Purpose: Neoadjuvant endocrine treatment (NET) has become a useful tool for the downstaging of luminal-like breast cancers in postmenopausal patients. It enables us to increase breast- conserving surgery (BCS) rates, provides an opportunity for us to assess in vivo NET effectiveness, and allows us to study any biological changes that may act as valid biomarkers. The purpose of this study was to evaluate the safety and effectiveness of NET, and to assess the role of Ki67 proliferation rate changes as an indicator of endocrine responsiveness. Methods: From 2016 to 2020, a single-institution cohort of patients, treated with NET and further surgery, was evaluated. In patients with Ki67 ≥ 10%, a second core biopsy was performed after four weeks. Information regarding histopathological and clinical changes was gathered. Results: A total of 115 estrogen receptor-positive (ER+)/HER2-negative patients were included. The median treatment duration was 5.0 months (IQR: 2.0–6.0). The median maximum size in the surgical sample was 40% smaller than the pretreatment size measured by ultrasound (p < 0.0001). The median pretreatment Ki67 expression was 20.0% (IQR: 12.0–30.0), and was reduced to 5.0% (IQR: 1.8–10.0) after four weeks, and to 2.0% (IQR: 1.0–8.0) in the surgical sample (p < 0.0001). BCS was performed on 98 patients (85.2%). No pathological complete responses were recorded. A larger Ki67 fold change after four weeks was significantly related to a PEPI score of zero (p < 0.002). No differences were observed between luminal A- and B-like tumors, with regard to fold change and PEPI score. Conclusions: In our cohort, NET was proven to be effective for tumor size and Ki67 downstaging. This resulted in a higher rate of conservative surgery, aided in therapeutic decision making, provided prognostic information, and constituted a safe and well-tolerated approach.

Purpose: The aim of this study is to describe our initial experience using magnetic seeds (Magseed^®) to guide breast-conserving surgery in non-palpable breast lesions and compare the use of magnetic seed with wires to guide breast-conserving surgery in terms of clinical and pathological characteristics. Methods: We performed a retrospective study including all breast-conserving surgeries for non-palpable breast lesions under 16 mm from June 2018 to May 2021. We compared breast-conserving surgeries guided with magnetic seeds (Magseed^®) to those guided with wires, analyzing tumor and patient characteristics, surgical time, and pathological results of the surgical specimens. Results: Data from 225 cases were collected, including 149 cases guided by magnetic seeds and 76 cases guided by wires. The breast lesion was localized in every case. Both cohorts were similar regarding clinical and pathological characteristics. We found significant statistical differences (p < 0.02) in terms of the median volume (cm³) of the excised specimen, which was lower (29.3%) in the magnetic seed group compared with the wire group (32.5 [20.5–60.0]/46.0 [20.3–118.7]). We did not find significant differences regarding surgical time (min) or the affected or close margins. Conclusion: In our experience, the use of magnetic seed (Magseed^®) is a feasible option to guide breast-conserving surgery of non-palpable lesions and enabled us to resect less breast tissue.

Purpose: Neoadjuvant endocrine treatment (NET) has become a useful tool for the downstaging of luminal-like breast cancers in postmenopausal patients. It enables us to increase breast conserving surgery (BCS) rates, and provides an opportunity for assessing in vivo NET effectiveness and studying any biological changes that may act as valid biomarkers. The purpose of this study was to evaluate the safety and effectiveness of NET, and to assess the role of Ki67 proliferation rate changes as an indicator of endocrine responsiveness.Methods: From 2016 to 2020, a single-institution cohort of patients treated with NET and further surgery was evaluated. In patients with Ki67≥10%, a second core biopsy was performed after four weeks. Information regarding histopathological and clinical changes was gathered.Results: A total of 115 estrogen receptor positive (ER+)/HER2 negative patients were included. The median treatment duration was 5.0 months (IQR: 2.0-6.0). Median maximum size in the surgical sample was 40% smaller than pretreatment size measured by ultrasound (p<.0001). Median pretreatment Ki67 expression was 20.0% (IQR: 12.0-30.0), and was reduced to 5.0% (IQR: 1.8-10.0) after four weeks, and to 2.0% (IQR: 1.0-8.0) in the surgical sample (p<.0001). BCS was performed on 98 patients (85.2%). No pathological complete responses were recorded. A larger Ki67 fold-change after four weeks was significantly related to a PEPI score of 0 (p<.002). No differences were observed between luminal A- and B-like tumors with regard to fold-change and PEPI score.Conclusions: In our cohort, NET has proven effective for tumor size and Ki67 downstaging. This results in a higher rate of conservative surgery, aids in therapeutic decision-making, provides prognostic information, and constitutes a safe and well-tolerated approach

