Prognostic Value of Distant Micrometastasis in Breast Cancer: A Population-Based Propensity Score-Matched Study

Distant micrometastasis of breast cancer [M0(i+)] is an emerging metastatic stage that has not yet received widespread attention. This study aimed to compare M0(i+) patients with traditional M0 patients and to evaluate the prognostic value of M0(i+).

Using data from the SEER database (2018-2022), we identified 283,503 M0 patients and 585 M0(i+) patients. Propensity score matching (PSM) was applied to control for confounding factors in intergroup comparisons. Overall survival (OS) and breast cancer-specific mortality (BCSM) were assessed using the Kaplan-Meier method, Cox regression analysis, the cumulative incidence function (CIF), and the Fine-Gray competing risk model.

After matching, no statistically significant differences were observed between the M0(i+) and M0 groups in either OS or BCSM (OS: HR=0.803, P=0.369; BCSM: HR=1.273, P=0.450). Nevertheless, the M0(i+) group showed a slightly worse survival trend in BCSM, while demonstrating a survival advantage in OS. This advantage may be influenced by the higher proportion of non-cancer-related deaths in this group.

The prognosis of M0(i+) patients is similar to that of M0 patients, supporting the rationale for combining the two stages in the AJCC staging system. Although the differences did not reach statistical significance, M0(i+) may still carry potential adverse prognostic implications. In light of prior literature, further refinement of the M0(i+) classification should be considered, with particular attention to the prognostic role of bone marrow micrometastasis.