Abstract PS4-11-29: Pathological Complete Response and Breast-Conserving Surgery in Patients Undergoing Neoadjuvant Chemotherapy for Breast Cancer in an Institution fully Funded by the Brazilian Public Health System in Bahia, Brazil

Abstract Introduction: Breast cancer (BC) incidence has increased in developing countries, showing high mortality rates despite lower incidence compared to developed nations. In 2023, 73,610 new BC cases were reported in Brazil, with 80% of patients relying solely on the Brazilian public health system (SUS) to get treated. Limited early detection in primary care contribute to 40% of cases being diagnosed at advanced stages. The prevalence of aggressive subtypes such as triple-negative (TN) and HER2+ (HER2) differ between regions in the country. The state of Bahia has a predominantly Black/mixed-race population (81%), however, data on tumor subtype distribution and neoadjuvant treatment outcomes are scarce. Objective: To evaluate pathological complete response (pCR) rates in the breast and axilla, following neoadjuvant chemotherapy (NACT) in BC patients treated at an institution fully funded by SUS in Bahia. Secondary objectives included assessing pCR rates by tumor subtype and breast-conserving surgery (BCS) rates. Methods: This case series study included 286 women who underwent surgery after NACT between Jan/2018 and Dec/2021. Patients with multifocal/multicentric tumors, bilateral BC, metastasis, or who received NACT elsewhere were excluded. Tumors were categorized into molecular subtypes (luminal, TN, HER2/HR+, HER2/HR−). Use of trastuzumab with number doses were documented for HER2 cases. Surgeries were classified as conventional BCS, oncoplastic BCS, or mastectomy (with/without reconstruction). pCR was defined as the absence of invasive disease in the breast and axilla. Results: Mean age was 51 ± 12 years; 82% self-identified as Black or mixed-race. Tumor subtypes: 41% luminal, 31% TN, 13% HER2/HR+, 15% HER2/HR−. TN was more frequent in mixed-race (36%), and HER2 in Black women (33%). 66% had locally advanced disease; 93% received ACT-based chemo. Among HER2+ cases, 63% received trastuzumab, but only 32% got 4+ doses. Overall, pCR was 22% and by subtype: HER2/HR− (49%), TN (26%), HER2/HR+ (25%), luminal (8%), by race: Black (27%), mixed-race (20%), White (17%). Mastectomy was done in 64% (19% with reconstruction); 36% had BCS, with 22% using oncoplastic techniques. In non-inflammatory cases, 42% had conservative surgery. Among cT3 patients with BCS, 41% had reconstructive surgery. Considering Luminal subtype, oncoplastic techniques were used in 36% of BCS. Discussion: This is the first study to assess pCR in a predominantly Black/mixed-race population in Brazil. The high prevalence of aggressive tumor subtypes and advanced-stage diagnoses likely influenced the pCR outcomes. Despite the recommendation for trastuzumab use since 2013, access remains limited and inconsistent in the SUS. Compared to international studies, pCR rates by subtype were lower, probably reflecting disparities in access to modern targeted therapies and immunotherapy. Higher pCR among Black/mixed-race women is likely linked to the greater frequency of responsive subtypes. BCS occurred in 42% of non-inflammatory breast cancer cases, lower than international figures. Oncoplastic techniques helped expand BCS eligibility in cT3 tumors and also in Luminal tumors, which tend to shrink less, benefiting from these surgical techniques. Conclusion: This study provides important insight into breast cancer treatment outcomes in an institution served exclusively by SUS in Brazil in a region with a predominantly Black/mixed-race population. The findings emphasize the urgent need for public health strategies that ensure earlier diagnosis and improve equitable access to targeted therapies, aiming to reduce disparities and improve outcomes for underserved populations. Citation Format: D. C. Camarotti, E. D. Moreira, M. F. Assunção. Pathological Complete Response and Breast-Conserving Surgery in Patients Undergoing Neoadjuvant Chemotherapy for Breast Cancer in an Institution fully Funded by the Brazilian Public Health System in Bahia, Brazil 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 PS4-11-29.