Progression-free survival 2 (PFS2) as a surrogate endpoint for overall survival (OS) in breast cancer randomized controlled clinical trials.

A moderate negative correlation was observed (Spearman ρ = –0.505, P = 0.248), suggesting that trials with greater PFS2 benefit may have experienced smaller changes in OS estimates with longer follow-up, although this trend did not reach statistical significance ( Supplementary Figure S8 , available at https://doi.org/10.1016/j.esmoop.2026.106062 ).

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