Abstract PR008: Duration of endocrine treatment for DCIS impacts second events: Insights from a large registry of cases at two academic medical centers
Authors: O'Keefe Thomas , Yau Christina , Iaconetti Emma , Jeong Eliza , Brabham Case , Kim Paul , McGuire Joseph , Griffin Ann , Esserman Laura , Harismendy Olivier , Hirst Gillian 
Published: December 2022

Abstract
Background: Ductal carcinoma in situ (DCIS) incidence has risen with increasing use of screening mammography. It is unclear who benefits from both the amount and type of adjuvant treatment (radiation therapy, (RT), endocrine therapy (ET)) versus what constitutes over-treatment. Our goal was to identify the effects of adjuvant RT, or ET+/- RT versus breast conservation surgery (BCS) alone in a large multi-center DCIS registry of retrospective patients with long follow up. Methods: Data was extracted on women with a primary diagnosis of DCIS (N=2979) between 1985 - 2017 treated in 2 University of California (UC) medical centers. ET treatment and duration was confirmed by chart review for 1916 patients for this analysis. Receipt of ET (N=404) was stratified to &gt; 2 years or &lt; 2 years. Association between treatment type and second events was assessed using Cox regression. Competing risks models were used to assess effect of treatment type on different type of second events (ipsilateral DCIS or invasive, or contralateral combined). Time-varying coefficients were used as appropriate. Results: Median follow up time was 8.2 years for the 1916 patients analyzed. The cumulative second event (any type) rate was 25% at 15 years. In univariate and multivariate analysis adjusting for clinical variables, all treatments reduced the risk of second events compared to BCS only. Further stratifying ET receipt by duration demonstrated significant risk reduction (HR=0.12, P=0.04) only in women taking &gt;2 years of ET, but not in women in the &lt;2year ET group (HR=1.3, P=0.55). In the competing risk model, RT significantly reduces the risk of both ipsilateral DCIS and invasive cancer, and ET &gt; 2 years significantly reduces the risk of ipsilateral invasive cancer. Conclusions: In our registry we show that women with DCIS who took less than 2 years of adjuvant endocrine therapy have a similar second event rate as BCS. In contrast, women who took more than 2 years of ET show a significantly reduced second event rate, similar to those who received either RT or combined ET+RT, which was independent of age, tumor size, grade, or period of diagnosis. This highlights the importance of ET duration for risk reduction of second events following surgery for newly diagnosed DCIS.
Citation Format: Thomas O'Keefe, Christina Yau, Emma Iaconetti, Eliza Jeong, Case Brabham, Paul Kim, Joseph McGuire, Ann Griffin, Laura Esserman, Olivier Harismendy, Gillian Hirst. Duration of endocrine treatment for DCIS impacts second events: Insights from a large registry of cases at two academic medical centers [abstract]. In: Proceedings of the AACR Special Conference on Rethinking DCIS: An Opportunity for Prevention?; 2022 Sep 8-11; Philadelphia, PA. Philadelphia (PA): AACR; Can Prev Res 2022;15(12 Suppl_1): Abstract nr PR008.