Quantitative SPECT/CT Imaging of 177Lu with In Vivo Validation in Patients Undergoing Peptide Receptor Radionuclide Therapy
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The purpose of this study is to extend an established SPECT/CT quantitation protocol to 177Lu and validate it in vivo using urine samples, thus providing a basis for 3D dosimetry of 177Lu radiotherapy and improvement over current planar methods which improperly account for anatomical variations, attenuation, and overlapping organs.
In our quantitation protocol, counts in images reconstructed using an ordered subset-expectation maximization algorithm are converted to kilobecquerels per milliliter using a calibration factor derived from a phantom experiment. While varying reconstruction parameters, we tracked the ratio of image to true activity concentration (recovery coefficient, RC) in hot spheres and a noise measure in a homogeneous region. The optimal parameter set was selected as the point where recovery in the largest three spheres (16, 8, and 4 ml) stagnated, while the noise continued to increase.
Urine samples were collected following 12 SPECT/CT acquisitions of patients undergoing [177Lu]DOTATATE therapy, and activity concentrations were measured in a well counter. Data was reconstructed using parameters chosen in the phantom experiment, and estimated activity concentration from the images was compared to the urine values to derive RCs.
In phantom data, our chosen parameter set yielded RCs in 16, 8, and 4 ml spheres of 80.0, 74.1, and 64.5 %, respectively. For patients, the mean bladder RC was 96.1 ± 13.2 % (range, 80.6–122.4 %), with a 95 % confidence interval between 88.6 and 103.6 %. The mean error of SPECT/CT concentrations was 10.1 ± 8.3 % (range, −19.4–22.4 %).
Our results show that quantitative 177Lu SPECT/CT in vivo is feasible but could benefit from improved reconstruction methods. Quantifying bladder activity is analogous to determining the amount of activity in the kidneys, an important task in dosimetry, and our results provide a useful benchmark for future efforts.
Key wordsQuantitative SPECT [177Lu]DOTATATE Peptide receptor radionuclide therapy Dosimetry
The authors would like thank our clinic’s technologists and members of the nursing staff, each of whom greatly aided our patient data collection. We also extend our thanks to Michal Cachovan for his thoughtful discussion and technical contributions.
Conflict of Interest
James Sanders, Joachim Hornegger, and Torsten Kuwert have an ongoing research collaboration with Siemens Molecular Imaging in the field of SPECT/CT. Professor Kuwert receives honoraria from Siemens Molecular Imaging for occasional lectures pertaining to SPECT/CT research. Philipp Ritt has no conflict of interest.
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All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethics standards.
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