ArticleBMC cancer2026
Redefining M1: a novel three-tiered classification stratifies prognosis in metastatic breast cancer.
Article in BMC cancer, 2026. The graph could read no effect estimate from its abstract, so it casts no vote on the map. Not yet cited in PubMed.
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Abstract
backgroundThe current AJCC staging system for breast cancer categorizes all distant metastases under a monolithic M1 designation, failing to capture profound heterogeneity in outcomes. This study aimed to develop and validate a refined M-stage classification that integrates metastatic burden, anatomic site, and brain metastasis status to improve prognostic stratification.
methodsIn this multi-center, retrospective study, 2,615 patients with metastatic breast cancer (diagnosed 2005–2020) were classified into three proposed categories: M1a (Oligometastasis: 1–5 lesions, ≤ 2 organs, no brain metastases), M1b (Oligometastasis: 1–5 lesions, ≤ 2 organs, includes 1–3 treatable brain metastases), and M1c (Extensive Metastasis: > 5 lesions, ≥ 3 organs, or untreatable brain metastases). Overall survival (OS) was compared using Kaplan–Meier curves and Cox proportional hazards models. Prognostic performance was assessed using Harrell’s C-index and compared to the traditional binary M1 stage.
resultsThe cohort was stratified into M1a (n = 672, 25.7%), M1b (n = 418, 16.0%), and M1c (n = 1,525, 58.3%). A clear prognostic gradient was observed: median OS was 98.3 months for M1a, 62.1 months for M1b, and 32.7 months for M1c (log-rank p < 0.001). The proposed classification was an independent prognostic factor on multivariable analysis (M1a vs. M1c adjusted HR = 0.49, p < 0.001). It demonstrated superior prognostic discrimination compared to the traditional M1 stage (C-index 0.67 vs. 0.59, Δ = 0.08, p < 0.001), reclassifying 41.7% of patients into more precise risk categories.
conclusionThis novel three-tiered M-stage classification robustly stratifies metastatic breast cancer into prognostically distinct groups, outperforming the current binary system. It provides a practical framework for refining prognostication, personalizing therapy, and enriching future clinical trials.
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