Radionuclide or Radioisotope? How Nuclear Medicine Actually Uses the Terms

Walk into any nuclear medicine department and you will hear “isotope” and “radionuclide” used as if they meant the same thing. Usually nobody is confused. But the two words are not synonyms, and the difference shows up in exactly the places nuclear medicine cares about most: generators, theranostic pairs and metastable states such as technetium-99m.

When we renamed the section on our homepage from Isotopes to Radionuclides, we checked the reasoning against how authors in The Journal of Nuclear Medicine (JNM) actually use the two words. This post sets out the physics, then what twelve JNM articles published between 2018 and 2026 show, and finally the convention we follow on XNucMed.

The physics: a species versus a relationship

IUPAC defines a nuclide as a species of atom characterised by its mass number, atomic number and nuclear energy state [1]. A radionuclide is simply a nuclide that is radioactive. The term describes one atomic species on its own: “¹⁸F is a radionuclide” needs no reference to any other atom.

Isotope is a relational term, a bit like “sibling”. Nuclides are isotopes of one another when they have the same atomic number Z (so they are the same element) but a different neutron number N. ¹²³I, ¹²⁴I and ¹³¹I are isotopes of iodine. ¹⁸F and ⁶⁸Ga are not isotopes of each other; they are two radionuclides of different elements.

That gives a simple rule of thumb:

• Use radionuclide as the default standalone noun: “Which radionuclide is on this tracer?”, “targeted radionuclide therapy”.
• Use isotope when you tie the atom to its element: “Iodine-131 is a β⁻-emitting isotope of iodine.”

Four cases where “isotope” does not fit

1. Cross-element comparisons. A department stocking ⁹⁹ᵐTc, ⁶⁸Ga and ¹⁷⁷Lu holds three radionuclides of three different elements, not “three isotopes”.
2. Nuclear isomers. ⁹⁹ᵐTc and ⁹⁹Tc both have Z = 43 and N = 56. What separates them is energy: ⁹⁹ᵐTc is a metastable excited state that decays by isomeric transition, emitting the 140 keV gamma photon behind most SPECT imaging. Because the definition of a nuclide includes the nuclear energy state, ⁹⁹ᵐTc and ⁹⁹Tc are distinct nuclides. “Isotope” cannot tell them apart.
3. Parent–daughter decay. In a generator, ⁶⁸Ge decays to ⁶⁸Ga and ⁹⁹Mo decays to ⁹⁹ᵐTc. Parent and daughter are different elements, so ⁶⁸Ge is the mother nuclide of ⁶⁸Ga, which is exactly the phrase used in JNM’s overview of radiometal chemistry [2].
4. Isobars. ¹³⁴Ce (Z = 58) decays by electron capture to ¹³⁴La (Z = 57). They share a mass number but not an element, so they are isobars, not isotopes. Bobba et al. accordingly describe them as a “radionuclide pair” [3].

Common medical nuclides at a glance

What the JNM literature shows

We read twelve JNM articles, from 2018 reviews to 2026 research papers. Their usage falls into five consistent patterns.

1. “Radionuclide” is the standard technical noun

• Lopes van den Broek et al. compared ⁸⁹Zr, ⁶⁴Cu and ¹²⁴I for brain immuno-PET and frame the whole study as one of radionuclide selection, from the title to the conclusion. “Radioisotope” does not appear in the paper at all [4]. With three different elements under comparison, radionuclide is the only correct choice.
• The OpenDose3D dosimetry software paper lists the radionuclides the software supports, thirteen of them including ¹⁷⁷Lu and ⁹⁰Y [5].
• Yang et al.’s review of α-emitter radiolabeling uses “α-emitting radionuclides” in its title [6].
• The OpenDose resource reports S values for 1,252 radionuclides in its abstract, although, as pattern 5 shows, not consistently [7].

2. “Isotope” is reserved for members of a single element

• Zimmermann’s review of copper counts five copper isotopes (⁶⁰Cu, ⁶¹Cu, ⁶²Cu, ⁶⁴Cu and ⁶⁷Cu), calls ⁶⁴Cu the most widely used radioisotope in this family, and describes theranostic pairs as isotopes of the same element. When the discussion widens beyond copper, the vocabulary switches to new radionuclides and the radionuclide armamentarium [8]. It is a textbook example of the formal distinction.
• Rodriguez et al. place ¹⁴⁹Tb in a theranostic “quadruplet” with the other terbium isotopes, ¹⁵²Tb, ¹⁵⁵Tb and ¹⁶¹Tb, and note that terbium offers four elementally matched theranostic nuclides [9].
• Yang et al. observe that the isotopes of astatine all have short half-lives, the longest being 8.1 h, and that none is stable [6]. Alpha emitters across the periodic table are radionuclides; the variants of element 85 are isotopes of At.
• Brandt et al. define an isotopic pair (or true pair) as different isotopes of the same element used for diagnosis and therapy, such as the β⁺ emitter ⁸⁶Y and the β⁻ emitter ⁹⁰Y, which are chemically identical [2].

3. “Nuclide” for decay chains and cross-element pairs

• Brandt et al. describe ⁶⁸Ga as produced by the decay of its long-lived mother nuclide ⁶⁸Ge, never a “mother isotope” [2].
• Bobba et al. call ¹³⁴Ce/¹³⁴La a radionuclide pair, which is physically exact for an isobaric parent–daughter couple [3].
• Chaple and Lapi’s title calls ⁴³,⁴⁴Sc, ⁵²Mn and ⁴⁵Ti “PET radionuclides”, and the authors note that each of these nuclides has unique imaging characteristics [10]. Three elements, so nuclide is the right word.

