In vivo binding of [68Ga]-DOTATOC to somatostatin receptors in neuroendocrine tumours — impact of peptide mass

Abstract: Objectives: The aim of this pilot study was to explore the impact of peptide mass on binding of [68Ga]-DOTATOC to neuroendocrine tumour somatostatin receptors in vivo using a tracer of variable specific radioactivity (SRA) and to show the logistic feasibility of sequential PET scans in the same patient. Material and Methods: Nine patients with gastroenteropancreatic neuroendocrine tumours were included. Six of them underwent three sequential PET-CT examinations with intravenous injections of [68Ga]-DOTATOC proceeded by 0, 50 and 250 or 500 μg of octreotide, administered 10 min before the tracer. Three patients were examined by dynamic and static PET/CT for pharmacokinetic and dosimetric calculations. The [68Ga]-DOTATOC synthesis included preconcentration and purification of the generator eluate and microwave heating in a semi-automated in-house procedure. Results: [68Ga]-DOTATOC synthesis and quality control were accomplished within 30 min and radiochemical purity was >95%. The tracer accumulation in the tumours varied and depended on the total amount of the administered peptide. In five of six patients, the highest tumour-to-normal tissue ratio was found when 50 μg of octreotide was preadministered. One patient showed a continuously increasing tumour uptake. Dosimetrically, a large variation in organ doses was found (kidney: 0.086–0.168 mSv/MBq; liver: 0.026–0.096 mSv/MBq; spleen: 0.046–0.226 mSv/MBq). The effective dose (0.015, 0.0067 and 0.0042 mSv/MBq) was correlated to the total amount of decays. Discussion: Three sequential PET-CT examinations using 68Ga-based tracer was carried out in 1 day. The use of high SRA [68Ga]-DOTATOC and unlabelled octreotide indicates an optimal mass leading to better image contrast. [68Ga]-DOTATOC-PET-CT employing variable SRA may be utilised for accurate quantification of tumour uptake with subsequent dosimetry for personalized therapy management. [Copyright &y& Elsevier]