Insights from a multi-institutional registry show duration of endocrine treatment for DCIS impacts second events.

Ductal carcinoma in situ (DCIS) incidence has risen rapidly with the introduction of screening mammography, yet it is unclear who benefits from both the amount and type of adjuvant treatment (radiation therapy, endocrine therapy) versus what constitutes over-treatment. Our goal was to identify the effects of adjuvant radiation therapy, or endocrine ± radiation therapy versus breast conservation surgery alone in a large multi-center registry of retrospective DCIS cases (N = 1916) with median follow up of 7.0 (IQR: 8.43) years. We show that patients with DCIS who took less than 2 years of adjuvant endocrine therapy alone have a similar second event rate as breast conservation surgery. However, patients who took more than 2 years of endocrine therapy show a significantly reduced second event rate, similar to those who received either radiation or combined endocrine + radiation therapy, which was independent of age, tumor size, grade, or period of diagnosis. This highlights the importance of endocrine therapy duration for risk reduction.