Background: Personalized molecular radiotherapy based on theragnostics requires accurate quantification of the amount of radiopharmaceutical activity administered to patients both in diagnostic and therapeutic applications. This international multi-center study aims to investigate the clinical measurement accuracy of radionuclide calibrators for 7 radionuclides used in theragnostics: 99mTc, 111In, 123I, 124I, 131I, 177Lu and 90Y.
Methods: In total, 32 radionuclide calibrators from 8 hospitals located in the Netherlands, Belgium and Germany were tested. For each radionuclide, a set of four samples comprising two clinical containers (10-mL glass vial and 3-mL syringe) with two filling volumes were measured. The reference value of each sample was determined by two certified radioactivity calibration centers (SCK CEN and JRC) using two secondary standard ionization chambers. The deviation in measured activity with respect to the reference value was determined for each radionuclide and each measurement geometry. In addition, the combined systematic deviation of activity measurements in a theragnostic setting was evaluated for 5 clinically-relevant theragnostic pairs: 131I/123I, 131I/124I, 177Lu/111In, 90Y/99mTc and 90Y/111In.
Results: For 99mTc, 131I, and 177Lu, a small minority of measurements were not within ±5% range from the reference activity (percentage of measurements not within range: 99mTc: 6%, 131I: 14%, 177Lu: 24%) and almost none were outside ±10% range. However, for 111In, 123I, 124I and 90Y more than half of all measurements were not accurate within ±5% range (111In: 51%, 123I: 83%, 124I: 63%, 90Y: 61%) and not all were within ±10% margin (111In: 22%, 123I: 35%, 124I: 15%, 90Y: 25%). A large variability in measurement accuracy was observed between radionuclide calibrator systems, type of sample container (vial vs syringe), and source-geometry calibration/correction settings used. Consequently, we observed large combined deviations (percentage deviation > ±10%) for the investigated theragnostic pairs, in particular for 90Y/111In, 131I/123I and 90Y/99mTc.
Conclusions: Our study shows that substantial over- or under-estimation of therapeutic patient doses are likely to occur in a theragnostic setting due to errors in the assessment of radioactivity with radionuclide calibrators. These findings underline the importance of thorough validation of radionuclide calibrator systems for each clinically-relevant radionuclide and sample geometry.