Abstract PO4-17-06: Real World Data: A single institute experience with neoadjuvant endocrine therapy for ER+ breast cancer — clinical response rates, clinical predictors of response and long-term outcomes following breast conserving surgery

Abstract Background: Neoadjuvant endocrine therapy (NET) in postmenopausal women (PMW) with large or locally advanced oestrogen receptor (ER)-rich breast cancer allows more women to be treated by breast-conserving surgery (BCS). A comprehensive analysis of the factors affecting clinical response to NET and the long-term safety of this strategy has been studied. Patients: A retrospective cohort was studied of 435 PMW (median age: 77 years; range: 50-98) with ER-positive breast cancer treated with NET (aromatase inhibition) (median duration: 6.8 months; range 3-43 months), between 2001-15. Clinical response was monitored throughout NET treatment with periodic 3D ultrasound measurements of the primary tumour. All patients went on to have surgery, 55% had adjuvant radiotherapy and all had adjuvant ET for at least 5 years. All patients had routine follow-up (median follow-up 12.3 years). To-date, this represents the largest cohort of patients treated with 3 months NET. Results: Clinical response to NET was excellent, with only 3.2% of tumours progressing on-treatment based on RECIST 1.1 criteria. Mean percentage reduction in tumour on NET was 71%. Response rate (partial/complete response) was 59% and response continued until 12 months and then levelled off. Following NET, over 92% of patients were suitable for BCS and 61% achieved clinical TNM down-staging. Multivariate analysis revealed that only ER level (response highest in patients with Allred scores 7 and 8), and percentage change in tumour volume by 6 weeks were significant predictors of overall clinical response to NET. Clinical response to NET was not associated with node status or tumour grade. BCS following NET had a local disease control rate of 89% (95%CI±0.06) at 10 years, but actuarial local recurrence at 10 years was only 7% in those having radiotherapy compared with 30% in the group who did not receive radiotherapy (P 0.0001). In addition, the overall recurrence rate (P=0.028) was improved by radiotherapy irrespective of nodal status. Radiotherapy did not improve overall breast cancer-specific survival (BCSS). Systemic recurrence and BCSS was higher with greater nodal involvement. Most patients in this study died of other causes (14-year crude all-cause mortality was 71%). Conclusion: NET followed by BCS is an effective strategy in PMW with ER-rich breast cancer. Excellent clinical response rates are seen even in women with node-positive or high-grade disease. Less than 5% showed evidence of progression. Early response to NET predicts for long-term tumour volume reduction and NET can achieve continued tumour shrinkage for up to 1 year. NET followed by BCS appears a safe treatment approach and offers good long-term outcomes when BCS is followed by adjuvant radiotherapy. Citation Format: Arran K Turnbull, Carlos Martinez-Perez, Charlene Kay, Rebecca Swan, Lorna Renshaw, J Michael Dixon. Real World Data: A single institute experience with neoadjuvant endocrine therapy for ER+ breast cancer — clinical response rates, clinical predictors of response and long-term outcomes following breast conserving surgery abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-17-06.