Abstract
1023
Objectives The availability of imaging analogs for therapeutic radionuclides has made it possible to use imaging radionuclides for dosimetry of therapeutic agents. We investigate the sensitivity associated with using different radionuclide pairs for this purpose. There are a number of such theranostic radionuclide pairs. Including isotopes of each other such as 86Y for 90Y or chemically similar elements as 68Ga for 177Lu. The applicability of such imaging/therapy pairs will depend on the pharmacokinetics (PK) of the radiopharmaceutical (RP).
Methods Using clinical data, we examined the 68Ga/177Lu pair for dosimetry of radiopeptide therapy. Two to four 68Ga PET scans from 17 patients were used in the analysis. To evaluate sensitivity, the measured activity in the last acquired time point for the normal tissues was increased or decreased by up to 10%. Integration was performed by a hybrid method. Fitted expressions were integrated when possible. Alternatively numerical integration was used, and extrapolation was based on the last two time points. If the data showed uptake, physical decay was used beyond the last time-point.
Results The sensitivity of 68Ga based dosimetry for 177Lu radiopeptide therapy was organ dependent. For example, a 5% increase and decrease in the last measured time point for the kidneys resulted in a 90% higher and 2.2% lower calculated time-integrated activity, respectively. The spleen still being in its uptake phase showed less variation.
Conclusions For the particular radionuclide pair examined, the sensitivity of absorbed dose estimates depended upon the RP half-life, imaging acquisition times p.i., and if the normal tissue was in an uptake or excretion phase over the measurement time-period. The short half-life of 68Ga limited the possibility of measuring late PK necessary for accurate dosimetry of a long-lived RP.