Prospective comparison of early interim (18)F-FDG-PET with (18)F-FLT-PET for predicting treatment response and survival in metastatic breast cancer

BACKGROUND: To compare the value of interim (18)F-FLT-PET and (18)F-FDG-PET for predicting treatment outcomes in patients with metastatic breast cancer after salvage therapy. METHODS: Patients with metastatic breast cancer received PET/CT using (18)F-FLT and (18)F-FDG at baseline, after the 1st and 2nd cycle of systemic chemotherapy. The clinical response was classified according to Response Evaluation Criteria in Solid Tumors 1.1 based on contrast-enhanced CT after 3 months of systemic chemotherapy. The metabolic response on PET was assessed according to European Organization for Research and Treatment of Cancer criteria or PET Response Criteria in Solid Tumors (PERCIST) and was correlated to the clinical response, overall survival (OS), and progression-free survival (PFS). RESULTS: Twenty-five patients entered final analysis. On (18)F-FDG-PET, clinical responders after 2 chemotherapy cycles (post-2c) had a significantly greater reduction of maximal standardized uptake value (SUV) and the peak SUV corrected for lean body mass (SULpeak) of the tumor than non-responders (P = 0.030 and 0.003). Metabolic response determined by PERCIST on post-2c (18)F-FDG-PET showed a high area under the receiver operating characteristics curve of 0.801 in predicting clinical response (P = 0.011). Patients who were metabolic responders by PERCIST on post-2c (18)F-FDG-PET had a significantly longer PFS (53.8% vs. 16.7%, P = 0.014) and OS (100% vs. 47.6%, P = 0.046) than non-responders. Survival differences between responders and non-responders in the interim (18)F-FLT-PET were not significant. CONCLUSIONS: (18)F-FLT-PET failed to show an advantage over (18)F-FDG-PET in predicting the treatment response and survival in patients with metastatic breast cancer. Assessment of treatment outcome by interim (18)F-FDG-PET may aid treatment. TRIAL REGISTRATION: The study was retrospectively registered on 02/06/2020 on Clinicaltrials.gov (identifier NCT04411966).