Today, the majority of clinically node positive (cN+) breast cancer patients is treated with neoadjuvant systemic therapy (NST). Axillary staging and treatment after NST in cN+ patients are areas of controversy. Patients with a pathological complete response (pCR) of the axillary lymph nodes are not expected to benefit from axillary lymph node dissection (ALND). Hence, less invasive axillary staging procedures are being introduced to avoid unnecessary ALND. However, evidence supporting the safety of replacing ALND by less invasive techniques in terms of oncologic safety and impact on quality of life (QoL) is lacking.

The MINIMAX is a multicenter registry study that includes node positive breast cancer patients, who are treated with NST (chemotherapy and ± immunotherapy), in order to gain insight in the oncologic safety and impact on QoL of less and more invasive axillary staging and treatment strategies.

Patients who are included in this study will complete Patient Reported Outcome Measures (PROMs) at baseline (time of diagnosis), and 1 and 5 years after diagnosis to assess impact on QoL.

A database will be built by the Netherlands Cancer Registry. Data on patient-, tumor-, pre-NST staging-, post-NST staging- and treatment-characteristics will be retrieved from patients' records by trained data registrars of the Netherlands Comprehensive Cancer Organisation (IKNL) using electronic case report forms (eCRFs). Five-year survival and recurrence will be evaluated to determine oncologic safety.

The results will be incorporated in the national guidelines. In case of an equilibrium between less and more invasive strategies, the data of this study will at least be extremely suitable to be used in the shared decision making process.

Inclusion Criteria:

• Female patient with unilateral invasive breast cancer and cN1-3
• Pathologically proven positive axillary lymph node
• Planned to undergo neoadjuvant chemotherapy (± immunotherapy), followed by staging and treatment of the breast and axilla

Exclusion Criteria:

• Clinically node negative breast cancer before NST
• Bilateral invasive breast cancer
• Neoadjuvant endocrine therapy
• Distant metastases (including oligometastatic disease)
• History of invasive breast cancer
• Other malignancies, except for basal/squamous cell skin cancer, and in situ carcinoma of the cervix or breast
• Axillary surgery or radiotherapy before NST (this includes SLNB prior to NST)

Female patients with unilateral invasive breast cancer and cN1-3, with a minimum of one pathologically proven positive axillary lymph node, treatment with neoadjuvant chemotherapy (± immunotherapy), followed by staging and treatment of the breast and axilla.