Oestrogen receptor testing and initiation of adjuvant endocrine therapy in women with ductal carcinoma in situ: a population-based study

Introduction Use of adjuvant endocrine therapy (ET) for ductal carcinoma in situ (DCIS) varies widely in clinical practice. We investigated oestrogen receptor (ER) testing, the initiation of ET, and the effect of ET on outcomes for DCIS in New Zealand. Methods Women with DCIS (2000–2022) were identified from the New Zealand Breast Cancer Foundation National Register and linked to the national pharmaceutical data. Logistic regression identified factors associated with ER testing, and cumulative incidence of breast cancer events was estimated with death as a competing risk. Results Among 5813 women with DCIS, 894 patients (15.4%) had ER testing, with the rate increasing from 11.1% in 2000 to 23.3% in 2006, before declining to 13.9% in 2022. The Waikato region contributed 65.6% of ER tests. In Waikato, testing was more likely among women who received adjuvant radiotherapy after breast-conserving surgery, likely reflecting a site-specific clinical practice. In other regions, testing was more frequent among older women, Māori ethnicity, those with symptomatic presentation, and those with DCIS size > 20 mm, potentially reflecting selective testing. Among 729 patients with ER-positive DCIS, only 183 (25.1%) commenced ET, of whom 80.3% were in Waikato. ET users had the lowest cumulative risk of breast cancer event, but differences were not statistically significant. Conclusions ER testing and initiation of ET for DCIS remained low, except in Waikato, where clinical centres participated in ET trials and ER testing became more common. Standardised ER testing and greater clinician participation in clinical trials may lead to improved evidence-based care. Clinical trial number Not applicable.