The Breast, Vol.68, Suppl.1 - April 2023

An Official Journal of the European Society for Medical Oncology An Associate Journal of the Australasian Society of Breast Disease Affiliated with European Society of Breast Cancer Specialists Official Journal of the Breast Centres Network THE BREAST Volume 68 Supplement 1 April 2023 ISSN 0960 9776 IN THIS SUPPLEMENT PRIMARY THERAPY OF EARLY BREAST CANCER ABSTRACTS OF THE 18TH ST.GALLEN INTERNATIONAL BREAST CANCER CONFERENCE 15–18 MARCH 2023

PRIMARY THERAPY OF EARLY BREAST CANCER Evidence, Controversies, Consensus 14th St.Gallen International Breast Cancer Conference Vienna, Austria, 18–21 March 2015 Conference Chairpersons Alan S. Coates, Melbourne, Australia Richard Gelber, Boston, USA Michel Gnant, Vienna, Austria Aron Goldhirsch, Lugano, Switzerland / Milan, Italy Martine Piccart, Brussels, Belgium Hans-Jo¨ rg Senn, St.Gallen, Switzerland Beat Thu¨ rlimann, St.Gallen, Switzerland 8th 15–18 March 2023, Vienn /Austria Understanding breast cancer complexity to improve patients outcomes Conference Co-Chairs: Univ. Prof. Dr. Michael Gnant, Medical University of Vienna / Austria Prof. Dr. med. Beat Thürlimann, Breast Centre, Kantonsspital St.Gallen / Switzerland Prof. Dr. Walter Weber, University Hospital Basel / Switzerland Scientific Programme-Chairpersons: Harold Burstein, Boston/MA, USA Giuseppe Curigliano, Milano/Italy Michael Gnant, Vienna/Austria Philip Poortmans, Antwerp/Belgium Meredith Regan, Boston/MA, USA Hans-Jörg Senn (†), St.Gallen/Switzerland Beat Thürlimann, St.Gallen/Switzerland Walter Weber, Basel/Switzerland Eric Winer, New Haven/CT, USA Conference Founder and Honorary Chair: Prof. Dr. med. Hans-Joerg Senn (* 28 March 1934, † 13 January 2023), Foundation St.Gallen Oncology Conferences (SONK), St.Gallen/Switzerland Publication of this Abstract Supplement is sponsored by St.Gallen Oncology Conferences

THE BREAST Editor-in-Chief Fatima Cardoso, Champalimaud Clinical Centre, Lisbon, Portugal Co-Editors Nehmat Houssami, University of Sydney, Sydney, Australia Giuseppe Curigliano, University of Milano, European Institute of Oncology, Milan, Italy Specialty Editor E. Negri, Milan, Italy N. Houssami, Sydney, Australia S. Delaloge, Paris, France F. Penault-Llorca, Clermont-Ferrand, France M.J. Cardoso, Lisbon, Portugal O. Gentilini, Milan, Italy E. Senkus-Konefka, Gdansk, Poland O. Pagani, Rennaz, Switzerland P. Poortmans, Paris, France S. Di Cosimo, Milan, Italy G. Pravettoni, Milan, Italy H. Rugo, San Fransisco, USA Epidemiology and prevention: Imaging, screening and early diagnosis: Familial and Hereditary Breast Cancer: Pathology: Surgery and Oncoplastic Surgery: Medical oncology: Radiation Oncology: Translational Research: Psycho-oncology and Quality of Life and Survivorship: Social Media Editor: E. Elder New South Wales, Australia C. Markopoulos Athens, Greece B. Thürlimann St Gallen, Switzerland H.J. Senn St Gallen, Switzerland C. Tinterri Milan, Italy Australasian Society for Breast Disease Representative EUSOMA – European Society of Breast Cancer Specialists Representative St Gallen Oncology Conferences Representatives Breast Centres Network Representative An Associate Journal of the Australasian Society for Breast Disease: Affiliated with the European Society of Breast Cancer Specialists: Official Journal of Breast Centres Network: Amsterdam • Boston • London • New York • Oxford • Paris • Philadelphia • San Diego • St Louis International Advisory Board K. Ruddy Rochester, USA V. Sacchini New York, NY, USA K. Sandelin Stockholm, Sweden F. Sardanelli Milan, Italy G. Schwartsmann Porto Alegre, Brazil V. Semiglazov St. Petersburg, Russia L. Solin Philadelphia, USA V. Speirs Leeds, UK E. Tagliabue Milan, Italy A. Tagliafico Genova, Italy D.A. Vorobiof Johannesburg, S.A. M.G. Wallis Cambridge, UK N. Williams London, UK B. Xu Beijing, China Y. Yarden Tel Aviv, Israel S. Zackrisson Malmö, Sweden J. Zˇ gajnar Ljubljana, Slovenia B. Anderson Seattle, USA B. Arun Houston, Texas, USA H.A. Azim Marseille, France G. de Bock Groningen, The Netherlands J. Bogaerts Brussels, Belgium E. Brain Saint-Cloud, France M. Brennan Sydney, Australia R.E. Coleman Sheffield, UK J. Cortes Barcelona, Spain A. Costa Milan, Italy T. Cufer Ljubljana, Slovenia R. Dent Prato, Italy V. Diéras, MD Rennes, France J. M. Dixon Edinburgh, Scotland S. Duffy London, UK L.E.M. Duijm Nijmegen, Netherlands N. El Saghir Beirut, Lebanon W. Gatzemeier Milan, Italy J.Gligorov Paris, France S. Hurvitz Santa Monica, USA S.-A. Im Seoul, Republic of Korea G. Jerusalem Liège, Belgium S.-B. Kim Seoul, Republic of Korea H. de Koning Eindhoven, The Netherlands S. Kyriakides Nicosia, Cyprus B. Linderholm Gothenburg, Sweden L. Lusa Ljubljana, Slovenia B. Mann Parkville, Australia L. Marinovich Bentley, Australia S. Nakamura Tokyo, Japan S. Ohno Fukuoka, Japan M. Paesmans Brussels, Belgium O. Pagani Lugano, Switzerland A. Partridge Boston, USA F.A Peccatori Milan, Italy F. Penault-Llorca Clermont-Ferrand, France K. Pritchard Toronto, Canada M.M Regan Boston, Massachusetts, USA I. Rubio Barcelona, Spain (†) St Gallen, Switzerland

The Breast: Aims and Scope The Breast is an international, multidisciplinary journal for researchers and clinicians, which focuses on translational and clinical research for the advancement of breast cancer prevention, diagnosis and treatment of all stages. 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St. Gallen Oncology Conferences (SONK) Rorschacherstrasse 150, 9006 St. Gallen / Switzerland, Primary Therapy of Early Breast Cancer Evidence, Controversies, Consensus 19th St. Gallen International Breast Cancer Conference 2025 11 – 15 March 2025, Vienna/Austria Save the date!

