2.1. Patients

Clinical data were extracted from a multicentric retrospective database comprising 23,134 patients who underwent primary surgery for early BC in 15 different French academic centers ({"type":"clinical-trial","attrs":{"text":"NCT03461172","term_id":"NCT03461172"}}NCT03461172 ^{9 , 10 , 11 , 12 , 13 , 14} ). Patients <75 years who underwent BCS and sentinel lymph node biopsy (SLNB) without neoadjuvant therapy, with SN micro‐metastases involvement (>0.2 mm to <=2 mm) confirmed by pathologic examination, with or without cALND were included. Exclusion criteria were the absence of SLNB; patients with SN macro‐metastases, pN0(i+)sn, pN0sn, SLNB, or ALND after NAC; SN micro‐metastasis lost of follow‐up; patients receiving mastectomy or with unknown type of surgery; Neo adjuvant chemotherapy; or age ≥ 75 years. SLNB was performed for patients with invasive BC < = 5 cm, axillary cN0, detected by combined isotopic and colorimetric method or only isotopic detection, with peri‐tumoral or retro areolar injection. Pathologic examination included immunohistochemically analysis in case of a negative result on standard hematoxylin and eosin analysis on serial sections. ¹⁵ Positive endocrine receptor (ER) status and HER2 status were determined according to French guidelines (estrogen receptor and/or progesterone receptor by immunochemistry [IHC] with a 10% threshold for ER positivity; IHC HER2 3+ and/or HER2 amplification by in situ hybridization). Adjuvant chemotherapy (AC) was administered according to guidelines used in each center and whole breast radiotherapy (WBR) was systematically performed after BCS. Regional node irradiation was not systematically realized.