Patients younger than 51 years of age, with stage T1-2N01-2aM0 breast cancer, and where the tumor can be locally excised with acceptable cosmetic result, will be randomized between a 16 Gy boost dose to the tumorbed and a 26 Gy boost dose to the tumor bed, after 50 Gy WBRT. Patients will be stratified based on age, tumor size, lymph node metastases, estrogen receptor status, interstitial or external boost irradiation, and institution. In principle frozen tumor samples and blood samples will be stored of each patient.