Purpose: Neoadjuvant endocrine treatment (NET) has become a useful tool for the downstaging of luminal-like breast cancers in postmenopausal patients. It enables us to increase breast conserving surgery (BCS) rates and provides an opportunity for assessing in vivo NET effectiveness and studying any biological changes that may act as valid biomarkers. The purpose of this study was to evaluate the effectiveness of NET as well as to assess the role of Ki67 proliferation rate changes as an indicator of endocrine responsiveness. Methods: From June 2016 to January 2022, a single-institution cohort of patients treated with NET and further surgery was evaluated. In patients with Ki67≥10%, a second core biopsy was performed after four weeks. Information regarding histopathological and clinical changes, as well as surgical management, was gathered. Results: A total of 168 estrogen receptor positive (ER+)/HER2 negative patients were included. The median age at diagnosis was 69.5 years old (IQR: 16.0). The median treatment duration was 5.0 months (IQR: 4.0). Median maximum size in the surgical sample was 44.0% smaller than pretreatment size measured by ultrasound (p<.0001), showing an inverse linear relationship with treatment duration. Median pretreatment Ki67 expression was 20.0% (IQR: 18.0) and was reduced to 5.0% (IQR: 8.0) after four weeks, and to 2.0% (IQR: 7.25) in the surgical sample (p< 0.0001). Other significant downgrading changes were observed with respect to tumor grade (p< 0.0001) and progesterone receptor (PR) expression (p< 0.0001). BCS was performed on 145 patients (86.3%). One case of pathological complete response was recorded. A larger Ki67 fold-change after four weeks was significantly related to a PEPI score of 0 (p< 0.002). No differences were observed between luminal A- and B-like tumors with regard to fold-change and PEPI score. No treatment abandonment was produced during the study. Conclusions: In our cohort, NET has proven effective for tumor size and Ki67 downstaging. This results in a higher rate of conservative surgery, aids in therapeutic decision-making, provides prognostic information, and constitutes a safe and well-tolerated approach Citation Format: Covadonga Marti, Laura Frias, Adolfo Loayza, Elisa Moreno, Marcos Meléndez, Jose Ignacio Sanchez-Mendez. Real World Data on Neoadjuvant Endocrine Treatment and Ki67 Assessment in ER+/HER2- Breast Cancer. Results from a Single-Institution Prospective Cohort Study [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P5-09-03.

Human epidermal growth factor receptor 2 (HER2)-enriched breast cancers (BC) present the highest rates of pathological response to primary systemic therapy (PST), but they are also the ones that tend to be larger at diagnosis, with microcalcifications and, often, with axillary involvement. If we do not have a reliable method to predict the degree of response, we may not be able to transfer the benefits of PST to surgery. The post-PST surgery planning is guided by the findings in the magnetic resonance imaging (MRI), whose predictive capacity, although high, is far from optimal. Thus, it seems interesting to find other variables to improve it. A retrospective observational study including women with HER2 BC treated with PST and further surgery was conducted. Information regarding clinical, radiological, and histopathological variables was gathered from a total of 132 patients included. Radiological complete response (rCR) was achieved in 65.9% of the sample, and pathological complete response (pCR), according to Miller and Payne criteria, in 58.3% of cases. A higher Ki67 value, the absence of Hormonal Receptors expression, and an rCR was significantly related to a pCR finding. This information impacts directly in surgery planning, as it permits adjustment of the breast resection volume.