Background: Endocrine therapy (ET) initiation in people within 1 year of diagnosis of estrogen-receptor-positive (ER+) ductal carcinoma in situ (DCIS) has ranged from 21-45% in prior studies; however, these studies did not evaluate initiation post-2011 and may not reflect current patterns of use. ET use following ER+ DCIS diagnosis can reduce the relative risks of recurrence and contralateral breast cancer by 30-50%. We evaluated ET initiation over time and by demographic, tumor, and treatment characteristics in the Kaiser Permanente Breast Cancer Survivors Study between 2001-2016. Methods: We included people aged 20+ years diagnosed with unilateral DCIS with at least 1 year of cancer-free follow-up from three U.S. integrated health systems. We excluded people with ER-negative disease, no surgery, and those who received chemotherapy or unknown radiation therapy. ET dispensings (tamoxifen and aromatase inhibitors) were extracted from electronic pharmacy records within 1 year of a person’s initial DCIS breast cancer diagnosis. Using generalized linear models with a log link and Poisson distribution, we estimated relative rates (RRs) of ET use (vs none) over time and by demographic, tumor, and other treatment characteristics. Results: Among 1,782 pre- and postmenopausal people, 577 (32%) initiated ET within 1 year of diagnosis. From 2001-2016, ET use did not change over time (34% vs 36%), tamoxifen use declined slightly (34% vs 29%), and aromatase inhibitor use increased slightly (0% vs 7%). People were less likely to initiate ET if they were ≥65 years, Black, had borderline or unknown ER status, were not tested for ER, had breast conserving surgery (BCS) without radiation, or had a unilateral mastectomy. Conclusions: Only one-third of people with DCIS diagnosed in a community healthcare setting and potentially eligible for ET, initiated treatment within 1 year of their diagnosis. Previous studies have shown long-term benefits of ET use in people with DCIS; however, we saw no increase in use over time. Concerns about side-effects, overdiagnosis and overtreatment of DCIS, and confusion about treatment guidelines may explain relatively low ET initiation.

¹Adjusted for diagnosis age and year, ER status, and study site; ²Also adjusted for surgery.