Dosimetry in peptide receptor radionuclide therapy using (177)Lu-DOTATATE is based on patient imaging during the first week after administration and determination of the activity retention as a function of time for different tissues. For calculation of the absorbed dose, it is generally assumed that the long-term activity retention follows the pattern determined from the first week. This work aimed to investigate the validity of this assumption by performing additional patient measurements between 5 and 10 wk after administration. A further aim was to investigate to what extent absorbed dose values were affected when including these measurements, also taking into account the radionuclide impurity of (177m)Lu and build-up of secondary (177)Lu from the (177m)Lu decay.
Methods: A combination of methods was used: planar γ-camera imaging as part of the clinical dosimetry protocol, determination of the whole-body activity between 5 and 9 wk after injection using spectrometric NaI(Tl) and HPGe detectors, and imaging between 5 and 10 wk after injection for assessment of the activity distribution. From these measurements the long-term retention of activity was determined and the relative influence on absorbed doses calculated.
Results: The most important finding was a clearly visualized tumor uptake in images from between 5 and 7 wk after injection and in 1 patient also kidney and spleen uptake in images acquired on day 33. As a consequence, the total-body time-activity curve had a tail, which was not completely captured by imaging during the first week. The absorbed doses to total body and tumors obtained when including these late time points were on average 5%-6% higher than those obtained when using data acquired during the first week. The contributions to the absorbed dose from (177m)Lu and secondary (177)Lu were negligible.
Conclusion: At approximately 5-7 wk after injection, there was a measureable amount of (177)Lu-DOTATATE in patients, which is mainly governed by retention in tumors. For tumor dosimetry, imaging at a later time than the routinely used 7 d may be warranted. The contribution to the absorbed dose from the radionuclide impurity of (177m)Lu was negligible.
Keywords: 177Lu; dosimetry; peptide receptor radiotherapy.
© 2015 by the Society of Nuclear Medicine and Molecular Imaging, Inc.