Background: Medullary breast cancer (MBC) is a rare tumor associated with a better prognosis compared with other breast cancers. The role of adjuvant chemotherapy has not been extensively studied. Methods: Female patients with invasive MBC reported to the National Cancer Data Base from 2004 to 2012 were analyzed. Overall survival (OS) and treatment were studied using the Kaplan–Meier method and the Cox proportional hazard model. Patients who had node-negative (N0), non-metastatic (M0) tumors 10 to 50 mm in size (T1cN0M0 and T2N0M0) treated with and without chemotherapy were analyzed using propensity score matching. Results: Of 3739 patients with MBC, 2642 (71%) had T1b-T2N0M0 disease treated with and without chemotherapy. Multivariable analysis showed that for all MBC patients, the significant predictors of OS were age older than 65 years, one or more comorbidities, tumor larger than 2 cm, positive nodes, distant metastasis, and treatment with chemotherapy or radiation therapy. Patients with T1cN0M0 and T2N0M0 had improved OS if they received chemotherapy (p < 0.0005). Patients with T1bN0M0 who received chemotherapy did not show better OS than those who did not. Patients with T1c-T2N0M0 were then matched by propensity score based on age, presence of comorbidities, tumor size, and treatment methods used. After matching, the group receiving chemotherapy showed an improved OS (hazard ratio [HR], 0.40; 95% confidence interval [CI], 0.26–0.62; p < 0.0005) compared to the group that did not receive chemotherapy. Conclusions: For patients with T1c-T2N0M0 MBC, chemotherapy significantly improves OS.
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