Sample size
ER-positive, tumor, ER-positive breast DCIS
DCIS, TUMOR
This study is designed to evaluate whether the efficacy of low-dose tamoxifen in reducing IBTR is not inferior to that of whole breast irradiation in patients with low-risk and ER-positive breast DCIS. We hypothesize that in all patients who meet the cohort 1 criteria of the ECOG E5194 study and have ER-positive status in the tumor, the 7-year IBTR rate in those who underwent BCS alone will be < 10.5% (assumption: 6–7%). In our previous study, breast DCIS patients who underwent BCS alone had the ECOG E5194 cohort 1 criteria, in whom the 7-year IBTR rate was 5% and 8% in the ER-positive and ER-negative groups, respectively [In total, 385 patients are needed in each group to provide 90% β-power with an α type I error of 5% (margin, IBTR difference, 5%); this permits a loss of 5% of patients due to follow-up at 5 years (statistical methods are shown in Table Statistical methods