The aim of this multicenter retrospective cohort study is to determine the safety of omission of axillary lymph node dissection in patients with residual micrometastases after neoadjuvant chemotherapy.

Chemotherapy before surgery, known as neoadjuvant chemotherapy (NAC), is often used to treat patients with advanced breast cancer or aggressive early-stage breast cancer. Research shows that about one in four early-stage breast cancer patients receives this treatment.

For patients undergoing NAC, standard of care includes checking the lymph nodes under the arm (axillary lymph nodes) using a procedure called Sentinel lymph node biopsy to determine if the cancer has spread. In cases where the cancer has spread to the lymph nodes but then shrinks or disappears after chemotherapy, special techniques like dual tracer mapping, targeted axillary dissection, or the Marking Axillary Lymph Nodes with Iodine Seeds procedure are used to ensure accurate biopsy results.

For patients whose cancer shrinks completely in the lymph nodes, it is generally safe to skip further surgery to remove more lymph nodes, a procedure called axillary lymph node dissection (ALND). However, if cancer remains in the lymph nodes, more lymph nodes are typically removed because there is a high chance that cancer is still present.

An increasing number of surgeons are starting to omit ALND, especially when only tiny amounts of cancer (micrometastases) remain. To ensure this practice is safe, real-world data on patient outcomes over time is needed.

This multicenter retrospective cohort study aims to assess the safety of omitting ALND in patients with residual micrometastases after NAC for breast cancer. The study analyzes data of breast cancer patients treated between 2013 and 2024 at more than 50 centers that are part of the Oncoplastic Breast Consortium.

The results of this study will provide valuable information to help doctors determine the best treatment approach for their patients.

Inclusion Criteria:

• Consecutive women and men aged 18 years or older with a diagnosis of stage I-III breast cancer (any histological and receptor subtype)
• For Clinically Node negative (cN0) at presentation: any axillary staging technique including palpation is allowed. Dual tracer mapping is not required for Sentinel Lymph Node Biopsy (SLNB)
• For Clinically Node positive (cN+) at presentation: Biopsy proven confirmation is required. Staging techniques include: SLNB with dual mapping or targeted axillary dissection (Targeted Axillary Dissection, TAD: imaging-guided localization of sampled node in combination with Sentinel Lymph Node procedure with or without dual mapping) or the Marking Axillary Lymph Nodes with Iodine Seeds (MARI) procedure
• Completed neoadjuvant chemotherapy
• Residual micrometastases detected on SLNB or TAD or MARI (on frozen section or final pathology)
• Concomitant presence of Isolated Tumor Cells (ITCs) and micrometastases in other sentinel lymph nodes is allowed
• Received axillary treatment with completion axillary lymph node dissection (ALND), axillary Radiotherapy (ART), both or none
• At least 1-year follow-up (inclusion should end in May 2023)
• Had surgery at any time point until May 2023 at the latest
• Prior history of stage I-III (not stage IV) breast cancer is allowed

Exclusion Criteria:

• Did not undergo SLNB/TAD/MARI (e.g., went straight to ALND or ART)
• Presence of ITCs alone or macrometastases on the sentinel nodes (or TAD nodes or MARI node)
• Stage IV disease at presentation
• Inflammatory breast cancer at presentation
• Neoadjuvant endocrine therapy
• Micrometastases detected by One-step nucleic acid amplification (OSNA)

The study assesses consecutive patients with residual micrometastases after neoadjuvant chemotherapy, treated with or without axillary dissection. The data originate from participating Oncoplastic Breast Consortium (OPBC) centers as well as from the US and Brazil, and have been collect within the clinical routine between 2013 and 2024.