Competing Interests: Declaration of interests CEC reports grants from Breast Cancer Now, Cancer Research UK (CRUK), Addenbrooke's Charitable Trust, and the National Institute for Health and Care Research (NIHR); participation in the Independent Data Monitoring Committees for the UK PivotalBoost trial (chair), the PROTIS phase III sinonasal proton versus IMRT trial (member), the TORPEdO Proton Beam Therapy trial (member); and a leadership role for the Lancet Breast Cancer Commission (chair). CHB reports grants and research support from AbbVie, Nektar Therapeutics, Pfizer, Polyphor, Amgen, Daiichi Sankyo, Sanofi, Exelixis, Regeneron, Novartis, Henlius, Shanghai Henlius Biotech, GSK, Janssen, OBI Pharma, Eli Lilly, Seagen, Checkpoint Therapeutics, Roche, Bristol Myers Squibb, MSD, Merck Serono, AstraZeneca, Novocure, Aveo Oncology, Takeda Pharmaceuticals, TRIO Pharmaceuticals, PharmaMar, Celgene, Myovant, PPD, Syneos Health, DOCS, Labcorp, ICON, IQVIA, Parexel, Nuvisan, PSI, and Medpace; ownership or stocks in Tummi and MEDSir; and participation in advisory boards and consulting activities for Boehringer Ingelheim, GSK, Novartis, Pfizer, Genentech, Eisai, Bayer, MSD, AstraZeneca, Zodiac, Eli Lilly, Sanofi, and Daiichi Sankyo. DAC reports research grants and support from Exact Sciences and Novartis; consulting fees from Eli Lilly, Novartis, Seagen, Daiichi Sankyo, Erytech Pharma, Carnall Farrar, Sapience, and Sanofi; payment or honoraria for lectures and presentations from Exact Sciences, Gilead, Eli Lilly, and Pfizer; participation on data safety monitoring or advisory boards for Novartis, AstraZeneca, and Grail; unremunerated leadership roles for Make Seconds Count (chair of charity trustees) and the Breast International Group (chair of board of not-for-profit research organisation); and receipt of funding for analysis and medical writing for Eli Lilly and Novartis. FC reports payment or honoraria for lectures and presentations and support for attending meetings and travel from Amgen, Astellas/Medivation, AstraZeneca, Celgene, Daiichi Sankyo, Eisai, GE Oncology, Genentech, Gilead, GSK, IQVIA, Macrogenics, Medscape, MSD, Merus, Mylan, Mundipharma, Novartis, Pfizer, Pierre Fabre, prIME Oncology, Roche, Sanofi, Samsung Bioepis, Seagen, Teva Pharmaceuticals, and Touch Independent Medical Education; institutional financial support for clinical trials from Amgen, AstraZeneca, Boehringer Ingelheim, Bristol Myers Squibb, Daiichi Sankyo, Eisai, Fresenius, Genentech, Gilead, GSK, Ipsen, Incyte, Nektar Therapeutics, Nerviano, Novartis, Macrogenics, MediGene, MedImmune, MSD, Millenium, Pfizer, Pierre Fabre, Roche, Sanofi, Sonus, Tesaro, Tigris, Wilex, and Wyeth; a leadership role for the ABC Global Alliance and ABC Consensus Conference and Guidelines (president); and membership of ESMO, ESO, EORTC-BCG, IBCSG, SOLTI, ASCO, AACR, EACR, SIS, and ASPIC. WC reports honoraria from AstraZeneca, Pfizer, MSD, and Eisai. PAF reports a former role in the scientific advisory committee of Breast Cancer Trials Australia and New Zealand cooperative group (chair). RJ reports support from the Susan G Komen Foundation; grants from the National Institutes of Health (NIH), the Doris Duke Charitable Foundation, the Greenwall Foundation, and the American Cancer Society; serving as an expert witness for Kleinbard and Hawks Quindel Law; roles in the Board of Directors of ASCO (former member), ASTRO's Ethics Committee (chair), and the Women in Radiation Oncology Affinity Group (chair); membership on the Medical Advisory Board for Blue Cross Blue Shield Association; stock options for their advisory board role in Equity Quotient; and personal fees from the NIH, Doris Duke Charitable Foundation, and the Greenwall Foundation. FMK reports research grants from the ABC Global Alliance, Breast Cancer Now, Merck Serono WHO, MSD, Avon Cosmetics, the US Cancer