4. “Isotope” for production and supply

• Chaple and Lapi describe calcium or titanium targets as typically enriched for the desired isotope [10], which is correct, because enrichment concerns one isotope of the target element.
• Rodriguez et al. discuss the need to scale up isotope production as a challenge specific to each isotope [9].
• Even papers that otherwise say radionuclide switch to isotope for supply. Bobba et al. credit the U.S. Department of Energy Isotope Program for their ¹³⁴Ce [3], and Lopes van den Broek et al. thank the hospital department that supplied their ⁸⁹Zr isotope [4].

5. Interchangeable use, as idiom and prose variety

• Rodriguez et al. title their review “Emerging Therapeutic Isotopes”, although it covers lutetium, copper, terbium and several α emitters, which strictly makes them radionuclides. Inside, the heading on β-emitting radionuclides in clinical use is followed by one on emerging β-emitting isotopes, and ⁶⁷Cu is called both a reemerging theranostic radionuclide and an ideal radioisotope for theranostics [9].
• Salerno et al. use both words in a single sentence: “Among PET isotopes, ¹⁸F is the most commonly used PET radionuclide” [11].
• Meyer et al.’s title is “Tandem Isotope Therapy”, and the introduction calls ¹⁷⁷Lu the most commonly used therapeutic isotope, but the results count mice per radionuclide [12]. Idiom survives in titles; the quantitative sections revert to the precise term.
• Berens et al.’s abstract moves between radionuclide therapy, radioactive isotopes, radionuclides and radioisotopes within a few consecutive sentences [13].
• OpenDose calls the same 1,252 entries radionuclides in its abstract and radioisotopes in its results, and describes ICRP Publication 107 as providing data for 1,252 radioisotopes [7].
• Chaple and Lapi close by calling their nuclides useful additions to the PET isotope toolbox [10].

Summary of conventions

How we apply it on XNucMed

We follow the same convention as the careful parts of the literature:

• Radionuclides whenever we compare species across elements. The eight tiles on our homepage (¹⁷⁷Lu, ⁹⁰Y, ¹³¹I, ²²⁵Ac, ⁶⁸Ga, ¹⁸F, ⁶⁴Cu and ⁹⁹ᵐTc) span eight elements, and one of them is a nuclear isomer.
• Isotope for production, supply chains and same-element families. Hence our Isotope supply focus area, and phrases such as “medical isotope production”.
• The author’s own words when we quote or summarise a paper, even where they are loose.

It is a small distinction. But precise language makes the literature easier to read, and makes it easier to notice when a headline is being careless.

References

1. IUPAC. Nuclide. In: Compendium of Chemical Terminology (the “Gold Book”). goldbook.iupac.org/terms/view/N04257
2. Brandt M, Cardinale J, Aulsebrook ML, Gasser G, Mindt TL. An overview of PET radiochemistry, part 2: radiometals. J Nucl Med. 2018;59(10):1500–1506. doi:10.2967/jnumed.117.190801
3. Bobba KN, Wadhwa A, Bidkar AP, et al. Evaluation of ¹³⁴Ce/¹³⁴La-PSMA-617 for PET imaging and Auger electron therapy of prostate cancer. J Nucl Med. 2025;66(8):1252–1257. doi:10.2967/jnumed.125.269751
4. Lopes van den Broek S, Bratteby K, Aguilar X, et al. Radionuclide selection influences imaging outcomes in immuno-PET with a brain-penetrating anti–amyloid-β antibody. J Nucl Med. 2026;67(3):463–470. doi:10.2967/jnumed.125.271194
5. Fragoso-Negrín JA, Vergara-Gil A, Hakim AR, et al. OpenDose3D: a free, open-source clinical dosimetry software for patient-specific dosimetry. J Nucl Med. 2026;67(2):276–282. doi:10.2967/jnumed.125.269539
6. Yang H, Wilson JJ, Orvig C, et al. Harnessing α-emitting radionuclides for therapy: radiolabeling method review. J Nucl Med. 2022;63(1):5–13. doi:10.2967/jnumed.121.262687
7. Chauvin M, Borys D, Botta F, et al. OpenDose: open-access resource for nuclear medicine dosimetry. J Nucl Med. 2020;61(10):1514–1519. doi:10.2967/jnumed.119.240366
8. Zimmermann R. Are ⁶¹Cu, ⁶⁴Cu, and ⁶⁷Cu really happening? J Nucl Med. 2026;67(7):1032–1035. doi:10.2967/jnumed.126.272416
9. Rodriguez C, Liles GK, Lapi SE, Lewis JS. Expanding the theranostic toolbox: emerging therapeutic isotopes. J Nucl Med. 2026;67(suppl 2):5S–16S. doi:10.2967/jnumed.125.269487
10. Chaple IF, Lapi SE. Production and use of the first-row transition metal PET radionuclides ⁴³,⁴⁴Sc, ⁵²Mn, and ⁴⁵Ti. J Nucl Med. 2018;59(11):1655–1659. doi:10.2967/jnumed.118.213264
11. Salerno I, Dunn N, White H, et al. Quantitative comparison of SPECT and PET performance for clinical theranostic applications. J Nucl Med. 2026;67(3):423–428. doi:10.2967/jnumed.125.270987
12. Meyer C, Stuparu A, Lueckerath K, et al. Tandem isotope therapy with ²²⁵Ac- and ¹⁷⁷Lu-PSMA-617 in a murine model of prostate cancer. J Nucl Med. Published online October 5, 2023. doi:10.2967/jnumed.123.265433
13. Berens L, Meier J, Kao CM, Chen CT. Preliminary investigation of using TOPAS-nBio to simulate DNA double-strand breaks from volumetric radioisotope sources [abstract]. J Nucl Med. 2022;63(suppl 2):2816. jnm.snmjournals.org/content/63/supplement_2/2816