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, β-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. Methods: 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. Results: 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. Conclusion: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.