ESR1 and ductal breast carcinoma in situ: One subgroup analysis based on NSABP B‐2426 retrospectively evaluated the HR status and proposed that postoperative tamoxifen in ER‐positive DCIS patients significantly decreased overall SBEs (HR, 0.58; 95% CI, 0.42–0.81; p = 0.002) and invasive SBEs (HR, 0.53; 95% CI, 0.34–0.82; p = 0.005).