Setting a precedent: Development of a core outcome set for locoregional treatment outcome reporting in neoadjuvant breast cancer trials.

e12570 Background: Accurate information about locoregional (LR) treatments following neoadjuvant systemic therapy (NST) for breast cancer is essential for interpretation of oncological outcomes, but reporting is currently poor. We aimed to develop a core outcome set (COS) to improve quality and consistency of LR outcome reporting in trials. Methods: The COS was developed in three phases according to COS-STAD guidance: 1. Generation of a long list of relevant outcome domains from a systematic literature review and stakeholder interviews. 2. Prioritisation of outcome domains using two rounds of an online Delphi survey. 3. An in-person consensus meeting to agree the final COS. Results: In total, 159 unique LR outcomes were identified from phase 1 and categorised into 101 (69 surgical/32 RT) outcomes for survey inclusion. 470 international participants (206 surgeons/144 medical oncologists/98 radiation oncologists) took part in survey Round 1, of whom 336 (71.5%) (153 surgeons/90 medical oncologists/ 77 radiation oncologists) participated in Round 2. After Round 2, 31 outcomes, combined into 15 summary outcomes, were scored as ‘consensus in’, 60 as ‘consensus out’ and 10 as ‘no consensus’. 23 professionals and 5 patient advocates attended the consensus meeting, where ‘consensus in’/’consensus out’ items were ratified, and ‘no consensus’ items discussed. A final 15 item COS for LR outcome reporting was agreed and ratified (Table). Conclusions: A COS for LR treatment reporting in NST trials has been robustly developed using an internationally collaborative approach. Widespread implemention will improve the quality and value of future breast cancer NST trials. Final COS 1 Type of breast and axillary surgery planned before starting NST 2 Proportion of patients not having surgery after NST due to disease progression, treatment toxicities or other comorbidities 3 Number/proportion of patients with a complete response to NST not having surgery to the breast and/or axilla, and how response was assessed 4 How response to NST in the breast and axilla was assessed 5 Type of initial breast and axillary surgery performed after NST 6 Proportion of patients with involved margins after initial and final surgery, and number of procedures required 7 Total number of excised and involved axillary lymph nodes, with extent of involvement 8 Further axillary treatment in patients with ypN+ disease after sentinel node biopsy/targeted axillary dissection (SLNB/TAD) 9 Proportion of patients in whom breast and/or axillary surgery was downstaged 10 Proportion of patients having radiation therapy 11 Indications for radiation therapy at trial level 12 Details of breast/chest wall and nodal targets 13 Details of dose and fractionation to breast/chest wall and nodal areas 14 Receipt of boost; indications for boost (at trial level) 15 Morbidity of locoregional treatments (short/long term as defined in protocol)