THE BREAST Speakers’ Abstracts Wednesday, 15 March 2023 Session 1: News in breast cancer care since SGBCC 2021 S1 Thursday, 16 March 2023 Session 2: Genomics, Transcriptomics, ctDNA for disease monitoring and risk stratification S2 Thursday, 16 March 2023 Session 3: Immunology in early breast cancer S3 Thursday, 16 March 2023 Special lecture I and panel discussion S3 Thursday, 16 March 2023 Session 4: Hereditary Breast Cancer S4 Thursday, 16 March 2023 Session 5: the future treatment landscape in the adjuvant setting S5 Friday, 17 March 2023 Session 6: Tailoring breast surgery and radiotherapy S6 Friday, 17 March 2023 Session 7: Controversies in the treatment of the axilla S7 Friday, 17 March 2023 Session 8: Optimizing treatment in patients with HER2-positive breast cancer S8 Friday, 17 March 2023 Session 9: Optimizing treatment in patients with triple negative breast cancer S10 Saturday, 18 March 2023 Session 10: Global Perspective on breast cancer treatment S11 Saturday, 18 March 2023 Session 11: Optimizing treatment in patients with HR positive breast cancer S11 Saturday, 18 March 2023 Special lecture II S12 Academy Abstracts S12 VOLUME 68 S1 APRIL 2023

viii Poster Abstracts Adjuvant Systemic Therapy S15 Biology/Pathology/Basic Research S25 Epidemiology/Prevention/Diagnostics S37 Locally advanced disease S48 Radiotherapy/IORT S49 Neoadjuvant (pre-operative) therapy S56 Predictive and prognostic factors S69 Surgery/Sentinels/DCIS S85 Other S114 Survivorship issues S124 Diagnostics S132 Author Index S137 Expression of NY-ESO-1, MAGE-A3, PRAME and WT1 in different subgroups of breast cancer: An indication to immunotherapy? A. Tessari, L. Pilla, D. Silvia, M. Duca, B. Paolini, M.L. Carcangiu, L. Mariani, F.G. de Braud and S. Cresta 68 Variations in the breast reconstruction rate in France: A nationwide study of 19,466 patients based on the French medicoadministrative database C. Régis, J. Le, M.-P. Chauvet, M.-C. Le Deley and G. Le Teuff 74 Freq ency and characteristics of additionally detected ipsilateral breast lesions following recall at screening mammography J.R. Lameijer, A.M. Coolen, J. Nederend, A.C. Voogd, V.C. Tjan-Heijnen and L.E. Duijm 94 Risk factors of sentinel and non-sentinel lymph node metastases in patients with ductal carcinoma in situ of the breast: A nationwide study E.V. Holm-Rasmussen, M.-B. Jensen, E. Balslev, N. Kroman and T.F. Tvedskov 128 The presentation, managem nt and ou come of inflammatory breast cancer cases in the UK: Data from a multi-centre retrospective review E. Copson, A.M. Shaaban, T. Maishman, P.M. Moseley, H. McKenzie, J. Bradbury, A. Borley, M. Brzezinska, S.Y.T. Chan, J. Ching, R.I. Cutress, I. Danial, B. Dall, M. Kerin, A.J. Lowery, I.R. Macpherson, L. Romics, E. Sawyer, N. Sharmat, T. Sircar, R. Vidya, Y. Pan, D. Rea, L. Jones, D.M. Eccles and F. Berditchevski 133 Trace element concentrations in breast cancer patients N. Cabré, F. Luciano-Mateo, M. Arenas, M. Nadal, G. Baiges-Gaya, A. Hernández-Aguilera, I. Fort-Gallifa, E. Rodríguez, F. Riu, J. Camps, J. Joven and J.L. Domingo 142 Treatment outcomes of breast cancer liver metastasis treated with stereotactic body radiotherapy C. Onal, O.C. Guler and B.A. Yildirim 150 ErbB4 receptor polymorphism 2368A>C and risk of breast cancer M. Mansouri Bidkani, H. Tabatabaeian, S. Parsafar, N. Ghanei, M. Fazilati and K. Ghaedi 157 SHORT COMMUNICATIONS Ribociclib plus letrozole and concomitant palliative radiotherapy for metastatic breast cancer I. Meattini, I. Desideri, V. Scotti, G. Simontacchi and L. Livi 1 Therapeutic Drug Monitoring of endoxifen as an alternative for CYP2D6 genotyping in individualizing tamoxifen therapy A.H.M. de Vries Schultink, A.D.R. Huitema and J.H. Beijnen 38 Guide for Authors Submission to The Breast proceeds totally online via the Elsevier Editorial System page of this journal at Authors will be guided through the creation and uploading of the various files. Once the uploading is done, the system automatically generates an electronic (PDF) proof, which is then used for reviewing. All correspondence, including the Editor’s decision and request for revisions, will be by e-mail. Authors may send queries concerning the submission process, manuscript status, or journal procedures to the editorial office at Indexed/abstracted in: Index Medicus, MEDLINE, ABI/Inform, Current Awareness in Biological Sciences, Current Contents/Clinical Medicine, EMBASE, Excerpta Medica National Library of Medicine (MEDLARS and MEDLINE), Research Alert, SCISEARCH, Science Citation Index Expanded, Scopus Amsterdam • Boston • London • NewYork • Oxford • Paris • Philadelphia • San Diego • St Louis Available online at ScienceDirect YBRST_v42_iC_FM.indb iv 10/17/2018 12:42:4 to The Breast proceeds totally on ine via the Elsevier Editorial System page of this journal at h tp :// il be guided through the creation and uploading of the various files. Once th uploading is done, the syst m auto tically generates an ( ) pro f, which is then used for revi wing. All corresponde c , including the Editor’s decision and request for revisions, will be by e-mail. send queries concerni g the submiss on process, manuscript status, or j urnal procedur s to the editorial office at th

