Abstract PS4-12-20: Regional Variation in Guideline Concordance for Women with Triple-Negative and HER2+ Breast Cancer in Nova Scotia

Abstract Background: Triple-negative (TNBC) and human epidermal growth factor receptor 2 positive (HER2+) breast cancers are associated with an elevated risk of metastatic tropism and poor outcomes. Since 2020, systemic neoadjuvant therapy (NAT) has been recommended for both forms of the disease when tumor size at diagnosis is equal to or over 2 cm. The objective of this study was to quantify the proportion of patients in Nova Scotia (NS) with resectable TNBC and HER2+ breast cancer that received guideline concordant neoadjuvant therapy. Methods: We conducted a retrospective analysis using 2021-2023 data from the NS Breast Screening Program. Adult patients (18-80 years) with an incident diagnosis of non-metastatic, ≥T2, triple negative or HER2+ breast cancer were considered theoretically eligible for NAT and included in the analysis. The neoadjuvant intervention was considered given if the patient had a wait-time of ≥4 months between tissue diagnosis and surgery. Our primary outcome was the proportion of theoretically eligible patients who received guideline-concordant NAT. Secondary outcomes included the proportion of patients who experienced a pathologic complete response (pCR). Receipt of guideline concordant care was compared over time (2021-2023) and between health system administrative zones (Central, Western, Northern, Eastern). Results: A total of 291 women were included, with a median age of 59 (IQR: 48-67). Forty-five percent were treated in the Central Zone. Of the cohort, 49.5% were TNBC and 50.52% were HER2+. Overall, 64.6% of eligible patients appear to have received NAT. There were differences in receipt of NAT between administrative zones (Central 70.8%, Eastern 69.5%, Northern 55.6%, Western 52.1%, p=0.046). The overall pCR for the cohort was 37.8% and the response was consistent across the zones (% range: 34.2%-40.0%, p=0.951). Conclusion: This study highlights regional disparities in the adherence to NAT guidelines for TNBC and HER2+ breast cancers. Future efforts should focus on strategies to improve NAT guideline adherence, which will improve outcomes for NS breast cancer patients. Citation Format: H. Stewart, A. Mayo, T. Li, R. Hemsworth, G. Knapp, A. Drohan. Regional Variation in Guideline Concordance for Women with Triple-Negative and HER2+ Breast Cancer in Nova Scotia [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS4-12-20.