Relationships between uptake of [(68)Ga]Ga-DOTA-TATE and absorbed dose in [(177)Lu]Lu-DOTA-TATE therapy

BACKGROUND: Somatostatin receptor (68)Ga PET imaging is standard for evaluation of a patient’s suitability for (177)Lu peptide receptor radionuclide therapy of neuroendocrine tumours (NETs). The (68)Ga PET serves to ensure sufficient somatostatin receptor expression, commonly evaluated qualitatively. The aim of this study is to investigate the quantitative relationships between uptake in (68)Ga PET and absorbed doses in (177)Lu therapy. METHOD: Eighteen patients underwent [(68)Ga]Ga-DOTA-TATE PET imaging within 20 weeks prior to their first cycle of [(177)Lu]Lu-DOTA-TATE. Absorbed doses for therapy were estimated for tumours, kidney, spleen, and normal liver parenchyma using a hybrid SPECT/CT–planar method. Gallium-68 activity concentrations were retrieved from PET images and also used to calculate SUVs and normalized SUVs, using blood and tissue for normalization. The (68)Ga activity concentrations per injected activity, SUVs, and normalized SUVs were compared with (177)Lu activity concentrations 1 d post-injection and (177)Lu absorbed doses. For tumours, for which there was a variable number per patient, both inter- and intra-patient correlations were analysed. Furthermore, the prediction of (177)Lu tumour absorbed doses based on a combination of tumour-specific (68)Ga activity concentrations and group-based estimates of the effective half-lives for grade 1 and 2 NETs was explored. RESULTS: For normal organs, only spleen showed a significant correlation between the (68)Ga activity concentration and (177)Lu absorbed dose (r = 0.6). For tumours, significant, but moderate, correlations were obtained, with respect to both inter-patient (r = 0.7) and intra-patient (r = 0.45) analyses. The correlations to absorbed doses did not improve when using (68)Ga SUVs or normalized SUVs. The relationship between activity uptakes for (68)Ga PET and (177)Lu SPECT was stronger, with correlation coefficients r = 0.8 for both inter- and intra-patient analyses. The (177)Lu absorbed dose to tumour could be predicted from the (68)Ga activity concentrations with a 95% coverage interval of − 65% to 248%. CONCLUSIONS: On a group level, a high uptake of [(68)Ga]Ga-DOTA-TATE is associated with high absorbed doses at (177)Lu-DOTA-TATE therapy, but the relationship has a limited potential with respect to individual absorbed dose planning. Using SUV or SUV normalized to reference tissues do not improve correlations compared with using activity concentration per injected activity. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s13550-022-00947-2.