The goal of this observational study is to describe the real-world utilization patterns of a pertuzumab biosimilar and to evaluate its clinical outcomes in patients with breast cancer in both neoadjuvant and metastatic settings. It also aims to assess pathological complete response (pCR), disease-free survival (DFS) in the neoadjuvant cohort, progression-free survival (PFS) in the metastatic cohort, overall survival (OS), treatment response, and safety and tolerability (adverse events according to CTCAE) across both cohorts.

This is a non-interventional, prospective, multicenter, bicohort observational study conducted in a real-world setting. It aims to describe the clinical and therapeutic characteristics of patients treated with a pertuzumab biosimilar and to evaluate outcomes in both neoadjuvant and metastatic settings. Approximately 1,000 patients will be included, with 500 patients per cohort. Cohort 1 includes patients receiving a pertuzumab biosimilar in the neoadjuvant treatment of HER2-positive breast cancer, while Cohort 2 includes patients treated in the first-line metastatic setting for HER2-positive metastatic breast cancer.

Inclusion Criteria:

• Age ≥ 19 years at the time of inclusion;
• Histologically confirmed diagnosis of HER2-positive invasive breast cancer (IHC 3+ score or HER2 amplification by FISH/CISH/SISH);
• Decision to initiate treatment with a pertuzumab biosimilar in the neoadjuvant setting (neoadjuvant cohort) or as first-line therapy in the metastatic setting (metastatic cohort), in accordance with local indications;
• Treatment prescribed as part of routine clinical care (outside of a clinical trial);
• Patient informed and having provided written informed consent.

Exclusion Criteria:

• Treatment with pertuzumab in a therapeutic setting other than neoadjuvant or first-line metastatic (e.g., adjuvant, late recurrence beyond first-line, maintenance therapy alone, etc.);
• Treatment administered as part of an interventional clinical trial;
• History of severe allergy or known contraindication to pertuzumab;
• Medical records unavailable or inability to ensure at least 6 months of minimal follow-up.

Adult women with HER2-positive breast cancer receiving a pertuzumab biosimilar in the neoadjuvant setting or as first-line treatment in the metastatic setting.