Differences in tumor-to-normal organ SUV ratios measured with 68Ga-DOTATATE PET compared with 177Lu-DOTATATE SPECT in patients with neuroendocrine tumors

Background Our goal is to quantitatively compare radiotracer biodistributions within tumors and major normal organs on pretherapy Ga-68-DOTATATE PET to post-therapy Lu-177-DOTATATE single-photon emission computed tomography (SPECT) in patients receiving peptide receptor radionuclide therapy (PRRT). Methods PET/CT at similar to 60 min postinjection of Ga-68 DOTATATE and research Lu-177-SPECT/CT imaging similar to at 4h (SPECT1) and similar to 24h (SPECT2) post-cycle#1 were available. Manual contours of lesions on baseline CT or MRI were applied to co-registered SPECT/CT and PET/CT followed by deep learning-based CT auto-segmentation of organs. Tumor-to-normal organ ratios (TNR) were calculated from standardized uptake values (SUV)(mean) and SUVpeak for tumor, and SUVmean for non-tumoral liver (nliver), spleen and kidney. Results There were 90 lesons in 24 patients with progressive metastatic neuroendocrine tumor. The correlation between PET and SPECT SUV TNRs were poor/moderate: PET versus SPECT1 R-2 =0.19. 0.21, 0.29; PET versus SPECT2 R-2 =0.06, 0.16, 0.33 for TNRnliver,TNRspleen, TNRkidney, respectively. Across all patients, the average value of the TNR measured on PET was significantly lower than on SPECT at both time points (P<0.001). Using SUVmean for tumor, average TNR values and 95% confidence intervals (CI) were PET: TNRnliver =3.5 [CI: 3.0-3.9], TNRspleen =1.3 [CI. 1.2-1.5], TNRkidney = 1.7 [CI: 1.6-1.9]; SPECT1: TNRnliver = 1.0 [CI: 8.2-11.7], TNRspleen = 2.9 [CI: 2.5-3.4]. TNRkidney = 2.8 [CI: 2.3-3.3]; SPECT2: TNRnliver = 1 6.9 [CI: 1 4-1 9.9], TNRspleen =3.6 [CI: 3-4.2], TNRkidney = 3.6 [CI: 3.0-4.2]. Comparison of PET and SPECT results in a sphere phantom study demonstrated that these differences are not attributed to imaging modality. Conclusions Differences in TNR exist for the theranostic pair, with significantly higher SUV TNR on Lu-177 SPECT compared with Ga-68 PET. We postulate this phenomenon is due to temporal differences in DOTATATE uptake and internalization in tumor as compared to normal organs. (C) 2022 Wolters Kluwer Health, Inc. All rights reserved.