Isotopes may be referred to in the medical literature alone or as a component of a radiopharmaceutical administered for therapeutic or diagnostic purposes. The nomenclature for the isotopes incorporated in radiopharmaceuticals follows the international nonproprietary name (INN) drug nomenclature and therefore differs from that of isotopes that occur in elements alone.
An isotope referred to as an element rather than as part of the name of a chemical compound may be described at first mention by providing the name of the element spelled out followed by the isotope number in the same typeface and type size (no hyphen, subscript, or superscript is used). The element abbreviation may be listed in parentheses at first mention and used thereafter in the article, with the isotope number preceding the element symbol as a superscript.
Of the 13 known isotopes of iodine, only iodine 128 (128I) is not radioactive. The investigators used 128I to avoid the difficulty and expense of disposing of radioactive waste.
The symbol representing a single element should not be used as an abbreviation for a compound (eg, do not abbreviate the compound gallium citrate Ga 67 as 67Ga).
The nomenclature for the isotopes incorporated in radiopharmaceuticals follows US adopted name (USAN, the US nomenclature agency) or INN (the international nomenclature agency) style. The USP Dictionary of USAN and International Drug Names1 publishes USAN and INN, with USAN appearing in boldface type and INN in roman type. All USANs are reviewed by the INN Committee. The INN agent (if not intended to be marketed in the US) may not have a USAN. Almost all nomenclature is the same between the 2 groups. The single difference is stylistic. In INN style, the order is (1) name of the drug that contains the radioactive element, (2) isotope number, (3) element symbol, and (4) carrier name, if there is one. In USAN style, the order is (1) name of the drug that contains the radioactive element, (2) element symbol, (3) isotope number, and (4) carrier name, if there is one. In both, the isotope number appears as a superscript. The JAMA Network journals follow the INN style.
Because the nonproprietary name comprises all these components, the complete name should be provided at first mention unless the radiopharmaceuticals being referred to are a general category. Subsequently, a shorter term may be used, such as iodinated albumin or gallium scan.
Although the nonproprietary name for the radiopharmaceutical may appear to contain redundant information, maintaining consistent terms is important for clarity. For example, technetium Tc 99m is contained in more than 40 nonproprietary radiopharmaceuticals, from technetium Tc 99m albumin to technetium Tc 99m teboroxime.1 The isotope number appears in the same type (not superscript) as the rest of the drug name, and it is not preceded by a hyphen. A few commonly used drugs appear below. For drugs not listed here, consult the most recent edition of the USP Dictionary.1
cyanocobalamin Co 60
fibrinogen I 125
fludeoxyglucose F 18
indium In 111 altumomab pentetate
indium In 111 satumomab pendetide
iodohippurate sodium I 131
sodium iodide I 125
technetium Tc 99m sestamibi
Strontium chloride Sr 89 can be used to treat pain from skeletal metastases.
In an earlier study, 50 patients underwent lung imaging with technetium Tc 99m sulfur colloid.
The patient underwent an exercise stress test with injection of thallous chloride Tl 201 (thallium stress test).
Placement of seeds labeled with radioactive iodine I 125 (125I) did not interfere with lymphoscintigraphy or intraoperative identification of sentinel lymph nodes. The 125I seeds were used in 10 patients in this 12-patient study.
In a discussion that does not refer to administration of a specific drug, the more general term may be used.
For a patient recuperating from a myocardial infarction who wishes to begin an exercise program, a treadmill test with or without thallium imaging may be useful to determine whether the patient is at high risk for recurrent ischemia.
At the beginning of a sentence, the name rather than the element symbol should be used (even if the abbreviation has been previously used).
The patient was treated with sodium iodide I 131 after she was diagnosed with hyperthyroidism. Iodine 131 levels were then monitored by measuring the amount of radioactivity in the patient’s urine.
14.9.3 Radiopharmaceutical Compounds Without Approved Names.
Compounds may be combined with radioisotopes for research purposes. Such compounds would not receive an INN if no commercial use is intended. In lieu of an INN, standard chemical nomenclature should be followed (see 14.9.1, Elements, or consult the CRC Handbook of Chemistry and Physics2 for more information).
After first mention, the name of the substance can be abbreviated. Use the superscript form of the isotope number to the left of the element symbol. Enclose the isotope symbol in brackets and close up with the compound name if the nonradioactive isotope of the element is normally part of the compound.
glucose labeled with radioactive carbon (14C) [or glucose tagged with carbon 14]
[14C]glucose (not glucose C 14)
Use no brackets and separate the element and compound name with a hyphen if the compound does not normally contain the isotope element.
amikacin labeled with iodine 125
If uncertain as to whether the isotope element is normally part of a compound, consult the USP Dictionary1 for drugs and The Merck Index3 for other compounds.
14.9.4 Radiopharmaceutical Proprietary Names.
In proprietary names of radiopharmaceuticals, isotope numbers may appear in the same position as in the approved nonproprietary names, but they are usually joined to the rest of the name by a hyphen and are not necessarily preceded by the element symbol. Follow the USP Dictionary1 or the usage of individual manufacturers.
sodium iodide I 131
iothalamate sodium I 125
14.9.5 Uniform Labeling.
The abbreviation ul (for uniformly labeled) may be used without expansion in parentheses:
Similarly, terms such as carrier-free, no carrier added, and carrier added may be used. In general medical publications, these terms should be explained at first mention because not all readers will be familiar with them.
14.9.6 Hydrogen Isotopes.
Two isotopes of hydrogen have their own specific names, deuterium and tritium, which should be used instead of hydrogen 2 and hydrogen 3, respectively. In text, the specific names are also preferred to the symbols 2H or D (for deuterium, which is stable) and 3H (for tritium, which is radioactive). The 2 forms of heavy water, D2O and 3H2O, should be referred to by the approved nonproprietary names deuterium oxide and tritiated water, respectively.
14.9.7 Metastable Isotopes.
The abbreviation m, as in krypton Kr 81m or technetium Tc 99m, stands for metastable. The abbreviation should never be deleted because the term without the m designates a different radionuclide isomer.
Principal Author: Cheryl Iverson, MA
Thanks to Stephanie C. Shubat, MS, USAN Program, American Medical Association, Chicago, Illinois, for reviewing and providing comments.
1.USP Dictionary of USAN and International Drug Names. 53rd ed. US Pharmacopoeia; 2017.
2.Haynes WM, ed. CRC Handbook of Chemistry and Physics. 95th ed. CRC Press; 2014.
3.O’Neil MJ, ed. The Merck Index: An Encyclopedia of Chemicals, Drugs, & Biologicals. 15th ed. Royal Society of Chemistry Publishing; 2013.