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Background: Recent data suggest that HER2-low breast cancer (BC) may represent a distinct entity. Approximately 55-60% of BC are considered as Her2-low, of which 80% are Luminal-like tumors. Recent studies support potential clinic-pathological and molecular features differences between Estrogen Receptor (ER) positive HER2-low and ER positive Her2-0 disease. Among patients (pts) with high genomic risk (Oncotype DX RS) HER2-low expression was associated with a significant improvement in overall survival compared to Her2-0. The objective of our study is to compare disease characteristics and outcomes between HER2-low and Her2-0 in estrogen receptor (ER) positive, early (e) BC. (Murtai R et al. The Breast. 60(2021)62-69. Methods: A single center retrospective study of all pts. with ER positive, Her2 negative eBC, for whom Oncotype DX test was performed between 1/Nov/2012 and 14/Febr/2019. The pts were separated into HER2-low (immunohistochemistry (IHC) +1 or + 2 and in situ hybridization not amplified) or HER2-0. Clinic-pathological features included were: demographics, tumour size, nodal status, histologic grade, Her2 expression, ER and progesterone receptor (PR), Ki-67, presence of lymph-vascular (LV1) tumor cell invasion and Oncotype recurrence score (RS) result. Results: A total of 344 pts were screened, of whom 297 pts were included (Exclusion for: Her2 “negative” expression (45 pts); HER2 positive disease by IHC (1 pts); metastatic disease (1 pts)). The distribution of HER2-0 and HER2-low subgroups was 121 pts (41%) and 176 pts (59%). The pathological characteristics according to Her2 expression status are summarized in Table 1. Median age was 57 year (38-79), similar between both groups: Her2-0 58 yr (41-78) and 56 yr (38-79) in Her2 low. The postmenopausal status was 76 pts (62,8%) Her2-0 Vs 113 pts (64,2%) in Her2 low. 1 pts was male. Proliferation Index: Ki67% < 20 was: 65 pts (53,7%) Her2-0 Vs 86 pts (48,5%) in Her2 low; 65 pts (13,5%) had Lympho-vascular invasion in Her2 low; 115 pts (95%) were ER positive in Her2-0 Vs 157 pts (89,2%) in Her2 low; PR positive > 20 was 106 pts (87,6%) in Her2-0 Vs 137 (78,7%) in Her2 Low. The median follow-up was 72 month (SD+- 22,52). 5 pts had a recurrence: 4 pts were Her2 low (1 local and 3 distant metastasis) and 1 Her2-0 with distant metastasis. Most of the pts received adjuvant hormone therapy. There were no statistically significant differences between both groups owing to neither the clinic-pathologic features nor the recurrence score. It was not reached the mínimum number of events for a survival analysis. Conclusions: Our results show that HER2-low eBC pts have similar characteristics and survival rates compared to HER2-0 BC pts without significant differences. Table 1. Baseline pathological characteristics. Citation Format: Elena Galve, Fernando Pikabea-Diaz, Borja Lopez-de-San-Vicente-Hernandez, Covadonga Figaredo-Berjano, Jairo Legaspi-Folgueira, Maria Angeles Sala-Gonzalez, Juan Fernando Arango-Arteaga, Sara Fernandez-Ferrer, Pablo Leonardo Loaiza-Jaramillo, Pablo Casado-Cuesta, Marina Temino-Frances, Anne Bilbao-Penas, Purificacion Martinez-del-Prado. Clinical and pathologic characteristics in early breast cancer Her2-low and high risk Oncotype DX RS [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 P1-07-09.

Introduction: Oncotype DX (ODX) Recurrence Score (RS) is a gene-expression assay that provides prognostic and predictive information in hormonal-receptor positive (HR +), human epidermal growth factor receptor 2 negative (HER2-), axilary lymph node negative (LN-) disease. It estimates the 10 year-risk of distant recurrence and it is recommended to asses this issue and the potential benefit of adjuvant systemic therapy in early stage breast cancer (ESBC). (JCO 2016, NCCN 2019). Objective: To evaluate the impact of RS on our adjuvant treatment decision making and follow-up (FU). Materials and methods: Observational, prospective single institution study. Eligible patients (pts) were diagnosed between September 2012 and December 2018 with HR+, HER2 -, N0 and N+ (N1mic) ESBC for who underwent ODX assay. Criteria for ordered ODX assay were established by the group of experts from the Basque Country Health System. Treatment recommendations were documented before and after the conventional RS results. We reviewed clinical and pathology reports for FU and events. Descriptive statistics were used to summarize patient and tumor characteristics and changes in oncologists’ treatment recommendations. Mc Nemar’s test was used to identify how these tests impact in decisión. Results: N=337 pts. Median age was 57 years (SD 9,11); women 328 pts (97%); premenopausical 114 (34%); conservative surgery 271 pts (80%); sentinel node 318 pts (94 %); pN0: 248 pts (74%), pN1mic: 89 (26%); tumor size: pT1a-b: 41 pts (12 %), pT1c: 234 pts (69%) and pT2: 62 pts (18%); histologic type: ductal 280 pts (83%); histological grade: G1 49 pts (16%), G2 250 pts (74 %), G3 34 pts (10%); Ki 67 index >=13: 221 pts (66 %); progesterone receptors: positives: 304 pts (90%); presence of lymphovascular invasión:36 pts (11%). Treatment decision before assay was hormonal therapy (HT) in 242 pts (72 %) and chemotherapy (CT) in 95 pts (28 %). Conventional RS were: low risk RS30 (HR): 22 pts (7%). TAILORx RS cutpoints were: LR RS 25: 52 pts (15%). Post-assay treatment: 278 pts (82.5%) received HT and 59 pts (17.5 %) CT. Treatment recomendation pre and post ODX assay changed in 17,5% of pts: in 10,5% from CT to HT and 7% from HT to CT. The McNemar's test was significant value (0.000). With a median FU of 35 months (range 1-77) there were 5 events: (1.5%): 1 local (0.3%) and 4 distant (1.2%). Conclusions: In our cohort, the 82.5% of pts received HT. The treatment decisión changed 17.5% of pts. The ODX avoided CT in 10.5% of 38 pts (28% pre and 17.5% post). At this time, insufficient median FU and number of events are available to examine long-term prognosis and CT benefit. Citation Format: Maria Purificacion Martínez del Prado, Borja López de San Vicente, Juan Fernando Arango Arteaga, Jairo Legaspi Folgueira, Ane Zumárraga Cuesta, Fernando Pikabea Diaz, Patricia Novas Vidal, María López Santillán, Laura Sande Sardina, Maitane Nuño Escolástico, María Teresa Abad Villar, Maria Ángeles Sala Gonzalez, Covadonga Figaredo Berjano, María Teresa Pérez Hoyos, Elena Galve Calvo. Oncotype DX assay’s real world data in early stage breast cancer patients. Outcomes of a cohort treated in the Basurto University Hospital, Spain [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-14-05.