Speakers’ Abstracts Wednesday, 15 March 2023 14:15–15:30 Session 1: News in breast cancer care since SGBCC 2021 SA 1.1 Surgery of patients with early breast cancer: Quo vadis? W. P. Weber1. 1Department of Surgery, Basel University Hospital, Basel, Switzerland Several randomized-controlled trials (RCTs) have shown that axillary lymph node dissection (ALND) does not relevantly improve survival or risk of recurrence compared to observation or axillary radiation in selected clinically node-negative patients with positive sentinel lymph nodes (SLNs). These findings enhanced ongoing trends in clinical practice toward de-escalation of axillary surgery in SLNpositive patients in the adjuvant setting. More recently, the SLN procedure has also been investigated as surgical technique to determine nodal pathologic complete response (pCR) after neoadjuvant chemotherapy (NACT). This concept is changing current surgical standards in patients with clinically node-positive breast cancer. Clipping of the sampled node with selective imaging-guided localization and removal has been shown to improve the accuracy of the SLN procedure in this setting. However, ALND remains standard care in most patients with palpable nodal disease in the upfront surgery setting and in patients with residual disease after NACT. Two RCTs are currently addressing some of these remaining indications for ALND. Attempts to improve breast surgery by routine combination of oncologic and plastic surgery techniques are increasingly supported by scientific evidence of effective patient benefit. Oncoplastic breast conserving surgery allows extension of breast conservation to larger tumors with lower re-excision rates compared to conventional breast conserving surgery with high quality of life and patient satisfaction. Simplification of implant-based breast reconstruction after nipple- or skin sparing mastectomy by pre-pectoral positioning was suggested to be safe and effective in contemporary clinical practice, but confirmation by ongoing RCTs is pending. In the setting of postmastectomy radiation, autologous reconstruction with support for immediate and delayed-immediate reconstructive approaches is preferred over implants due to lower risk of long-term complications. Finally, de-escalation of breast surgery by effective neoadjuvant systemic therapy is on the way. SA 1.2 Access to evidence based radiation therapy for patients with early breast cancer P. Poortmans, M.D., Ph.D.1. 1IridiumNetwerk and University of Antwerp, Wilrijk Antwerp, Belgium Radiation therapy (RT) for breast cancer continues evolving to offer the most personalised approach for every single patient by optimising treatment indications, dose and volume prescriptions, target volume definitions and integration with surgery and systemic treatments within the overall treatment approach. This way, the burden related to the treatment has been strongly reduced over the last decades in a stepwise manner, resulting in a reduction of both early and late side effects and a decrease in the number of visits for treatment sessions, all without compromising on treatment efficacy. As additional benefit, this led to a reduction in the workload and thereby costs for the community. However, implementation of these evidence-based changes in clinical practice are challenging, depending on a number of multifactorial and complex factors. As an example, adaptation of hypofractionated RT for breast cancer has been extremely slow and incomplete up to today, more than 10 years after publication of supporting long-term level-1 evidence. Barriers to its adoption include perceived–not real - concerns about late normal side effects, lack of experience, resilience to incorporate it into some of the (inter) national guidelines for a broad selection of patients, and outdated reimbursement systems that are based on a payment per session, making the use of hypofractionated RT unsustainable for the hospital management. As a benefit from a worldwide disaster, the COVID-19 pandemisery challenged this resistance against hypofractionation as a tool to minimise exposure of both patients and health care professionals to the virus, with international groups of experts publishing guidelines for extending its use, also incorporating the very recent outcomes of the FAST-Forward trial, supporting a further reduction of the number of fractions to 5 in 1 week for whole breast/chest wall RT only. Today, we still witness a broad variation in the adaptation of hypofractionation, with some countries taking the lead, while others remain reluctant for above-mentioned reasons, notwithstanding a sharp increased support in international guidelines. Therefore, an important effort needs to be done to support equitable access to optimal breast cancer RT on a global level. In the presence of the lack of resources in many LMICs, the use of hypofractioned RT will for sure be an important asset in improving health care capacity. Unfortunately, radiation oncologists in LMICs are in general even less favouring the use of hypofractionated breast RT, due to the abovementioned factors. Innovative and creative solutions are therefore highly needed to provide incentives for delivering high-value treatments irrespective of confounding factors such as the number of fractions, and to improve attractivity for patients by shortening and streamlining the overall RT-related workflow, within a multidisciplinary organisation. SA 1.3 What’s new in systemic treatment of patients with early breast cancer K. Miller1. 1Department of Medicine Hematology/Oncology Division, Indiana University School of Medicine, Indianapolis, IN USA This abstract is not available. The Breast 62S1 (2023) S1–S14 Contents lists available at ScienceDirect The Breast j ou r na l homepage : www. e l sev i e r . com/ b r st 0960 9776/ © 2023 Elsevier Ltd. All rights reserved.

