MIRD formalism

The Medical Internal Radiation Dose (MIRD) formalism, developed by the MIRD Committee of the Society of Nuclear Medicine and Molecular Imaging, is the standard framework for calculating the absorbed dose to a target region of the body from a radionuclide distributed in one or more source regions. In its simplified organ-level form, the mean absorbed dose to a target region equals the time-integrated activity in a source region multiplied by an 'S value': D = A-tilde x S, where the S value (units of Gy per Bq-s, or a convenient multiple) is a radionuclide- and geometry-specific constant representing the absorbed dose rate per unit activity, computed from the energies and yields of all emitted radiations and the fraction of that energy absorbed in the target given the source-target anatomy (the absorbed fraction).

The other half of the calculation is the time-integrated activity (historically called cumulated activity), obtained by integrating the time-activity curve, the measured activity in a source region plotted against time after injection, over the treatment or imaging period; dividing by the administered activity gives the time-integrated activity coefficient (formerly 'residence time'). S values have been tabulated for many radionuclides and source-target organ pairs using standardised anthropomorphic reference phantoms (MIRD Pamphlets 11 and 21, and successor tools such as MIRDcalc), and voxel-level S values extend the same schema to patient-specific 3D dosimetry from SPECT/CT or PET/CT.

The MIRD schema underpins essentially all clinical and research dosimetry for radioligand therapy, including personalised activity prescription, and it is functionally similar to (and cross-validated against) the ICRP's own internal dosimetry system, both built on the same underlying physics of time-integrated activity and absorbed fraction.