Correlations between [68Ga]Ga-DOTA-TOC uptake and absorbed dose from [177Lu]Lu-DOTA-TATE

Abstract Background: Peptide receptor radionuclide therapy (PRRT) with [177Lu]Lu-DOTA-TATE is used to treat gastroenteropancreatic neuroendocrine tumours (GEP-NET), by targeting somatostatin receptors (SSTRs). High SSTR expression assessed on [68Ga]Ga-DOTA-TOC PET/CT images decides patient eligibility for [177Lu]Lu-DOTA-TATE PRRT. This study aimed to investigate potential correlations between therapeutic absorbed dose to tumours and the pre-treatment [68Ga]Ga-DOTA-TOC uptake.Methods: This retrospective study included 301 tumours from 54 patients with GEP-NET. The tumours were segmented on pre-therapeutic [68Ga]Ga-DOTA-TOC PET/CT, and post-therapy [177Lu]Lu-DOTA-TATE SPECT/CT acquired 24 and 168 hours post administration (also 4 hours for some patients). The SPECT/CT images were used for absorbed dose calculations. Results: Mean SUVmean and SUVmax from [68Ga]Ga-DOTA-TOC PET was 24.0 (3.6-84.4) and 41.0 (6.7-146.5), and the mean absorbed dose from [177Lu]Lu-DOTA-TATE was 26.9 Gy (2.4-101.9). An absorbed dose prediction model was formed based on a weak, but significant (R2 = 0.44, p<0.05), linear relationship between SUVmean and [177Lu]Lu-DOTA-TATE activity concentration at 24 hours, together with a population-averaged pharmacokinetics. The relative error between predicted and calculated absorbed dose was 0.31+/-0.90.Conclusions: Depending on the required accuracy, [68Ga]Ga-DOTA-TOC PET imaging may estimate the [177Lu]Lu-DOTA-TATE absorbed dose. While a linear correlation was found, differences between [68Ga]Ga-DOTA-TOC and [177Lu]Lu-DOTA-TATE uptake gave especially high variance.