Rationale: Whole breast irradiation with 50 Gy after lumpectomy has been shown to reduce the local recurrence rate with a factor 3 to 4. An additional boost of 16 Gy to the tumorbed has been shown to improve the local control rate even further. However, this boost dose appears to reduce not only the local recurrences in the tumorbed, but also elsewhere in the breast, suggesting that the boost may not always be delivered at the correct region. In addition, even with the boost dose of 16 Gy, the local recurrence rate in young patients is still quite high - with the majority of recurrences in the tumorbed. Consequently, further improvement of the local control, especially in young women, is still an important aim to pursue, in order to maintain breast conserving therapy (BCT) as an acceptable treatment option in young women. One way to improve local control may be to increase the dose [Young Boost Trial], but also to improve the definition of the tumorbed, i.e. the target volume for boost irradiation.