Whole breast irradiation (WBI) after breast conserving surgery for early-stage breast cancer halves the recurrence risk and reduces the breast cancer death by about one sixth. A sequential boost (SeqB) dose to the tumour bed further improves local control, but also increases the risk of late skin toxicity and cosmetic changes. At Ghent University Hospital WBI is prescribed in 15 fractions of 2.67 Gy according to the START-B hypofractionation scheme. A sequential boost is typically given in 4 to 8 extra fractions which prolongs the overall treatment time. The boost dose can also be delivered within the 15 fractions of WBI, the so-called simultaneous integrated boost (SIB). SIB shortens the overall treatment time which is convenient for the patient and the radiotherapy department. In this study we wish to test the hypothesis of acceptable skin toxicity and reduced cost with SIB using hypofractionated prone intensity modulation radiotherapy IMRT with topographical dose painting, a technique recently developed in our group. Patients are randomized between SeqB and SIB.