SA 1.4 Translational research priorities for patients with early breast cancer F. André1. 1Gustave Roussy Cancer Center, Villejuif, France This abstract is not available. SA 1.5 Hot Topics in Survivorship, Patient Reported Outcomes and Quality of Life A. Partridge1. 1Medical Oncology, Dana-Farber Cancer Institute, Boston, United States Advances in the early detection and treatment of breast cancer have led to a large and growing population of breast cancer survivors. Increasingly, we have recognized that improvements in treatment and survivorship care can improve QOL and psychosocial effects of breast cancer, and are often associated with improved long-term survival (e.g., depression is associated with reduced endocrine therapy adherence which is associated with increased risk of recurrence). Unfortunately, recent studies testing the repurposing of anti-inflammatorymedications (e.g., aspirin and celecoxib), as well as agents aiming to modify the energy metabolism of cells (i.e., metformin) have been unsuccessful to date in improving breast cancer outcomes. However, research aiming to tailor therapy and better select patients for whom the benefits outweigh the risks, have allowed large numbers of early breast cancer patients to avoid unnecessary treatment and associated physical and psychosocial toxicities, including the recent Mindact, TailorX, and RxPonder trials, as well as efforts at local therapy de-escalation. However, several studies have suggested that we are overtreating many patients surgically with negative impact particularly on patient-reported outcomes including psychosocial, body image, and sexual well-being. Efforts to understand and improve patient decision-making and support at diagnosis and in follow-up are ongoing and clearly needed. Given many women require adjuvant therapies to improve risk of future breast cancer events and overall survival, research focusing on the identification of biomarkers of risk for long-term comorbidities including secondary cancers and cardiotoxicity as well as other complications from treatment is of paramount importance. Impactful studies have focused recently on the prevention and management of potential toxicity from hair loss and neuropathy to fertility and cardiotoxicity. Recent data regarding depression, fatigue, cognitive dysfunction all suggest not only predictors and patterns of symptomatology in survivorship, but that integrative therapy, psychotherapeutic cognitive behavioral techniques, and energy balance interventions have particular promise for a wide range of symptoms as well as potential for long-term cancer risk reduction. At the same time, an increasing number of studies are beginning to test the delivery of these interventions as part of comprehensive cancer survivorship care so that all survivors can benefit from state-of-theart support and care, ensuring that the diverse needs of survivors are met. Thursday, 16 March 2023 09:00–10:00 Session 2: Genomics, Transcriptomics, ctDNA for disease monitoring and risk stratification SA 2.1 ctDNA dynamics for early assessment of recurrence risk N. Turner1. 1Institute of Cancer Research, Royal Marsden Hospital, London, United Kingdom This abstract is not available. SA 2.2 Molecular Imaging in Breast Cancer M. van Kruchten1, E. G. de Vries1. 1Medical Oncology, University Medical Center Groningen, Groningen, Netherlands Molecular imaging can visualize biology in vivo and can be performed by labeling a ligand with radionuclides for PET imaging. PET imaging provides whole-body information, and tracer tumor and normal tissue uptake can be quantified. Best know is FDG-PET with [18F]-fluorodeoxyglucose (FDG) tracer imaging glucose tumor consumption. For breast cancer staging (ESMO & NCCN Guidelines), a bone or sodium fluoride scan or FDG/PET is used to detect bone metastases. FDG-PET/CT may be performed with a diagnostic CT and may be helpful when standard staging studies are equivocal or suspicious and in identifying unsuspected regional nodal disease and/or distant metastases. In newly diagnosed patients with metastatic breast cancer (MBC), 18F-FDG PET compared to bone scan analysis to assess bone lesions resulted in clinically relevant management differences in 16% of patients. The bone scan delivered insufficient information in >1/4th of patients, leading to additional 18F-FDG PET requests. (van Es et al, J Nucl Med 2021). FDG-PET is part of the RECIST criteria only to detect disease progression. The estrogen receptor (ER) status can be determined with 16α-[18F] fluoro-17β-estradiol ([18F]FES)-PET. We showed a high diagnostic accuracy of qualitative and quantitative [18F]FES-PET assessment to predict ER expression by immunohistochemistry in patients with newly diagnosed MBC. (Van Geel et al, J Clin Oncol 2022). The FDA approved 202118F-FES for PET imaging of ER-positive lesions as an adjunct to biopsy in patients with recurrent or MBC. Repeat [18F]FESPET also served to prove antihormonal treatment effects on the ER. Monoclonal antibodies and antibody-related therapeutics are interesting for theranostic approaches because antibodies are designed against specific targets. We started labeling trastuzumab and detected more tumor lesions than with conventional imaging, and noticed major heterogeneity in tumor tracer uptake. Moreover, pretreatment imaging of HER2 targeting, combined with early metabolic response assessment, did improve the understanding of tumor heterogeneity in MBC. It may be used for selecting patients whowill/will not benefit from T-DM1. (Gebhart, et al. Ann Oncol 2016). For, immune checkpoint inhibitor antibodies, there are only a few approved biomarkers to select patients for immunotherapy, which do not consider the heterogeneity of tumor characteristics across lesions within a patient. We showed in small studies that programmed cell death protein 1 (PD-1) antibody, and programmed death-ligand 1 (PD-L1) antibody tumor tracer uptake predicted tumor response and patient survival (Bensch, et al. Nat Med, 2018, Kok, et al. Ann Oncol 2022). Also, CD8+ T cell presence can be visualized. We showed safety of the CD8-specific, one-armed antibody PET tracer 89ZED88082A in patients with solid tumors before and ∼30 days after starting immunotherapy, with tracer uptake in normal lymphoid tissues, and tumor lesions across the body. Higher tumor SUVmax was associated with longer overall survival. Re-imaging patients on treatment showed no change in average tumor tracer uptake compared to baseline, but individual lesions showed diverse changes independent of tumor response. This suggests enormous heterogeneity in CD8+ T cell distribution and pharmacodynamics within and between patients (Kist de Ruijter, et al. Nat Med 2022). For all novel tracers, it will be critical that they undergo appropriate clinical testing to obtain a firm role in the clinic. Conflict of Interest statement: Institutional Financial Support for advisory boards/consultancy: • Daiichi Sankyo, NSABP and Crescendo Biologics. Institutional Financial Support for clinical trials or contracted research: • Amgen, Genentech, Roche, Synthon, Bayer, Servier, G1 Therapeutics, Regeneron, GE Healthcare, AstraZeneca Speakers’ Abstracts / The Breast 62S1 (2023) S1–S14 S2

