Removing breast cancer with a margin of surrounding normal breast tissue (breast conserving surgery) is an alternative to a mastectomy for many women with early breast cancer. Breast conserving surgery is almost always followed by a course of radiotherapy. This treatment includes all of the remaining tissue of the breast from which the cancer was removed. 

For some women the results of radiotherapy can be improved by an additional dose (boost) of radiotherapy given to just the part of the breast where the cancer was (the tumour bed). Studies have suggested the benefit from this is greater for younger women, especially those under 40. 

Another situation where a boost treatment might be used is when there is a possibility the cancer hasn't been completely removed at the time of the operation, or if the margins of healthy tissue around the tumour were not completely clear of cancer. Usually if there is a suspicion that any tumour has been left behind further surgery will be offered but for some women the additional boost radiotherapy may be an alternative.