Patients will be evaluated at the time of acceptance to participate in the study after signing the informed consent and will be tracked every three months during treatment with second-line monochemotherapy, coinciding with routine assessments within normal clinical practice. When the patient completes treatment with second-line chemotherapy he/she will enter the post-treatment period, with follow-up at six and 12 months after completion of treatment. Also, patients will be followed to assess survival until the end of the study (30 months after enrolment of the last patient in the study). Follow-up time has been established based on the median survival of patients with MBC