SA 2.3 Multi-omic machine learning prediction of treatment response C. Caldas1. 1 Cancer Research Uk Cambridge Institute, University Of Cambrige, United Kingdom This abstract is not available. SA 2.4 Assessing clinical utility and clinical validity of biomarkers in clinical trials M. Regan1. 1IBCSG Statistical Center, Dana-Farber Cancer Institute, Boston, United States The development of biomarkers for clinical care—whether for risk stratification, treatment selection and/or disease monitoring— requires a multi-step progression of rigorous studies, similar in concept to drug development. As described by the BEST (Biomarkers, Endpoints and other Tools) Glossary ( books/NBK338448), analytic validity refers to the technical performance of a biomarker assay and clinical validity refers to establishing a meaningful relation between a biomarker and the relevant clinical measure or outcome. Establishing clinical validity is often accomplished via prospective-retrospective studies using clinically-annotated archival specimens, unless the biomarker must be determined in real-time for example as has been required for detecting circulating tumor cells (CTCs). Prospective studies are required to demonstrate clinical utility of a biomarker, that is, to demonstrate that a biomarker’s use leads to a net improvement in outcome or provides clinically-beneficial information about detection, treatment or management of disease. Study design concepts and challenges are discussed. Thursday, 16 March 2023 10:00–11:00 Session 3: Immunology in early breast cancer SA 3.1 Understanding the anti-cancer immune response: Innate and adaptive responses to cancer cells S. Loi1,2. 1Division of Cancer Research, Peter MacCallum Cancer Centre, Melbourne, Australia; 2The Sir Peter MacCallum Department of Medical Oncology, University of Melbourne, Parkville, Australia Breast cancer is a leading cause of cancer-related deaths among women, yet the complex role the immune system plays in both the promotion and inhibition of its growth remains poorly understood. The immune system has two branches: the innate and the adaptive immune system. The innate immune system is the body’s first line of defence against infections and foreign elements, able to respond quickly to any potential threat. The adaptive immune system, on the other hand, is slower to respond but importantly can develop immune memory which protects rapidly against antigen rechallenge. Both the innate and adaptive immune systems most certainly play a role in the response to breast cancer. The innate immune system is activated by the presence of cancerous cells, leading to the production of cytokines and the recruitment of immune cells to the site of the tumor. The adaptive immune system is activated by tumor antigens, leading to the activation of immune cells that can specifically target cancer cells. How do these relate to our understanding of the treatment of early breast cancer? It is clear now that T cells, part of the adaptive immune arm, play an important role suppression of the growth of triple negative breast cancer (TNBC), as seen with the success of PD-1 targeting therapies as well as adoptive cell therapies. It is also clear that the immune infiltrate consists of both tumor specific as well as non-tumor antigen specific T cells (known as bystander T cells), which likely also have an important part to play in the immune response. This also suggests that many of the T cells in the tumor microenvironment don’t require activation or antigen. However undoubtedly innate immunity is important as well: for example, natural killer (NK) cells, part of the innate immune system, can recognize and kill cancer cells, has been shown to be important in ADCC, and contribute to the efficacy of monoclonal antibodies such as trastuzumab. Other innate immune cells, such as macrophages and dendritic cells, can also present tumor antigens to the adaptive immune system, which can then trigger an adaptive immune response against the cancer cells. The full complement is likely critical for a development of an effective anti-tumor response as well as ongoing immune memory to protect against recurrence. Also of relevance in breast cancer is the fact that the immune response is generated in the ipsilateral axillary lymph nodes: therefore treatment of the axilla with radiotherapy, as lymphocytes are extremely sensitive to radiation, which may be of clinical relevance in an era of immune agents. In summary, the immune systemplays a complex role in the response to breast cancer, and we are only just beginning to see its benefit for breast cancer patients. A better understanding of this relationship is crucial for the development of effective T cell as well as other immune-based therapies in early stage breast cancer. Conflict of Interest statement: SL receives research funding to institution:Novartis, BMS, MSD, Puma Biotech,Eli Lilly,Nektar Therapeutics,Astra Zeneca and SeaGen. Consultant (not compensated): SeaGen, Novartis, BMS, MSD, AstraZeneca, Eli Lilly, Pfizer, Gilead Therapeutics and Roche-Genentech. Consultant (paid to institution): Aduro Biotech, Novartis, GSK, Roche-Genentech, Astra Zeneca, Silverback Therapeutics, G1 Therapeutics, PUMA Biotech, Pfizer, Gilead Therapeutics, SeaGen, Daiichi Sankyo, MSD, Amunix, Tallac Therapeutics, Eli Lilly and BMS. Portugal SA 3.2 Tumor microenvironment in early breast cancer C. Denkert1. 1Philipps-University Marburg, Germany This abstract is not available. SA 3.3 Multiplexed analysis and spatial histology to predict response M. Kok1. 1Netherlands Cancer Institute, Amsterdam, Netherlands This abstract is not available. SA 3.4 The future of breast cancer immunotherapy (PD, PDL, vaccines) G. Curigliano1. 1University of Milano, Milan, Italy This abstract is not available. Thursday, 16 March 2023 12:45–13:30 Special lecture I and panel discussion SL 1 The future of innovation: Why are we conducting clinical trials in countries that are unlikely to be able to afford innovative drugs? S. Tolaney1. 1Dana-Farber Cancer Institute, Boston, USA This abstract is not available. 18th St.Gallen International Breast Cancer Conference / The Breast 62S1 (2023) S1–S14 S3

