Evaluation of Medical Treatments in MBC Patients According Biological Subtype and Line of Treatments

The primary objectives are to evaluate the duration of metastatic breast cancer treatments (chemotherapy, hormonal therapy and biological therapies) according to biological subtype (Luminal A, Luminal B, HER2 positive, triple-negative) and to evaluate the number of lines of metastatic breast cancer treatments according to biological subtype (Luminal A, Luminal B, HER2 positive, triple-negative). The secondary Objectives are to evaluate overall survival according to duration and to number of lines of metastatic breast cancer treatments and to identify predictive factors of number of lines of treatment as for example age, treatment response, biological subtype, metastatic sites, etc and to identify possible elements of different treatment management between participating sites. The aim of this retrospective and prospective study is to identify the duration of treatments (chemotherapy, hormonal therapy and biological therapies) according to biological subtype and line of treatment in metastatic breast cancer patients. An ancillary study will be conducted on part of population (HR+/HER2- patients newly diagnosed for mBC receiving first line CDk4/6 inhibitors). For the ancillary study, it is expected to enroll at least 400 patients, who will be asked to fill in some questionnaires at the following visits, scheduled as per clinical practice: * PROFFIT: baseline, day 1 cycle 3, day 1 cycle 5, every 6 months thereafter; * Patient-reported outcomes (PRO): EORTC-QLQ-C30, FACT-B, COST-FACIT at baseline, day 1 cycle 3, day 1 cycle 5, every 6 months thereafter. The ancillary study could evaluate: * the impact of first line CDk4/6 inhibitors on HR+/HER2- metastatic breast cancer patients' financial toxicity * retrospectively, the correlation between NLR and outcome in patients with breast cancer and in treatment with a CDk4/6 Inhibitor (as first line vs as second line) * the correlation between BMI and outcome in patients with breast cancer and in treatment with a CDk4/6 Inhibitor (as first line vs as second line).