Biokinetics and imaging with the somatostatin receptor PET radioligand 68Ga-DOTATOC:: preliminary data

Somatostatin (SMS) scintigraphy is widely used for the detection and staging of neuroendocrine tumours. Because of its superior imaging properties, there is growing interest in the use of positron emission tomography (PET) technology for SMS scintigraphy. This study addressed the production of gallium-68 DOTA-TOC, its biokinetics and its clinical performance in detecting SMS-positive tumours and metastases. A preparation protocol was developed, yielding 40% overall incorporation of Ga-68 into the peptide (DOTATOC). After column filtration, the radiochemical purity exceeded 98%. Eight patients with histologically verified carcinoid tumours were injected with 80-250 MBq of this tracer. PET acquisition was initiated immediately after administration and carried out until 3 h post injection. Images were quantitated using standardised uptake values and target to non-target ratios. Prior to Ga-68-DOTATOC PET, all patients underwent indium-111 octreotide planar and single-photon emission tomographic (SPET) imaging. Arterial activity elimination was bi-exponential, with half-lives of 2.0 (+/-0.3) min and 48 (+/-7) min. No radioactive metabolites were detected within 4 h in serum. Maximal tumour activity accumulation was reached 70+/-20 min post injection. Kidney uptake averaged <50% compared with spleen uptake. Of 40 lesions predefined by computed tomography and/or magnetic resonance imaging, Ga-68-DOTATOC PET identified 100%, whereas In-111-octreotide planar and SPET imaging identified only 85%. Tumour to non-tumour ratios ranged from >3:1 for liver (In-111-octreotide: 1.5: 1) to 100: 1 for CNS (In-111-octreotide: 10:1). With Ga-68-DOTATOC >30% additional lesions were detected. It is concluded that PET using Ga-68-DOTATOC results in high tumour to non-tumour contrast and low kidney accumulation and yields higher detection rates as compared with In-111-octreotide scintigraphy.