Thursday, 16 March 2023 13:30–14:30 Session 4: Hereditary Breast Cancer SA 4.1 Imaging, screening & surveillance for individuals with increased hereditary breast cancer risk S. Delaloge1. 1Gustave Roussy, Paris, France This abstract is not available. SA 4.2 Local treatment in patients with high risk hereditary breast cancer M.-J. Cardoso1. 1Champalimaud Foundation Breast Unit, Lisbon, Portugal This abstract is not available. SA 4.3 Management of patients harboring medium penetrance genes C. Singer1. 1Dept of OB/GYN and Comprehensive Cancer Center, Medical University of Vienna, Vienna, Austria With the widespread availability of Next Generation Sequencing (NGS) technologies, conventional BRCA1 and 2 analysis is progressively replaced by simultaneous testing of multiple genes. Gene panel analyses permit for a cost-effective and rapid detection of high- and moderate-penetrance gene mutations, and offer additional information on cancer risks. Genetic testing for breast cancer susceptibility gene panels is therefore widely used in high-, and increasingly also in average risk populations. While up to 5% of all individuals who undergo panel testing are diagnosed with mutations in one or more genes, the clinical validity of many of these gene variants remains controversial: evidence of an associationwith breast cancer is usually weak, and underlying risk estimates are often imprecise. Furthermore, studies demonstrating improved outcomes for tested individuals are still not available, which limits the potential benefit of panel testing. Although there is evidence of clinical utility in selected actionable mutations, the appropriate management of individuals harboring most moderate-penetrance genetic variants is subject to considerable debate. While some experts argue that multigene panel testing provides estimates of risks and guide genetic counseling, others caution that the application of guidelines originally established for the clinical care of BRCA1 and 2 mutation carriers to the management of patients with mutations in moderate penetrance genes could result in substantial overtreatment and adverse psychological effects. Cancer risks associated with mutations in moderate-penetrance genes are lower and different than those reported for high penetrance gene mutations, but the perceived risks are often overestimated by both patients and care-givers. There is no evidence that breast cancer patients with pathogenic mutations in moderate penetrance genes should be treated any different from non-mutation carriers, studies suggest otherwise: female mutation carriers in moderate penetrance genes are more likely to undergo bilateral mastectomy than women without a multi-gene panel mutation. Also, the transfer of screening recommendations for women with high penetrance genes such as BRCA1 or 2 to moderate penetrance genes carries an inherent risk of increased false-positive results. It is therefore important to not only consider gene-specific lifetime risks, but to also account for differences in incidence rates in particular age cohorts. More robust estimates of age-specific risks from prospective cohort and case-control studies, such as the PROMPT study, as well as the inclusion of additional risk determinants such as family history, or polygenic risk scores, are clearly needed in order to refine our screening and management approaches in women with moderate penetrance gene mutations. Conflict of Interest statement: Advisory board member for DaiichiSankyo, AstraZeneca, Novartis; Research and travel support from Novartis, Roche, Daiichi-Sankyo. AstraZeneca, Amgen, Pfitzer, Stemline. SA 4.4 Adjuvant systemic therapy for patients with germline BRCA1/2 mutations A. Tutt1. 1Institute of Cancer Research, King’s College London, United Kingdom More than 15 years ago the use of potent PARP1 inhibitors (PARPi), that both inhibit the catalytic activity of the PARP1 enzyme and trap PARP1 onto DNA, was shown to generate a “synthetic lethal” interaction with the malignant cell specific loss of function of genes crucial for homologous recombination (HR) gene function. This phenomenon has now been exploited with therapeutic intent in several contexts initially leading to approved single agent PARPi treatments for advanced forms of ovarian, prostate, pancreatic and breast cancers These approvals have in large part been associated a biomarker requirement for evidence of loss of function of HR. In the last year approval has been gained for the PARPi olaparib as a single agent adjuvant therapy in patients with germline BRCA1 or BRCA2 mutation associated HER2 negative early breast cancer following (neo)adjuvant chemotherapy based on the OlympiA trial. OlympiA restricted eligibility to patients with the germline “pathogenic” or “likely pathogenic” mutation in BRCA1 or BRCA2 but included patients with hormone receptor positive breast cancer. Since St Gallen 2021 OlympiA reported its primary analysis in June 2021 following an interim analysis showing 12 months of olaparib in the post-(neo)adjuvant chemotherapy setting improves both invasive disease free survival (IDFS) and distant disease free survival (DDFS) by approximately 40% with highly statistically significant hazard ratios (HR) of 0.58 and 0.57 respectively that met pre-specified early stopping boundaries. The publication of the initial results for OlympiA led to rapid updates of both genetic testing and treatment international guidelines for BRCA1 and BRCA2 mutation associated breast cancer to include use of olaparib in patients meeting the eligibility criteria for the trial. Since the IDMC driven early primary analysis was published, with median follow-up of 2.5 years, a second pre-planned event driven interim analysis of overall survival has been presented and published at a median follow-up of 3.5 years. This showed significant improvement in the olaparib group relative to the placebo group [hazard ratio 0.68; 98.5% confidence interval (CI) 0.47– 0.97; P = 0.009]. Four-year OS was 89.8% in the olaparib group and 86.4% in the placebo group (D 3.4%, 95% CI 0.1% to 6.8%). Effect sizes for the primary IDFS and secondary DDFS