The OBSERB study is a multi-center, non-blinded, pragmatic, parallel, randomized controlled trial in which patients with cT1-3N1 and ycN0 breast cancer.

The OBSERB study aims to compare the oncologic outcomes of radiotherapy without sentinel lymph node biopsy versus sentinel lymph node biopsy for the management of breast cancer which showed the cN1 was conversed as ycN0 after neoadjuvant chemotherapy.

To verify whether, the radiotherapy without SLN can be applied, when the cN1 was conversed as ycN0 after neoadjuvant chemotherapy.

To verify whether, the additional treatment decision can be established based on only the biology of primary tumor without the pathologic information of axillary lymph nodes To verify whether, the patients' quality of life can be improved by a less invasive surgical procedure, when the cN1 was conversed as ycN0 after neoadjuvant chemotherapy.

Inclusion Criteria:

1. women aged > 20 years
2. cT1-3N1M0, primary invasive
3. initially confirmed ALN by fine needle aspiration cytology before neoadjuvant chemotherapy
4. completion of neoadjuvant chemotherapy
5. tumors negative for LN metastasis confirmed by imaging after neoadjuvant chemotherapy

Exclusion Criteria:

1. synchronous distant metastases
2. previous other malignancy
3. bilateral breast cancer
4. previous primary systemic therapy
5. pregnancy or breastfeeding
6. pre-operative radiological evidence of multiple involved or suspicious axillary nodes
7. patient with psychiatric, addictive, or any disorder, which compromises ability to give informed consent for participation in this study.