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Background: In order to determine the optimal treatment strategy for oligometastatic breast cancer (OMBC), effective and safe treatments for metastatic sites and sensitive and specific imaging techniques are needed. But it is also essential to know the incidence of oligometastatic breast cancer and its clinical and biological characteristics [1]. Efficient imaging techniques and therapeutic tools exist, but knowledge of incidence, clinical and biological characteristics of OMBC is scarce. This is partly due to the lack of publications describing these data on recent, consecutive, and unselected series of OMBC. Methods: we retrospectively collected data from 998 patients diagnosed with synchronous or metachronous metastatic breast cancer (MBC) between January 2014 and December 2018 at our institution. The only criterion used to define OMBC was the presence of one to five metastases at diagnosis. Hormone receptor (HR) and HER2 receptor status, histology, SBR grade, number of metastases and organs affected were collected. Results: Of 998 MBC, 15.8% were OMBC (158/998). Among the series, 88% (139/158) of OMBC had 1 to 3 metastases and 86.7% (137/158) had only one organ involved. Among 158 patients, 52.5% (n=83) had bone metastases, 20.9% (n=33) had lymph node metastases, 14.6% (n=23) had liver metastases, 13.3% (n=21) had brain metastases, 8.2% (n=13) had lung metastases, and 3.8% (n=6) had others (skin, pancreas, adrenal). Among these 158 patients, 83.4% (n=131) had ductal breast carcinoma, 55.7% (n=88) had HR+/HER2- OMBC, 25.3% (n=40) had HER2+ OMBC and 19% (n=30) had HR-/HER2- OMBC. HR+/HER2- subtype was statistically associated with bone and bone only metastases (p=0.001), HER2+ subtype with brain metastases (p=0.001) and HR-/HER2- subtype with lymph node metastases (p=0.008). Visceral metastases (lung or liver) are not statistically associated with any biological subtypes. The proportion of OMBC with SBR grade III was statistically higher than in a series of 22,109 patients with MBC [2] (49.4% vs 35.2%; p< 0.001). Conclusion: OMBC is a heterogeneous entity. OMBC incidence is certainly much higher than the commonly used values. OMBC is not an indolent disease, and each subgroup, according to its biological and anatomical characteristics, may deserve a specific management. [1] Hellman S, Weichselbaum RR. Oligometastases. J Clin Oncol Off J Am Soc Clin Oncol 1995;13:8–10. https://doi.org/10.1200/JCO.1995.13.1.8. [2] Deluche E, Antoine A, Bachelot T, Lardy-Cleaud A, Dieras V, Brain E, et al. Contemporary outcomes of metastatic breast cancer among 22,000 women from the multicentre ESME cohort 2008–2016. Eur J Cancer 2020;129:60–70. https://doi.org/10.1016/j.ejca.2020.01.016. Citation Format: Jean Louis LACAZE, Clémence Brac de la Perrière, Mony Ung, Florence Dalenc, Vincent Nicolai, Eleonore De Maio, Marion Montastruc, Bastien Cabarrou, Nils Monselet, Ciprian Chira, Gauthier Glemarec, Thibaut Cassou-Mounat. Clinical and biological features of 158 consecutive and unselected oligometastatic breast cancers [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 P1-05-25.

Purpose: Data about incidence, biological and clinical characteristics of oligometastatic breast cancer (OMBC) are scarce. However, these data are essential in determining optimal treatment strategy. Gaining knowledge of these elements means observing and describing large, recent, and consecutive series of OMBC in their natural history. Methods: We collected data retrospectively at our institution from 998 consecutive patients diagnosed and treated with synchronous or metachronous metastatic breast cancer (MBC) between January 2014 and December 2018. The only criterion used to define OMBC was the presence of one to five metastases at diagnosis. Results: Of 998 MBC, 15.8% were classified OMBC. Among these, 88% had one to three metastases, and 86.7% had only one organ involved. Bone metastases were present in 52.5% of cases, 20.9% had progression to lymph nodes, 14.6% to the liver, 13.3% to the brain, 8.2% to the lungs, and 3.8% had other metastases. 55.7% had HR+/HER2- OMBC, 25.3% had HER2+ OMBC, and 19% HR-/HER2- OMBC. The HR+/HER2- subtype statistically correlated with bone metastases (p=0.001), the HER2+ subtype with brain lesions (p=0.001), and the HR-/HER2- subtype with lymph node metastases (p=0.008). Visceral metastases were not statistically associated with any OMBC subtypes (p=0.186). OMBC-SBR grade III was proportionally higher than in the ESME series of 22,109 MBC (49.4% vs. 35.2%, p< 0.001). Conclusion: OMBC is a heterogeneous entity whose incidence is higher than has commonly been published. Not an indolent disease, each subgroup, with its biological and anatomical characteristics, merits specific management.

3575 Background: BRAF mutations are present in 5-15% of mCRC. V600E BRAF mutations account for ~80% of cases and are mostly found in right-sided tumors. Non-V600E BRAF mutations are rare (~2% of mCRC), mostly left-sided. Although BRAF V600E mutations are associated with a dismal prognosis, some studies suggest that non-V600E BRAF mutations may be associated with a favorable outcome. The chemosensitivity of non-V600E BRAF-mutated mCRC has never been studied. Methods: From 2017 to 2018, all consecutive patients (pts) with non-V600E BRAF-mutated mCRC (next generation sequencing) treated in the participating centers were included. Survival analyses were performed using Kaplan-Meier method and LogRank test. Results: A total of 108 pts in 34 centers in France were included between October 2017 and August 2018 (median age, 66 years [range, 58-77]; ECOG performance status 0-1, 86%). The primary was mostly left-sided (66%). Main metastatic sites were the liver (73%), lungs (33%), lymph nodes (39%) and peritoneum (26%). D594 (34%), G469 (15%), K601 (11%), N581 (7%) and L597 (7%) were the most frequent mutations. A concomitant RAS mutation was found in 22% of pts. Microsatellite instability (MSI) was found in 3/67 pts (4.5%). First-line chemotherapy (CTx) (n = 69) efficacy was (overall response rate/disease control rate) 49%/77% (anti-EGFR-containing CTx [n = 20], 75%/85%; antiangiogenic-containing CTx [n = 22], 55%/73%). Median overall survival (mOS) was 25.6 months (95% CI : 17.1-43.8) overall; it was 8.0 months with best supportive care alone (n = 10), 16.0 months with palliative CTx alone (n = 63), and attained 105.1 months with curative-intent management of metastases (n = 35). mOS did not differ according to sidedness (p = 0.22), type of mutation (p = 0.52), or its functional impact on BRAF (p = 0.19). Conclusions: Non-V600E BRAF-mutated mCRC retain sensitivity to CTx + biologics and harbor a good prognosis (especially when amenable to curative-intent surgery), regardless of the type of mutation and its impact on BRAF kinase function. Contrarily to BRAF V600E mutations, non-V600E mutations may occur along with RAS mutations, but uncommonly with MSI.