Background: This study investigated the impact of pN1mi disease on the survival of T1 breast cancer patients and examined the clinical usefulness of the online PREDICT tool and updated staging system. Methods: The node stages of 2344 patients were divided into pN0, pN1mi, and pN1a. Clinicopathological parameters and survival outcomes were retrospectively analyzed. Data for 111 micrometastatic diseases were applied to the PREDICT version 2.0 and re-classified using the 8th edition of the cancer staging manual. Results: Univariable analyses demonstrated worse disease-free and overall survival rates for patients with node-positive cancer; however, the significance was not maintained in multivariable analyses. Chemotherapy improved outcomes in patients with node-positive and non-luminal A-like subtype cancers. The PREDICT tool demonstrated good performance when estimating the 5-year overall survival for pN1mi disease (area under the receiver operating characteristic curve, 0.834). According to the updated staging system, 74% of cases were down-staged to IA, and clearly splitting survival curves were identified. Conclusion: pN1mi disease alone did not adversely affect survival outcomes. Biologic and treatment factors determined outcomes in cases of small-volume node micrometastasis. The PREDICT tool or new staging classification could help predict the survival of patients with micrometastatic sentinel nodes.
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