Sentinel node (SN) biopsy in breast cancer has been demonstrated to be a reliable method, and several follow-up studies have shown that it is safe to refrain from completion axillary clearance (axillary lymph node dissection, ALND) in SN-negative patients. SN biopsy alone results in significantly less arm discomfort following the operation.

However, as a surprisingly low frequency of axillary relapse has been observed in patients without any axillary intervention (neither SN biopsy nor axillary clearance), or without completion ALND after a positive SN biopsy, the importance of ALND is being questioned, even for patients with SN metastases. A large, randomized study (ISBCG23-1) was not able to show any differences in 5-year disease-free survival between patients with SN micrometastases who had undergone ALND and those who had not.

This Swedish multicenter study will include patients with SN micrometastases. These women will not undergo ALND but will be registered in a study cohort. The patients will otherwise be treated in accordance with the national guidelines and will be clinically followed every year for five years, after 10 years and finally after 15 years. Hypothesis: To refrain from axillary clearance in breast cancer patients with sentinel node micrometastases does not impair survival.

From April 2017 onwards, only patients operated by mastectomy or those receiving neoadjuvant therapy and having a sentinel node biopsy before its initiation may be recruited into SENOMIC. The inclusion target has been met for those patients operated by breast-conserving therapy, and from now on, these patients will not be included in SENOMIC anymore. Recruitment of the above-mentioned patient selection will continue until 452 patients operated by mastectomy have been enrolled.

Inclusion Criteria:

• Patients with clinically node-negative, invasive breast cancer ≤ 5 cm (T1-T2)
• Histopathology results demonstrate SN micrometastases.
• Patients who undergo mastectomy (protocol change from April 2017 onwards)
• The patient must have given verbal and written consent.

Exclusion Criteria:

• Preoperatively diagnosed lymph node metastases.
• Sentinel node metastases > 2 mm.
• Metastases beyond the ipsilateral axillary lymph nodes at the time of surgery.
• History of previous breast cancer.
• Pregnancy.
• Bilateral breast cancer where any of the other exclusion criteria applies to either side.
• Medical contraindication for systemic adjuvant therapy