The value of PET/CT with 18F-FLT and 18F-FDG in the management of metastatic germ cell tumors (GCT): A pilot study

Abstract

e16142 Background: The purpose is to assess the ability of [F-18]-3’-Fluoro-3’-deoxythymidin (FLT), a cell proliferation marker, for early response monitoring and prediction of histology of residual tumor masses in patients (pts) with metastatic GCT in comparison to the standard tracer F-18-FDG, CT-scans and tumor markers. Methods: 11 male pts, age 23–48 yrs, with metastatic GCT were evaluated with both FDG- and FLT-PET/CT prior to chemotherapy (CTh), after the first cycle (early response) and 3 weeks after completion of induction CTh. PET was analyzed visually and quantitatively. The results were validated by histopathology of resected residual masses after CTh in 7 pts or by clinical follow-up for at least 6 mos in 4 pts. Presence of necrosis was judged as responder, as well as CR/PRm- within a minimum progression-free interval (PFI) of 6 mos. In case of multiple resections, the worse histology was taken into account. Regarding early tumor response EORTC criterias were used. Results: 8 out of 11 pts had a PFI > 6 mos (range, 206–1337 days). Examination of resected masses revealed necrosis in 3/7, teratoma in 2/7 as well as 2/7 pts with viable tumors. Prior to CTh the reference lesions showed increased FDG uptake (SUVrange/mean, 2.9–15.0/8.8) in all pts but moderate FLT uptake (SUVrange/mean, 1.7–9.7/3.7) in 10 out of 11 pts. SUVavg decrease in early response FDG monitoring was 64% in responders and 60% in non-responders (p = 0.8), as well as 57% vs. 48% for FLT (p = 0.5), respectively, and 85% vs. 72% (FDG, p = 0.1) and 67% vs. 65% (FLT, p = 0.8) in the final monitoring. Results of early and final response were inconsistent in 6/11 pts in FDG and in 4/10 pts in FLT-PET. In 2 pts with teratoma false negative results in both FDG- and FLT-PET have been seen. The sensitivities, specificities, positive and negative predictive values (%) of FDG and FLT-PET for early and final response monitoring were 60/33/43/50, 60/80/75/67, 20/100/100/60, and 0/100/0/50, respectively. Conclusions: PET negative residual masses after CTh of metastatic GCT still require resection, since the low negative predictive value of FDG-PET cannot be improved by application of the proliferation marker FLT. No significant financial relationships to disclose.