In this multicenter retrospective cohort study the aim is to determine the safety of omission of axillary lymph node dissection in patients with TNBC and HER2+ tumors with residual macrometastases (in the SLN/TAD/TAS or MARI node) after NAC.

Inclusion Criteria:

• Women and men with a diagnosis of stage I-III TNBC or HER2+ breast cancer at diagnosis. HER2+ is defined as an Immunohistochemistry (IHC) score of 3+ or positive FISH. TNBC is defined as ER and Progesterone Receptor (PR) IHC expression of 0 and HER2 negativity defined as either IHC expression of 0-1+ or lack of gene amplification (FISH < 2.0). Patients with ER low (1-10%) and/or PR low (1-10%) tumors are allowed.
• Any histological subtype
• For Clinical Nodal Stage (cN) 0 at presentation: any axillary staging technique including palpation with or without imaging is allowed. Dual tracer mapping is not required for SLN surgery.
• For cN+ at presentation: Percutaneous biopsy proven confirmation is required at diagnosis. Staging techniques after NAC include: SLN surgery with dual mapping or Targeted Axillary Dissection (TAD: imaging-guided localization of sampled node in combination with SLN procedure with or without dual mapping) or Tailored Axillary Surgery (TAS: removal of the sentinel lymph nodes as well as selective removal of all palpable disease and documentation of the removal of the initially biopsy-proven and clipped lymph node metastasis by specimen radiography) or the MARI procedure (Marking Axillary Lymph Nodes with Iodine Seeds).
• Received neoadjuvant chemotherapy
• Residual macrometastases (metastasis greater than 2 mm in diameter) detected on SLN surgery or TAD ot TAS or MARI (on frozen section or final pathology)
• Concomitant presence of Isolated Tumor Cells (ITCs) and micrometastases in other sentinel lymph nodes is allowed
• Following SLN surgery/TAD/TAS/MARI patients underwent completion ALND, nodal radiation therapy (RT), both or no further axillary treatment
• At least 1-year follow-up (had surgery at any time point until October 2024 at the latest) - Prior history of ductal carcinoma in situ (DCIS) is allowed

Exclusion Criteria:

• Did not undergo SLN surgery/TAD/TAS/MARI (e.g., went straight to ALND)
• Presence of ITCs or micrometastases alone in the sentinel nodes (or TAD nodes or MARI node or TAS nodes) without macrometastases
• HR+HER2- tumors (except ER low and or PR-low)
• Stage IV disease at presentation
• Inflammatory breast cancer at presentation
• Neoadjuvant endocrine therapy
• Macrometastases detected by Oncoplastic Breast Consortium (OSNA)

Patients who presented with cT1-4 N0-3 TNBC (including ER-low tumors) or HER2+ BC who underwent NAC followed by axillary staging with either SLN surgery, TAD, TAS or the MARI procedure and were found to have residual macrometastases will be included. Data from "Universitätsspital Basel" will be extracted from medical records of from an already existing database.