Local management of second ipsilateral breast cancer events after primary breast-conserving therapy: an international Delphi consensus.

Management of second ipsilateral breast cancer events (iBCEs) remains controversial, and there is a need to collate existing evidence and international guidance on patient selection and local treatment strategies. This project, endorsed by US and European surgical and radiation oncology societies, aimed to gather expert consensus on these issues. A questionnaire on second iBCE local treatment was developed and reviewed by a core group of eight experts, and Delphi methodology was applied over two rounds to 36 panellists, including radiation oncologists, breast surgeons, a plastic surgeon, and medical physicists. Consensus was predefined as agreement of 75% or higher. After two rounds, consensus was reached for 78 (80%) of 97 items. Panellists agreed that patient preferences are central to decision making (100%) and that a second breast-conserving therapy represents a reasonable option for selected patients (100%). Criteria associated with greater suitability for second breast-conserving therapy included an interval between surgeries of at least 60 months, low-risk accelerated partial breast irradiation classification, luminal molecular profile, and no grade 3 late toxicity related to the first breast-conserving therapy. HER2 (also known as ERBB2)-positive or triple negative subtypes were not viewed as absolute contraindications. Strong consensus was also observed regarding the importance of tumour-to-breast volume ratio, clear surgical margins, and tumour bed reirradiation. For patients undergoing mastectomy, immediate autologous reconstruction was preferred (94%) over implant-based approaches (75%). This international Delphi consensus offers structured guidance for the local management of second iBCE and supports shared decision making and individualised treatment planning.