Comparison of Axillary Sentinel Lymph Node Biopsy Versus no Axillary Surgery

Inclusion Criteria: * Written informed consent prior to beginning breast-conserving surgery, including expected cooperation of the patients for follow-up, must be obtained and documented according to the local regulatory requirements * Histologically confirmed unilateral primary invasive carcinoma of the breast (core biopsy or open biopsy). Multifocal or multicentric tumors are allowed if breast-conserving surgery is planned * Age at diagnosis at least 18 years * Preoperative imaging techniques with estimated tumor size of \<5 cm (iT1/iT2 irrespective of hormone sensitivity or HER2 status) * Clinically and sonographically tumor-free axilla prior to core biopsy (cN0/iN0) * In cases with cN0 and iN+, a negative core biopsy or fine needle aspiration (FNA) biopsy of the sonographically suspected lymph node is required before randomization * No clinical evidence for distant metastasis (M0) * Planned breast-conserving surgery (R0 resection) with postoperative external whole-breast irradiation (conventional fractionation or hypofractionation) Exclusion Criteria: * History of malignancy within last 5 years, except curatively treated basalioma of the skin and carcinoma in situ of the cervix * Time since core biopsy \>3 months (optimal \<1 month) * Previous and already (neoadjuvant) treated invasive breast carcinoma * Histologically non-invasive breast carcinoma * cT3/T4 or iT3/T4 tumors * pregnant or lactating patients * Planned total mastectomy (e.g. multicentric tumors ) * Planned intraoperative radiotherapy (e.g. Intrabeam) or postoperative partial breast irradiation (e.g. multicatheter technique) alone; both procedures are allowed as boost techniques * Male patients