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

Somatostatin 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.

Eighteen 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.

For 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%.

On 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.