Pain Relief Committee, and the Medical Research Council; consulting fees from Merck Serono and Instituto Tecnológico y de Estudios Superiores de Monterrey Mexico; and leadership roles for the Board of Directors of Women in Global Health (unpaid member), Tómatelo a Pecho (unpaid president and founder), the Mexican Health Foundation (unpaid Senior Economist), and the Board of Directors of the International Association for Hospice and Palliative Care (unpaid member). SAMcI reports grant funding from the NIHR, CRUK, and Breast Cancer Now; honoraria from Roche, MSD, AstraZeneca, Becton, Dickinson and Company; travel and conference support from Roche, Eli Lilly, and MSD; membership of advisory boards for Eli Lilly, MSD, Roche, and Novartis; and roles for the UK National Cancer Research Institute Breast Research Group (unpaid chair) and the Royal College of Surgeons Breast Surgical Specialty Lead (unpaid). K-AP reports clinical trial funding from AstraZeneca (paid to institution) and unremunerated previous participation in AstraZeneca advisory boards. LR reports grants from the European Commission Horizon 2020 and the German Research Foundation and a role in the International Association for Hospice and Palliative Care (Chair of Board of Directors). MKT reports grants from Addenbrooke's Charitable Trust and CRUK. FA reports grants from Novartis, Pfizer, AstraZeneca, Eli Lilly, Daiichi Sankyo, and Roche and consulting fees from MedImmune, Gilead, Relay Therapeutics, and Guardant Health. JEA reports speaker honoraria from AstraZeneca, Pfizer, Novartis, and Eisai and conference travel support from AstraZeneca. ISB reports participation in the Roche advisory board for ESMO. JMB reports research grants from Breast Cancer Now, AstraZeneca, MSD, Puma Biotechnology, Clovis Oncology, Pfizer, Janssen-Cilag, Novartis, Eli Lilly, Roche, CRUK, and the NIHR. MAF is funded by a Conquer Cancer Breast Cancer Research Foundation Career Development Award for Diversity and Inclusion supported by the Breast Cancer Research Foundation and reports financial support from the WeShare project. AF reports consultancy work for Oxford Immune Algorithmics; grants from Addenbrooke's Charitable Trust and the Ann McLaren Building Preclinical Imaging Suite of the University of Cambridge; and travel support from the University of Cambridge. DIR reports support from the University of Chicago Center for Global Health. DN reports research support from Cepheid; a grant from the US National Cancer Institute; and an unpaid leadership role with the WHO Global Breast Cancer Initiative. WER reports grants from the Cambia Health Foundation, the Robert Wood Johnson Foundation, and the Rita and Alex Hillman Foundation; and book royalties from Springer Publishing and Jones & Bartlett Learning. JR reports funding from the NIHR and support from AstraZeneca for attendance at a teaching event. DS reports a leadership role at the Jamaica Cancer Care and Research Institute (co-director). HS reports membership of the Breast Cancer Now Tissue Bank Advisory Board and voluntary patient membership of the Independent Cancer Patients' Voice charity; an honorarium from the CRUK Precision Grand Challenge; and support for travel expenses from Breast Cancer Now, CRUK, the Association of Breast Surgeons UK, and Breast International Group. IV-L reports grants from Resilience; consulting fees from Novartis, AstraZeneca, and Amgen; payment or honoraria for lectures and presentations from Novartis, Sandoz, Amgen, AstraZeneca, and Pfizer; and support for travel and attending meetings from Novartis. CV-G reports grants from AstraZeneca and Roche; consulting fees from Roche, Novartis, Pfizer, and Eli Lilly; honoraria from Roche, Myriad Genetics, and Novartis; and support for attending meetings and travel from Roche, MSD Oncology, and Pfizer. All other authors declare no competing interests.