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

OBJECTIVES

The aim of this pilot study was to explore the impact of peptide mass on binding of [(68)Ga]-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 [(68)Ga]-DOTATOC proceeded by 0, 50 and 250 or 500 microg of octreotide, administered 10 min before the tracer. Three patients were examined by dynamic and static PET/CT for pharmacokinetic and dosimetric calculations. The [(68)Ga]-DOTATOC synthesis included preconcentration and purification of the generator eluate and microwave heating in a semi-automated in-house procedure.

RESULTS

[(68)Ga]-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 microg 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 (68)Ga-based tracer was carried out in 1 day. The use of high SRA [(68)Ga]-DOTATOC and unlabelled octreotide indicates an optimal mass leading to better image contrast. [(68)Ga]-DOTATOC-PET-CT employing variable SRA may be utilised for accurate quantification of tumour uptake with subsequent dosimetry for personalized therapy management.