 × 10 4 -5 × 10 5 TBq. Widely used international documents on isotope production continue to use the Ci unit while discussing the security of isotope supply in order to avoid confusion [3]. 2. Did the use of Ci introduce grave risk of errors in therapeutic work? Nuclear medicine practice started in late forties when Ci was the unit of radioactivity and despite the introduction of Bq in the seventies most nuclear medicine physicians continue to use Ci in routine practice. This is evident from the fact that recommended dose of 89 SrCl 2 for bone palliation approved by FDA in 1993 would have been 150 MBq instead of the prescribed 148 MBq. A recent literature example indicated use of 74–111 MBq (2–3 mCi) of activity, which would have been 75– 110 MBq, if the Bq unit had been prescribed [4]. The use of Ci unit in practice and subsequent conversion to the Bq unit is done by many to comply with journal requirements. Such conversion is a source of error in therapy while prescribing as well as in documenting. 3. Is re-evaluation of the use of radioactivity units in nuclear medicine needed? The use of the Curie unit as the standard for radioactivity was adapted by the Radium Standards Committee chaired by Earnest Rutherford in 1910 [5]. Marie Curie was responsible for making the standard as one gram, rather than 10 -8 g of radium, as had been insisted by many other committee members. Her argument was that an infinitesimally small quantity of anything was altogether inappropri- ate. The 1975 resolution adapting Bq did not take into consideration Marie Curie's logical argument of lack of practical significance for a small unit. One Bq or 27 pCi of radioactivity is an insignificant quantity in practice. The Bq unit does not fall under the category of the 7 ‘base quantities’ of SI. These base units (metre, kilogram, second, ampere, kelvin, mole, candela) are dimensionally large and have practical significance. Unlike the above base quantities, Bq is an insignificantly small quantity having very little practical significance. Moreover Bq cannot be considered as a fundamental unit as it assumes significance only when factors such as the type and energy of radiation and half-life are known. There is no direct method to measure Bq, since any measured value must be corrected for detector efficiency. The Ci unit is directly related to a fun- damental unit of mass. The goal of this letter is to initiate a discussion to evaluate the merits for continuing recommended use of Bq or if Ci should be the preferred unit, for practical convenience not only in nucle- ar medicine, but also wherever high radioactivity levels are discussed. Maroor Raghavan Ambikalmajan Pillai Molecular Group of Companies, Puthuvype, Ernakulam, Kerala, 682 508, India Furn F. (Russ) Knapp Jr. Medical Radioisotope Program, Nuclear Security and Isotope Division, Oak Ridge National Laboratory (ORNL), Oak Ridge, TN, 37830-6229 http://dx.doi.org/10.1016/j.nucmedbio.2016.10.003 References [1] The International Systems of Units (SI). NBS special publication 330. Published by U.S. Department of Commerce, National Bureau of Standards; 1977 p31. Nuclear Medicine and Biology 45 (2017) 51–52 0969-8051/© 2016 Elsevier Inc. All rights reserved. Contents lists available at ScienceDirect Nuclear Medicine and Biology journal homepage: www.elsevier.com/locate/nucmedbio