endpoints were maintained. Recently a number of trials have investigated PARPi in a (neo) adjuvant breast cancer treatment (NACT) setting in patients with (or enriched for) HR deficiency. These have included phase II neoadjuvant PARPi studies such as the NeoTALA study using single agent talazoparib in germline BRCA1 and BRCA2 mutation carriers and the combination chemotherapy and olaparib therapy studies GeparOLA and PARTNER, enriched for HR deficient breast cancer, that have reported important signals with regard efficacy and tolerability. A recent phase III trial analysis that has included anaylsis of BRCA1/2 mutation status has influenced recent early breast cancer treatment guidelines. The BrighTNess trial was conducted in the neoadjuvant setting. BrighTNess recruited patients with biologically heterogeneous triple negative breast cancer (TNBC) and used the weak PARP1 trapping PARPi veliparib. BrighTNess did not show convincing evidence of benefit to the addition of veliparib to standard of care in TNBC NACT but has shown the benefit to the addition of carboplatin to sequential paclitaxel-AC NACT with significant improvements in event free survival (HR 0.57) compared to sequential paclitaxel-AC alone. The trial did not find that the effect sizes were significantly different in those with and without germline Speakers’ Abstracts / The Breast 62S1 (2023) S1–S14 S4

BRCA1 or BRCA2mutations. The lecturewill both explore the data and the implications for practice and some of the ongoing questions to be addressed by ongoing clinical trial and translational research initiatives. Conflict of Interest: Pharma Institutional payments: AstraZeneca -Financial support to my academic & hospital institutions for costs associated with global academic study chair and local site costs for OlympiA trial/Travel expenses related to any trial related travel/ Payments to Institution through Breast International Group for trial conduct in OlympiA trial and through CRO’s for commercial parp inhibitor trials/AstraZeneca- royalties paid to Institute of Cancer Research for the use of parp inhibitors in DNA deficient cancers, and personally to me through ICR rewards to Inventor’s scheme. Merck KGAA -Local site trial support costs associated with clinical trial DNA. Medivation -Financial support for research at ICR. Myriad Genetics -Financial support for research at ICR. Ad Boards/ Speaker Roles: Personal payments Pfizer -Personal fees/Advisory Board related to targeted therapies in DNA repair deficient. Vertex -Personal fees from /Advisory Board related to targeted therapies in DNA repair deficient. Artios -Personal fees/Advisory Board related to targeted therapies in DNA repair deficient. Prime Oncology -Personal fees/Advisory Board related to targeted therapies in DNA repair deficient. Inbiomotion -Personal fees/Scientific Ad Board function and stock options. MD Anderson -personal fees/Moon shot Breast Cancer scientific advisory board honoraria. Medscape Education honorarium from Merck educational grant -personal fees/speaker for a video series tentatively titled “Understanding the Rationale for PARP Inhibitors and their Combination with Immunotherapy in the Treatment of Various Solid Tumors. SABCS Mini Symposium -speaker honararium. EMPartners- -Personal fees/Advisory Board. Gilead: -Personal fees for Ad Board & payments to my institution. GBCC 2022 -Speaking role personal honorarium. Cancer Panel -Speaker personal honorarium. Research to Practice -Personal honorarium for research survey. AACR Team prize -Personal honorarium for Team prize. Penn Medicine -Speaker honorarium. AZ Breast Academy -Invited speaker honararium. Institutional payments VJ oncology -Roundtable- payment to institution. GE Healthcare -Meeting –payment to institution. AZ ESMO Symposium -Paid to ICR for speaker role. Gilead: -Personal fees for Ad Board & payments to my institution. Innovation In Breast Cancer Symposium -Speaker honorarium which went to KCL Institute. AZ UK/Asia/Ad Board -UK hon ICR others non -paid. Aicme -Speaker on CME. SABCS 2022. -Speaker honararium. Grants: Breast Cancer Now Charity -Grant funded to study homologous recombination deficient breast and other cancers, BCN receive payments through AstraZeneca related to PARP inhibitor patents. CRUK -Grant funded to study homologous recombination deficient breast and other cancers, CRUK receive payments through AstraZeneca related to PARP inhibitor patents/personal fees/Honoraria associated with function as Deputy Chair and reviewer for CRUK clinical research. Stock options: Inbiomotion - Personal fees/Scientific Ad Board function and stock options. Rewards/Patents: AstraZeneca with royalties paid to The Institute of Cancer Research (ICR) for the use of parp inhibitors in DNA deficient cancers, licensee– AstraZeneca. ICR rewards to inventor’s payments paid to Andrew Tutt’s research accounts at The Institute of Cancer Research and to personal accounts. Thursday, 16 March 2023 14:30–15:30 Session 5: the future treatment landscape in the adjuvant setting SA 5.1 Post-neoadjuvant options in triple negative disease: PARPi, capecitabine, checkpoint inhibitors J. Cortes1. 1International Breast Cancer Center IBCC, Barcelona, Spain This abstract is not available. SA 5.2 Risk assessment in ER positive disease: Who should receive CDK 4–6 inhibitors? A. De Michele1. 1University of Pennsylvania, Philadelphia, USA This abstract is not available. SA 5.3 What’s next? Antibody-drug conjugates for breast cancer therapy L. Sanz1,2, C. Saura1,2. 1Vall d’Hebron University Hospital, Barcelona, Spain; 2Vall d’Hebron Institute of Oncology, Barcelona, Spain The breast cancer (BC) treatment landscape has undergone a revolution in recent years. One of the most important advances has been the new class of targeted anticancer treatment, antibody-drug conjugates (ADCs). The first ADC approved to treat solid tumors appeared in 2013, with the FDA approval of Trastuzumab (T) emtansine (T-DM1) for HER218th St.Gallen International Breast Cancer Conference / The Breast 62S1 (2023) S1–S14 S5

