In vivo quantification of (177)Lu with planar whole-body and SPECT/CT gamma camera imaging.

Background: Advances in gamma camera technology and the emergence of a number of new theranostic radiopharmaceutical pairings have re-awakened interest in in vivo quantification with single-photon-emitting radionuclides. We have implemented and validated methodology to provide quantitative imaging of (177)Lu for 2D whole-body planar studies and for 3D tomographic imaging with single-photon emission computed tomography (SPECT)/CT.

Methods: Whole-body planar scans were performed on subjects to whom a known amount of [(177)Lu]-DOTA-octreotate had been administered for therapy. The total radioactivity estimated from the images was compared with the known amount of the radionuclide therapy administered. In separate studies, venous blood samples were withdrawn from subjects after administration of [(177)Lu]-DOTA-octreotate while a SPECT acquisition was in progress and the concentration of the radionuclide in the venous blood sample compared with that estimated from large blood pool structures in the SPECT reconstruction. The total radioactivity contained within an internal SPECT calibration standard was also assessed.

Results: In the whole-body planar scans (n = 28), the estimated total body radioactivity was accurate to within +4.6 ± 5.9 % (range -17.1 to +11.2 %) of the correct value. In the SPECT reconstructions (n = 12), the radioactivity concentration in the cardiac blood pool was accurate to within -4.0 ± 7.8 % (range -16.1 to +7.5 %) of the true value and the internal standard measurements (n = 89) were within 2.0 ± 8.5 % (range -16.3 to +24.2 %) of the known amount of radioactivity contained.

Conclusions: In our hands, state-of-the-art hybrid SPECT/CT gamma cameras were able to provide accurate estimates of in vivo radioactivity to better than, on average, ±10 % for use in biodistribution and radionuclide dosimetry calculations.