(C-082) Trends in the Utilization and Persistence on Adjuvant Hormonal Therapy among Women with Invasive Breast Cancer in the United Kingdom, 2004 to 2023.

Background: Adjuvant hormonal therapy (AHT) using tamoxifen or aromatase inhibitors (AI) improves long-term survival for women with hormone receptor-positive breast cancer. Over the past two decades, guideline changes have favoured the use of AIs in postmenopausal and some premenopausal women, longer treatment, and an upfront AI treatment strategy in postmenopausal women. Nevertheless, the extent to which these changes have impacted clinical practice in the United Kingdom (UK) remains unclear.

Objectives: To examine annual trends in AHT initiation among newly diagnosed women with invasive breast cancer in the UK from 2004 to 2023, stratified by age; assess trends in upfront versus sequential AI therapy in postmenopausal women; and evaluate changes in AHT duration over time.

Methods: We used the UK Clinical Practice Research Datalink to identify women aged ≥ 18 years newly diagnosed with breast cancer from 2004 to 2023. We calculated annual AHT initiation rates in women within one year of diagnosis, the prevalence of tamoxifen versus AI (anastrozole, letrozole, and exemestane) use, and the annual use of sequential versus upfront AI therapy among postmenopausal women. Upfront AI use was defined as initiation without prior tamoxifen, while sequential use followed it. Treatment persistence (continuous use with a 90-day grace period) was assessed overall and by treatment era (2004-2013 and 2014-2023) using the Aalen-Johansen estimator.

Results: Of 238,480 women newly diagnosed with breast cancer (mean age: 48.6 years, SD: 5.6), 176,567 (74.0%) initiated AHT, with a median initiation time of 2.5 months (IQR: 1.3, 6.2). Over a median follow-up of 5.3 years (IQR: 2.2, 9.7), 85,102 (35.7%) used tamoxifen, and 133,314 (55.9%) used an AI. Annual AHT initiation rates among newly diagnosed women remained stable (~55%). Among postmenopausal women, AHT initiation with AI increased from 13% in 2004 to 45% in 2023, while tamoxifen initiation declined from 44% to 8%. In premenopausal women, tamoxifen initiation held steady at ~30% annually, while AI initiation grew from 3% to 12%. Of 117,616 postmenopausal AI users, 83.9% initiated it upfront, while 16.1% used it sequentially. Upfront AI use increased from 81.9% in 2004 to 94.3% in 2023, with a decline in sequential strategy use. The median duration of any AHT was 5.2 years (IQR: 2.1, 5.5), with persistence rates of 96.7%, 83.9%, and 40.2% at 1, 5, and 10 years. The corresponding rates were 97.3%, 85.1%, and 40.0% in the earlier era, and 96.0%, 82.8%, and 40.9% in the later era.

Conclusions: AI use has increased among premenopausal women, and the upfront AI strategy has become predominant in postmenopausal women. The 10-year persistence rates weakly suggest a longer duration of AHT in recent times.