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

Abstract

BackgroundSomatostatin receptor 68Ga PET imaging is standard for evaluation of a patient’s suitability for 177Lu peptide receptor radionuclide therapy of neuroendocrine tumours (NETs). The 68Ga 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 68Ga PET and absorbed doses in 177Lu therapy.MethodEighteen patients underwent [68Ga]Ga-DOTA-TATE PET imaging within 20 weeks prior to their first cycle of [177Lu]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 68Ga activity concentrations per injected activity, SUVs, and normalized SUVs were compared with 177Lu activity concentrations 1 d post-injection and 177Lu 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 177Lu tumour absorbed doses based on a combination of tumour-specific 68Ga activity concentrations and group-based estimates of the effective half-lives for grade 1 and 2 NETs was explored.ResultsFor normal organs, only spleen showed a significant correlation between the 68Ga activity concentration and 177Lu 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 68Ga SUVs or normalized SUVs. The relationship between activity uptakes for 68Ga PET and 177Lu SPECT was stronger, with correlation coefficients r = 0.8 for both inter- and intra-patient analyses. The 177Lu absorbed dose to tumour could be predicted from the 68Ga activity concentrations with a 95% coverage interval of − 65% to 248%.ConclusionsOn a group level, a high uptake of [68Ga]Ga-DOTA-TATE is associated with high absorbed doses at 177Lu-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.