Although there is no doubt that the presence of lymph node metastases worsens prognosis of a patient, unambiguous evidence to support lymph node dissection is still lacking. For many solid tumors, the role of lymph node dissection is yet controversial, and may depend on the tumor type and the stage of patient presentation for diagnosis. Axillary surgery for breast cancer is now considered as staging procedure that does not seem to influence breast cancer mortality. Women with breast cancer have benefitted greatly from a series of carefully performed randomized controlled trial focusing on axillary surgery. The objective of INSEMA is to show that less axillary surgery is better, in that oncological outcomes are the same and less surgical intervention will result in fewer surgical complications.
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