positive metastatic BC (MBC), based on the EMILIA trial (NCT00829166). Subsequently, the Katherine trial (NCT01772472) evaluated the treatment of T-DM1 vs. standard adjuvant T for 14 cycles in patients who had residual disease after neoadjuvant treatment based on chemotherapy plus HER2-targeted therapy. The primary endpoint was invasive disease-free survival (IDFS), and this study demonstrated a 50% reduction in the risk of recurrence or death with T-DM1 vs. T (HR = 0.50; 95% CI, 0.39–0.64; P < 0.0001), thus making T-DM1 the standard adjuvant treatment in this population. However, when relapses at the CNS level were assessed, a very similar rate was observed in both arms. With the aim of reducing CNS relapse, the ongoing CompassHER2 RD trial (NCT04457596) has emerged for non-luminal patients with residual disease or nodal involvement, regardless of hormone receptor status. Eligible patients are randomized to receive T-DM1 plus Tucatinib, an oral HER2specific tyrosine kinase inhibitor with high CNS penetrance, or TDM1 and placebo. As another approach to optimize T-DM1 activity, the addition of immunotherapy is being studied and in the ASTEFANIA trial (NCT04873362), patients with residual disease receive T-DM1 with or without atezolizumab. Still targeting HER2-positive patients, another ADC was first approved by FDA in 2019: Trastuzumab deruxtecan (T-DXd), an anti-HER2 ADC. Recently, it has received both FDA and EMA approval for 2-line treatment of HER2+ MBC based on the DESTINY-Breast03 study (NCT03529110), where the benefit of T-DXd in progression-free survival (PFS) and overall survival (OS) was confirmed. Its efficacy has also been demonstrated in patients with advanced HER2-low BC in the DESTINY-Breast04 trial (NCT03734029). In the adjuvant setting, T-DXd is being evaluated against T-DM1 in patients with residual disease and high risk of recurrence after neoadjuvant therapy based on chemotherapy and T ± Pertuzumab (DESTINY-Breast05; NCT04622319). Moving to HER2-negative patients, Sacituzumab govitecan (SG), an ADC against trophoblast cell-surface Ag-2 (Trop-2), has shown benefit in terms of PFS and OS in advanced triple-negative BC (TNBC) population after at least two prior therapies, in the ASCENT trial (NCT02574455), and for patients with HR+/HER2-MBC who received prior endocrine therapy, iCDK4/6 and two to four lines of chemotherapy in the TROPiCS-02 study (NCT03901339) evaluating SG vs. physician’s choice of chemotherapy in terms of PFS. Given this drug’s impact, its efficacy in the adjuvant setting is also being evaluated in the SASCIA trial (NCT04595565), a phase 3 trial for patients with TNBC or HR+/HER2- BC with residual disease after neoadjuvant chemotherapy. Luminal BC patients are included not only based on non-pCR but also on the CPS+EF score. ADCs combinations with ICIs, iPARP, endocrine agents or other HER2targeted treatments are being studied in the different BC settings and other ADCs are in development with promising results. We hope in the coming years to see these new strategies incorporated into the treatment of early disease to cure more patients with BC. Conflict of Interest statement: I have served as consultant, participated in advisory boards or received travel grants from: AstraZeneca, AX’Consulting, Byondis B.V, Daiichi Sankyo, Eisai, Exact Sciences, Exeter Pharma, F.Hoffmann-La Roche Ltd, ISSECAM, MediTech, Merck Sharp & Dohme, Novartis, Pfizer, Philips, Pierre Fabre, PintPharma, Puma Biotechnology, SeaGen, and Zymeworks. SA 5.4 Integrating new oral SERDs in the adjuvant treatment D. Cameron1. 1Edinburgh Cancer Research, The University of Edinburgh, United Kingdom This abstract is not available. Friday, 17 March 2023 09:00–10:00 Session 6: Tailoring breast surgery and radiotherapy SA 6.1 Patient selection, dose and fractionation for external beam radiotherapy in patients with early breast cancer C. E. Coles1. 1Department of Oncology, University of Cambridge, Cambridge, United Kingdom Breast radiation therapy (RT) research over the last 30 years has clearly demonstrated that local control and survival is at least as effective with moderately fractionated RT compared with 2 Gy daily fractions, with similar or reduced normal tissue toxicity1–3. An additional consequence is reduction in the cost of breast RT and reduced burden for patients in terms of treatment visits4,5. Implementation of practice-changing breast hypofractionation has been challenging with slow progress in the past5. However, a positive consequence of the COVID-19 pandemic has been increased global collaboration and solidarity among the breast radiation oncology community to share experience of hypofractionation, including the 1-week FAST Forward schedule6. This has catalysed implementation of hypofractionated breast RT and several international consensus documents have emerged as a consequence7,8. This presentation will discuss the patient selection, dose and fractionation for external beam radiotherapy for whole breast, chest wall and partial breast RT. It will draw on evidence from key randomised trials and from international expert consensus. References 1. Haviland JS, et al. The UK Standardisation of Breast Radiotherapy (START) trials of radiotherapy hypofractionation for treatment of early breast cancer: 10-year follow-up results of two randomised controlled trials. Lancet Oncol 2013; 14(11): 1086–94. 2. Whelan TJ, et al. Long-term results of hypofractionated radiation therapy for breast cancer. N Engl J Med 2010; 362(6): 513–20. 3. Offersen BV, et al. Hypofractionated Versus Standard Fractionated Radiotherapy in Patients With Early Breast Cancer or Ductal Carcinoma In Situ in a Randomized Phase III Trial: The DBCG HYPO Trial. J Clin Oncol 2020; 38(31): 3615–25. 4. Glynn D, et al. Cost-effectiveness of 5 fraction and partial breast radiotherapy for early breast cancer in the UK: model-basedmultitrial analysis. Breast Cancer Res Treat (2022). https://doi-org.ezp. 5. Marta GN, et al. The use of moderately hypofractionated postoperative radiation therapy for breast cancer in clinical practice: a critical review. Critical Reviews in Oncology/Hematology (2020). 6. Murray Brunt A, et al. Hypofractionated breast radiotherapy for 1 week versus 3 weeks (FAST-Forward): 5-year efficacy and late normal tissue effects results from a multicentre, non-inferiority, randomised, phase 3 trial. Lancet 2020; 395(10237): 1613–26. 7. Meattini I, et al. European Society for Radiotherapy and Oncology (ESTRO) Advisory Committee in Radiation Oncology Practice (ACROP) consensus recommendations on patient selection and dose/fractionation for external beam radiation therapy in early breast cancer. Lancet Oncology 2022: 23: e21–31. 8. Burstein HJ, et al. Customizing local and systemic therapies for women with early breast cancer: the St. Gallen International Consensus Guidelines for treatment of early breast cancer 2021. Ann Oncol. 2021 Oct;32(10):1216–123. Speakers’ Abstracts / The Breast 62S1 (2023) S1–S14 S6