Altered biodistribution of [Ga]Ga-DOTA-TOC during somatostatin analogue treatment.

Purpose: The need for an interval between the administration of long-acting Somatostatin Receptor Analogues (SSA) and the [Ga]Ga-DOTA-TATE PET has been questioned based on recent literature in the new EANM guidelines. Here an earlier studies showed that SSA injection immediately before SSTR PET had minimal effect on normal organ and tumor uptake (1). However, data are scarce and there are (small) differences between [Ga]Ga-DOTA-TATE and [Ga]Ga-DOTA-TOC binding affinity, and it remains unknown whether these findings can be directly translated to scans with [Ga]Ga-DOTA-TOC as well. The purpose of this study was to assess the effect of SSA use on the biodistribution in a subsequent [Ga]Ga-DOTA-TOC PET/CT and compare this intra-individually across several cycles of SSA treatments.

Methods: Retrospectively, 35 patients with NENs were included. [Ga]Ga-DOTA-TOC PET at staging and after the 1st and 2nd cycle of SSA were included. SUVmean and SUVmax of blood, visceral organs, primary tumor and two metastases were determined. Also, the interval between SSA therapy and the PET scan was registered.

Results: Treatment with SSA resulted in a significantly higher bloodpool activity and lower visceral tracer uptake. This effect was maintained after a 2nd cycle of SSA therapy. Furthermore, there was an inverse relationship between bloodpool tracer availability and visceral tracer binding and a positive correlation between bloodpool tracer availability and primary tumor tracer uptake. With an interval of up to 5 days, there was a significantly higher bloodpool activity than at longer intervals.

Conclusion: Absolute comparison of the SUV on [Ga]Ga-DOTA-TOC PET should be done with caution as the altered biodistribution of the tracer after SSA treatment should be taken into account. We recommend not to perform a scan within the first 5 days after the injection of lanreotide.