Time-activity curve (TAC)

A time-activity curve (TAC) plots measured radioactivity (or activity concentration) in a region of interest, an organ, a tumour, blood, or the whole body, against time after a radiopharmaceutical is administered. It is generated from a series of quantitative images (dynamic PET frames, or sequential SPECT/CT or planar scans taken at several time points after a therapeutic injection) or from direct sampling such as blood or excreta counting.

The shape of the TAC reflects both the physical decay of the radionuclide and the biological uptake, retention and clearance of the labelled compound in that tissue; separating these two contributions is why dosimetry needs the effective half-life rather than the physical half-life alone. Integrating the area under a TAC over the intended dose-integration period gives the time-integrated activity (historically 'cumulated activity'), and dividing that by the administered activity gives the time-integrated activity coefficient, the direct input to the MIRD dose equation. Because TAC shape is usually inferred from only a handful of imaging time points (commonly three to four samples spanning a few hours to about a week after 177Lu therapy) and then fitted with an exponential or other analytical function, the number, timing and spacing of these samples, and the choice of fitting model, are recognised as major, site-dependent sources of variability in radioligand-therapy dosimetry.

Accurate TAC measurement matters clinically because both under- and over-sampling can bias the estimated absorbed dose to kidneys, marrow or tumour, which in turn affects dosimetry-guided activity prescription and estimates of nephrotoxicity or haematotoxicity risk.