Importance Optimizing treatment decisions in metastatic breast cancer (MBC) can alleviate patients’ burden and improve quality of life. Whether 18 F-fluorodeoxyglucose positron emission tomography (FDG-PET) can be used to better estimate outcomes is unknown. Objective To evaluate clinical utility of early metabolic change on FDG-PET for improving outcome estimation compared with standard diagnostic evaluation in patients with newly diagnosed MBC. Design, Setting, and Participants The multicenter IMPACT-MBC clinical cohort trial enrolled patients with nonrapidly progressive, newly diagnosed MBC from August 2013 to May 2018, before initiation of first-line systemic therapy. Baseline assessment included metastasis biopsy procedure and FDG-PET and CT imaging. Early FDG-PET was performed after 2 weeks of treatment, and CT response evaluation after 8 weeks. Clinical utility was defined as the ability of early FDG-PET to estimate progressive disease (PD) on CT, progression-free survival (PFS), and overall survival (OS). Data were analyzed from October 19, 2025, to February 13, 2026. Intervention Early FDG-PET or standard-of-care (SOC) biopsy-based treatment. Main Outcomes and Measures Clinical utility of molecular imaging to improve outcome estimation of standard diagnostics defined as the capacity to identify poor patient outcomes. Measures were PD at 8 weeks, PFS, and OS. Results The analysis included 200 patients (median range age, 61 32-84 years; 198 females 99% and 2 males 1%). Non-PD on early FDG-PET had a negative predictive value (NPV) of 94.7% (95% CI, 89.5%-97.4%) for non-PD on 8-week CT. This was similar in all MBC subtypes and bone-only disease. Patients with SOC treatment and non-PD on early FDG-PET (n = 133) had a median PFS of 19.4 (95% CI, 15.2-22.8) months and OS of 39.4 (95% CI, 33.7-48.3) months compared to 4.1 (95% CI, 3.3-15.5) months and 18.5 (95% 3 CI, 7.0-33.0) months, respectively ( P lt; .001 for both). Patients with non-PD on 8-week CT but with PD on early FDG-PET had a median (IQR) PFS and OS of 9.5 (4.1-18.1) and 19.4 (8.7-33.0) months compared to 22.3 (15.3-96.1) months and 40.1 (23.4-72.7) months without PD. Conclusions and Relevance In this clinical cohort trial of patients with nonrapidly progressive, newly diagnosed MBC before initiation of first-line systemic therapy, early FDG-PET after only 2 weeks of treatment identified patients with MBC with distinct long-term outcomes. Incorporating early FDG-PET can improve outcome estimation of standard CT assessment. Trial Registration ClinicalTrials.gov Identifier: NCT01957332
Geel et al. (Thu,) studied this question.