RECIST 1.1 and PERCIST for response assessment in metastatic breast cancer: feasibility and agreement in a real‑world retrospective cohort study

Background Accurate response assessment during systemic therapy of metastatic breast cancer (mBC) is essential to guide treatment decisions. We aimed to compare the applicability of RECIST 1.1 (CT) and PERCIST (FDG-PET) for response evaluation in mBC within a contemporary real-world setting. Methods In this retrospective cohort study, mBC patients undergoing systemic therapy between 2017 and 2025 were identified from a large imaging database. Paired CT and FDG-PET examinations were analyzed using RECIST 1.1 and PERCIST, respectively. We evaluated target lesion availability, agreement in response categorization, concordance of metastatic patterns—including oligometastatic disease—and time to progression. Results Fifty-one patients with 375 imaging examinations were included, yielding 151 paired CT and FDG-PET scans. All FDG-PET scans (151/151) had measurable target lesions according to PERCIST, whereas 65% of CT scans (98/151) lacked measurable target lesions per RECIST 1.1. Response category agreement reached 68% (53/78) among paired scans evaluable by both RECIST 1.1 and PERCIST. FDG-PET detected progression significantly earlier than CT (p < 0.05). In 23% of CT‑classified oligometastatic scans, FDG‑PET demonstrated a higher metastatic burden (p < 0.001). Conclusion In this real-world study of mBC, FDG-PET consistently enabled response assessment according to PERCIST, whereas CT frequently could not provide measurable target lesions according to RECIST 1.1. Moreover, FDG-PET revealed a higher metastatic burden than CT, with implications for classification of oligometastatic disease. These findings suggest that integrating FDG-PET could refine treatment individualization, potentially broaden trial eligibility, and better support clinical decision-making, though further validation is needed.