TABLE 1.

Terminology and Nomenclature Used in MIRD Schema (1–3)

			Unit^*
Parameter	Symbol	Definition	SI	Older	Comment
Administered activity	$\text{[math]}$	Activity of radiopharmaceutical administered to patient	MBq	mCi	Base SI unit of activity is becquerel (≡1 disintegration per second [dps]); older unit of activity, curie, corresponds to 3 × 10¹⁰ dps; 37 MBq equals 1 mCi and 37 kBq equals 1 μCi; it is important to distinguish activity administered (often mistakenly referred to as dose) from actual radiation dose
Source region	$\text{[math]}$	Region (such as tissue or organ) containing radionuclide and within which radiation is emitted as radionuclide undergoes decay	Not applicable	Not applicable
Target region	$\text{[math]}$	Region (such as tissue or organ) within which energy of emitted radiations is deposited	Not applicable	Not applicable	For systemically administered and distributed radiopharmaceutical, every tissue or organ is both source and target region
Time	t	Time after administration of radiopharmaceutical	h	h	Time of administration of radiopharmaceutical is designated as time zero, that is, as t = 0
Activity	$\text{[math]}$	Activity in source region $\text{[math]}$ at time t	MBq	mCi	Plot of activity in source region $\text{[math]}$ vs. time t is source-region time–activity curve
Time-integrated activity	$\text{[math]}$	Number of radioactive decays in source region $\text{[math]}$ from time t = 0 to time t = τ	MBq-s	μCi-h	$\text{[math]}$ ; integration time τ is commonly set to infinity, yielding total number of decays in source region $\text{[math]}$ for complete decay of administered radionuclide; time-integrated activity was originally called cumulated activity
Time-integrated activity coefficient^†	$\text{[math]}$	Number of radioactive decays in source region $\text{[math]}$ from time t = 0 to time t = τ per unit administered activity	MBq-s MBq⁻¹	μCi-h mCi⁻¹	$\text{[math]}$ ; time-integrated activity coefficient was originally called residence time
Absorbed dose	$\text{[math]}$	Absorbed dose to target region $\text{[math]}$ from time t = 0 to time t = τ	Gy	rad	Integration time τ is commonly set to infinity, yielding total absorbed dose to target region $\text{[math]}$ for complete decay of administered radionuclide
Absorbed dose coefficient	$\text{[math]}$	Absorbed dose to target region $\text{[math]}$ from time t = 0 to time t = τ per unit administered activity	Gy MBq⁻¹	rad mCi⁻¹	$\text{[math]}$
Mass	$\text{[math]}$	Mass of source region $\text{[math]}$ and target region $\text{[math]}$ , respectively	g	g
Energy	$\text{[math]}$	Energy (or mean energy) per particle or photon of ith radiation emitted by administered radionuclide	MeV	MeV
Number of radiations per decay	$\text{[math]}$	Number of ith radiation emitted per radioactive decay for administered radionuclide	Not applicable	Not applicable
Energy per decay	$\text{[math]}$	Energy (or mean energy) of ith radiation emitted by administered radionuclide per radioactive decay	MeV	MeV	$\text{[math]}$
Absorbed fraction	$\text{[math]}$	Absorbed fraction, that is, fraction of energy $\text{[math]}$ of ith radiation emitted within source region $\text{[math]}$ that is absorbed in target region $\text{[math]}$	Not applicable	Not applicable
Specific absorbed fraction	$\text{[math]}$	Specific absorbed fraction, that is, fraction of energy $\text{[math]}$ of ith radiation emitted within source region $\text{[math]}$ that is absorbed in target region $\text{[math]}$ per unit mass of target region $\text{[math]}$	g⁻¹	g⁻¹	$\text{[math]}$
S value	$\text{[math]}$	Absorbed dose to target region $\text{[math]}$ per radioactive decay in target region r	mGy MBq⁻¹ s⁻¹	rad μCi⁻¹ h⁻¹	$\text{[math]}$

↵* Although older units are presented for completeness, SI units should be used exclusively.
↵† In modern MIRD-schema nomenclature (1), dosimetric quantity normalized to administered activity is identified by term coefficient appended to quantity name.