Background: Treatment guidelines recommending omission of axillary surgery in breast cancer are largely based on the SOUND and INSEMA trials. However, the extent to which their study populations represent real-world patients remains unclear. We aimed to evaluate the real-world applicability and external validity of these trial populations.

Materials and methods: All consecutive patients treated for early breast cancer at a single university hospital between 2010 and 2018 were included. Patients with clinically node-negative disease were identified, and eligibility according to the SOUND and INSEMA inclusion criteria was determined. Clinicopathologic characteristics were compared between trial-eligible real-world patients and published trial populations.

Results: A total of 2787 consecutive patients with clinically negative axilla were included; 71% (1982/2787) fulfilled the INSEMA and 52% (1461/2787) the SOUND trial eligibility criteria. Patients eligible in the SOUND trial were largely representative of real-world patients in terms of clinicopathologic characteristics. In contrast, the INSEMA trial appeared more selected, with a higher proportion of biologically favorable tumors. Both trials predominantly included patients with small (< 2 cm) luminal breast cancers. Patients with larger tumors and more aggressive subtypes were underrepresented.

Conclusion: The SOUND and INSEMA eligibility criteria are broadly applicable to real-world patients with small luminal breast cancers. However, differences between trial populations and real-world patients highlight the need for careful consideration when applying SLNB omission beyond these lower-risk subgroups.

Keywords: axillary lymph node dissection; breast cancer; de‐escalation; sentinel lymph node biopsy; surgery.