68Ga-labeled bombesin studies in patients with gastrointestinal stromal tumors: comparison with 18F-FDG

Dynamic PET studies with a 68Ga-bombesin analog, DOTA-PEG2-[d-Tyr6, beta-Ala11,Thi13,Nle14] BN(6-14) amide (68Ga-BZH3; DOTA is 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid, and PEG is ethylene glycol [2-aminoethyl-carboxymethyl ether]), were performed on patients with gastrointestinal stromal tumors (GIST) to investigate the impact of complementary receptor scintigraphy on diagnosis and the potential of a radionuclide treatment. Furthermore, dynamic 18F-FDG studies were performed on the same patients.

This study comprised 17 patients with GIST. All patients were scheduled for therapy with imatinib because of unresectable primary or recurrent GIST or because of metastatic disease. Dynamic PET scans using 68Ga-BZH3 and 18F-FDG were obtained on 2 consecutive days. Multivariate analysis was used to evaluate the kinetic data. Standardized uptake values (SUVs) were calculated, and a compartmental model (2-tissue) and noncompartmental model were used for data evaluation of both tracers.

Fourteen of 17 patients (25/30 lesions) were positive for uptake on 18F-FDG imaging, whereas 68Ga-BZH3 demonstrated an enhanced accumulation in 7 of 17 patients (8/30 lesions). Thirteen lesions were confirmed by histologic examination, and the remaining 17 were confirmed by follow-up. One recurrent tumor in the stomach could not be delineated on 18F-FDG imaging but showed enhanced 68Ga-BZH3 uptake. The median SUV for 68Ga-BZH3 was 3.3, in comparison with 7.9 for 18F-FDG. Best-subset analysis demonstrated that the global SUV (55-60 min after injection) for 18F-FDG was primarily dependent on k3, followed by k1. Multivariate analysis did not show a significant correlation between the kinetic parameters (k1-k4, fractional blood volume, and SUV) for 18F-FDG and bombesin.

68Ga-BZH3 may be helpful for diagnostic reasons in a subgroup of patients with GIST, as in the case of negative 18F-FDG findings and suspicion of viable tumor tissue. The meaning of the enhanced 68Ga-BZH3 uptake is open at the moment.