Molecular imaging of PET/CT with Ga-68 DOTATOC and F-18 FDG in the detection of neuroendocrine gastroenteropancreatic tumors (GEP NETs)

Abstract

Aim: Functional imaging methods, such as somatostatin-receptor (SSR) scintigraphy in combination with anatomical techniques like CT can be regarded as advanced and elegant molecular imaging procedures enabling mapping of SSRs in GEP NETs. The SSR status of GEP NETs is of great relevance, because receptor-negative lesions may be poorly differentiated and characterised by aggressive growth and poor prognosis – with consequences for the choice of therapy. Ga-68 DOTA-D-Phe(1)-Tyr(3)-octreoide (Ga-68 DOTATOC) is a newly developed PET-radiotracer with a high specific binding to SSR2, a subtype of SSRs, most often expressed in GEP NETs. F-18 FDG is routinely applied in the work-up of malignant tumors and is of particular interest in the assessment of poorly differentiated tumors. This study compares the clinical performance of both PET-tracers in detecting SSR2-positive GEP-NETs. Material and Methods: Forty patients with metastatic GEP NETs were enrolled in this study. Both imaging methods were applied to each patient – in a time distance of less than 2 weecks (GE Discovery LS PET/CT: 360 MBq F-18 FDG, Scan: 1h p.i.; 100 MBq Ga-68 DOTATOC, Scan: 40 Min. p.i.). F-18 FDG and Ga-68 DOTATOC were compared with respect to tumor uptake differences. Results: In 34 patients, the uptake of Ga-68 DOTATOC was significantly higher than that of F-18 FDG (which was weak to zero); only two patients showed higher F-18 FDG accumu-lation compared to Ga-68 DOTATOC. GEP NETs with a high grade of differentiation tend to show high Ga-68 DOTATOC uptake and low F-18 FDG uptake, whereas in poorly differentiated GEP NETS the uptake of Ga-68 DOTATOC was significantly lower than that of F-18 FDG. In 4 patients we could demonstrate a mosaic pattern of tracer uptake: lesions with high Ga-68 DOTATOC and low F-18 FDG uptake neighboured by lesions having a reversed uptake picture, i.e. high F-18 FDG and low Ga-68 DOTATOC uptake. This phenomenon points to the possibility of different grades of differentiation within metastases of the same patient. Conclusion: Ga-68 DOTATOC- and F-18 FDG-PET/CT together could be an ideal tool to characterise the grade of differentiation of GEP NETs and their metastases, thus providing the most adequate choice of therapy for each patient.