In patients < 51 years of age, the LR rate was reduced with 50% after a 66 Gy dose to the tumor bed, compared to 50 Gy (5-year LRR 12% vs 5.9%, p < 0.02). However, the LRR in young women was still quite high (> 1% per year). Therefore the first aim of the study is to investigate whether an additional boost dose to the tumorbed (26 Gy) reduces the LRR further. Therefore, we will compare the effect of a low boost dose (16 Gy) with the effect of a high boost dose (26 Gy) on the LRR, but also on the cosmetic outcome.