Circulating Tumor Cells to Guide Chemotherapy for Metastatic Breast Cancer

Inclusion Criteria: * Women over the age of 18 years. * WHO performance status: 0 to 4. * Metastatic breast cancer. * Progression after 2 lines of chemotherapy with decision to initiate third-line chemotherapy. * Disease evaluable by CTC (CTC-positive before starting chemotherapy). * Histology: lobular or ductal adenocarcinoma. * Information of the patient and signature of the informed consent form by the patient or her legal representative. Exclusion Criteria: * Disease not evaluable by CTC (CTC-negative before starting chemotherapy). * History of other potentially metastatic cancer (stage III or IV cancer) different from breast cancer. * Histology other than lobular or ductal adenocarcinoma. * Pregnant woman, women likely to become pregnant or nursing mothers. * Persons deprived of their freedom or under guardianship. * Women unable to comply with the medical follow-up of the study for geographical, social or mental reasons.