The MASTER Study (MAmmary Cancer STatin ER Positive Study)

Patients must meet ALL of the following criteria to be eligible for randomization: Inclusion Criteria: 1. Women with estrogen receptor positive breast cancer who are candidates for (neo)adjuvant systemic therapy OR have received ≤3 years of adjuvant endocrine therapy. 2. Age \> 18 years. 3. Performance status of ECOG ≤ 2. 4. Prior to patient registration, written informed consent must be given according to ICH/GCP, and national/local regulations. Patients meeting ANY one of the following criteria are not eligible: Exclusion Criteria: 1. History of any prior (ipsi- and/or contralateral) invasive breast carcinoma. 2. Ongoing (prevalent) cholesterol-lowering therapy (statins, fibrates, ezetimibe, PCSK9 inhibitors). If so, the patient can be enrolled in the observational arm. 3. Evidence of hepatic dysfunction (alanine aminotransferase level more than three times the upper limit of the normal range) or renal dysfunction (creatinine level more than three times the upper limit of the normal range). 4. Predisposing factors for rhabdomyolysis, including hypothyroidism, reduced renal function, any muscle - or liver disease, or excessive alcohol consumption AND creatine kinase (CK) measured to less than five times the upper limit (CK only measured in case of predisposing factors). 5. No current medication with potent CYP3A4-inhibitors (e.g. ketokonazole, erythromycin) or gemfibrozile, cyclosporin or danazol. 6. Pregnancy or breast-feeding. 7. Psychological, familial, sociological or geographical condition potentially hampering compliance with the study protocol and follow-up schedule; these conditions will be discussed with the patient before registration in the trial. 8. History of allergic reactions attributed to compounds of similar chemical or biological composition to atorvastatin.