DCIS, a non-invasive condition in which abnormal cells are confined to the milk ducts, accounts for roughly one in four breast cancers detected through screening programmes. Standard treatment options include mastectomy or breast-conserving surgery (BCS), with or without adjuvant radiotherapy, and in some cases endocrine therapy with drugs such as tamoxifen or aromatase inhibitors to reduce the risk of recurrence. Because approximately 65 to 80 percent of DCIS lesions are oestrogen receptor positive, knowing a tumour’s ER status is central to deciding whether hormone therapy is appropriate. Yet, as the new study demonstrates, ER testing for DCIS was performed in only 15.4 percent of the 5,813 women included in the cohort.

The researchers drew their data from the Breast Cancer Foundation National Register, which began collecting information in Auckland and Waikato in 2000, expanded to Christchurch in 2009 and Wellington in 2010, and achieved nationwide coverage only from 2020 onwards. By linking the register to New Zealand’s national Pharmaceutical Collection database through encrypted National Health Index numbers, the team could identify who was dispensed tamoxifen or aromatase inhibitors after a DCIS diagnosis. Statistical analyses employed multivariable logistic regression to identify factors associated with ER testing and cumulative incidence functions, with death treated as a competing risk, to estimate the probability of breast cancer events over time.

The headline finding is geographic. Of the 894 women who had an ER test on record, 65.6 percent were treated in the Waikato region, home to a well-established breast cancer research infrastructure and one of New Zealand’s two original breast screening pilot programmes. In Waikato, ER testing rose sharply from 35 percent in the late 1990s to 82.4 percent by 2003 and remained high, reaching 84.8 percent in 2022. Everywhere else, testing stayed stubbornly low, creeping from just 4.5 percent in 2000 to 7.1 percent in 2022. The investigators attribute Waikato’s exceptional performance to the participation of local clinical centres in international trials evaluating endocrine therapy for DCIS, including the UK/ANZ DCIS trial and the IBIS-II DCIS trial, which exposed clinicians there to the evidence base early and embedded testing into routine care.

The factors associated with receiving an ER test also differed dramatically by region, offering a window into contrasting models of care. In Waikato, women who received adjuvant radiotherapy after breast-conserving surgery were more likely to be tested, a pattern the authors interpret as reflecting local protocols that promoted both treatments in tandem rather than selective ordering by individual clinicians. Elsewhere in the country, testing appeared highly selective: older women, Māori women, those presenting with symptoms rather than through screening, and those with larger DCIS lesions exceeding 20 millimetres were more likely to be tested, suggesting clinicians ordered the test only when they believed the result would influence management. Nationally, the overall testing rate rose from 11.1 percent in 2000 to a peak of 23.3 percent in 2006 before drifting back down to 13.9 percent in 2022.

Among the 729 women whose DCIS was confirmed as ER positive, only 183, or 25.1 percent, actually initiated endocrine therapy, and more than 80 percent of those initiations occurred in Waikato. Tamoxifen was the most common first-line treatment, used by 58.5 percent of women starting therapy, followed by aromatase inhibitors at 38.3 percent. Consistent with shifts seen internationally after the NSABP B-35 trial suggested anastrozole outperformed tamoxifen in postmenopausal women, aromatase inhibitor use in Waikato climbed steadily, exceeding tamoxifen after 2018 and rising from 12.5 percent of initiations in the mid-2000s to 55 percent by 2022. Overall initiation of endocrine therapy in Waikato grew from 20 percent in 2000–2001 to 62.5 percent in 2022.

Does the hormone therapy actually help? After a median follow-up of 4.8 years, women with ER-positive DCIS who received endocrine therapy showed the lowest cumulative risk of any breast cancer event, but the differences between groups did not reach statistical significance, likely because the number of treated patients and events was simply too small to detect a benefit. The authors caution that these results do not exclude a clinically meaningful effect. Supporting evidence from prior research suggests that good adherence reduces recurrence, that at least two years of treatment decreases second breast events, and that adding endocrine therapy to breast-conserving surgery lowers the risk of subsequent invasive breast cancer compared with surgery alone. For a subset of Waikato women with documented therapy use of at least two years, the five-year cumulative incidence of a breast cancer event was just 6.3 percent, though this estimate rested on very few events.

The international context makes New Zealand’s numbers stand out even more sharply. In the United States, ER testing for DCIS surged from 17.5 percent in the early 2000s to 93.1 percent by 2012–2014, propelled by the 2000 approval of tamoxifen for DCIS following the NSABP B-24 trial. European testing rates range from 30 to 85 percent. Since 2020, both the ASCO/CAP guideline and the National Comprehensive Cancer Network have recommended ER testing for all DCIS patients to guide endocrine therapy decisions. New Zealand’s own early breast cancer guideline, last updated in 2009, recommends endocrine therapy for ER-positive invasive cancers but leaves the decision for ER-positive DCIS to individual circumstances, leaving clinicians without a clear national mandate for testing.

The study’s authors argue that the regional variation they documented is not an argument for exporting Waikato’s model wholesale, but rather evidence that nationally consistent guidelines and clinical pathways are urgently needed. Without standardised ER testing, they note, clinicians and patients cannot make informed decisions about whether the potential recurrence-reduction benefits of endocrine therapy, weighed against side effects such as aromatase inhibitor-related musculoskeletal symptoms, are worth pursuing in an individual case. Options such as low-dose tamoxifen, which showed comparable efficacy with improved tolerability in the TAM-01 trial, may widen the appeal of treatment for patients prioritising quality of life. Greater clinician participation in clinical trials, the researchers suggest, could simultaneously strengthen local evidence and keep practitioners engaged with emerging data.

Limitations temper some conclusions. Registry data capture began at different times in different regions, and nationwide coverage was achieved only in 2020, making endocrine therapy initiation numbers outside Waikato too sparse for trend analysis. Patient preferences regarding adjuvant therapy were unavailable, and the observed higher likelihood of testing among Māori women outside Waikato may reflect unmeasured local practice patterns rather than true differences in tumour biology. Nevertheless, as the first population-based study of ER testing and endocrine therapy for DCIS in New Zealand, the work delivers a clear message: a two-decade evidence gap persists in routine care, and closing it will require updated national guidelines, standardised receptor testing, and a broader culture of clinical research participation to ensure that women with DCIS everywhere benefit from treatments proven in trials.

Subject of Research: Oestrogen receptor testing and adjuvant endocrine therapy use in ductal carcinoma in situ in New Zealand

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

Article References: Oestrogen receptor testing and initiation of adjuvant endocrine therapy in women with ductal carcinoma in situ: a population-based study. (n.d.). https://doi.org/10.1007/s10549-026-08072-7

Image Credits: AI Generated

DOI: 10.1007/s10549-026-08072-7

Keywords: ductal carcinoma in situ, oestrogen receptor testing, endocrine therapy, tamoxifen, aromatase inhibitors, breast cancer, New Zealand, regional variation, population-based study, clinical trials, recurrence, guidelines