Background Our goal is to quantitatively compare radiotracer biodistributions within tumors and major normal organs on pretherapy 68Ga-DOTATATE PET to post-therapy 177Lu-DOTATATE single-photon emission computed tomography (SPECT) in patients receiving peptide receptor radionuclide therapy (PRRT). Methods PET/CT at ∼ 60 min postinjection of Ga-68 DOTATATE and research 177Lu-SPECT/CT imaging ∼ at 4 h (SPECT1) and ∼ 24 h (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)meanand SUVpeakfor tumor, and SUVmeanfor 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 R2= 0.19, 0.21, 0.29; PET versus SPECT2 R2= 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 SUVmeanfor 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= 10 [CI: 8.2-11.7], TNRspleen= 2.9 [CI: 2.5-3.4], TNRkidney= 2.8 [CI: 2.3-3.3]; SPECT2: TNRnliver= 16.9 [CI: 14-19.9], 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 177Lu SPECT compared with 68Ga PET. We postulate this phenomenon is due to temporal differences in DOTATATE uptake and internalization in tumor as compared to normal organs.
- neuroendocrine tumor
- peptide receptor radionuclide therapy
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging