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Background: Adjuvant chemotherapy (AC) is an important, effective treatment for breast cancer. This study evaluates the effectiveness of post-mastectomy AC in treating patients with prognostic stage IB breast cancer. Method: We conducted a retrospective cohort-based study using Surveillance, Epidemiology, and End-Results database. Overall survival (OS) and breast cancer-specific survival (BCSS) were calculated using the Kaplan–Meier method. Multivariate Cox risk models were used to identify the impact of AC. Stratified analysis was performed according to molecular subtypes, anatomic stages, and other risk factors to evaluate the effect of AC on survival. Results: 28,825 women diagnosed with prognostic stage IB breast cancer were included. The 5-year OS was significantly higher in AC group than in non-adjuvant chemotherapy (NAC) group (P 0.05). Meanwhile, AC is not an independent prognostic factor for OS and BCSS in patients with lymph node micrometastases. Conclusion: Our study demonstrates that patients with prognostic stage IB do not fully benefit from AC. Individualized treatment management is required for patients with pT1a-1b/N0-1 tumors, lymph node micrometastases, or HR(+)/HER2(−) subtypes.

Adjuvant chemotherapy (AC) is an important, effective treatment for breast cancer. This study evaluates the effectiveness of post-mastectomy AC in treating patients with prognostic stage IB breast cancer. We conducted a retrospective cohort-based study using Surveillance, Epidemiology, and End-Results database. Overall survival (OS) and breast cancer-specific survival (BCSS) were calculated using the Kaplan–Meier method. Multivariate Cox risk models were used to identify the impact of AC. Stratified analysis was performed according to molecular subtypes, anatomic stages, and other risk factors to evaluate the effect of AC on survival. 28,825 women diagnosed with prognostic stage IB breast cancer were included. The 5-year OS was significantly higher in AC group than in non-adjuvant chemotherapy (NAC) group (P < 0.0001); however, the 5-year BCSS in AC group was significantly lower than in NAC group (P = 0.039). Multivariate analysis revealed that AC was a favorable prognostic factor for OS (P < 0.001), but not BCSS (P = 0.407). AC was not an independent prognostic factor for BCSS in patients with hormone receptor-positive, human epidermal growth factor receptor 2-negative (HR[+]/HER2[-]) subtype or pT1a-1b/N0-1 stage with HER2 overexpression (HER2[+]) subtype, regardless of whether HR was positive or negative (P > 0.05). Meanwhile, AC is not an independent prognostic factor for OS and BCSS in patients with lymph node micrometastases. Our study demonstrates that patients with prognostic stage IB do not fully benefit from AC. Individualized treatment management is required for patients with pT1a-1b/N0-1 tumors, lymph node micrometastases, or HR(+)/HER2(−) subtypes. [ABSTRACT FROM AUTHOR]