3. Justification Breast conserving surgery with the use of whole breast radiation after partial mastectomy is the preferred treatment for most women with early stage breast cancer. Conventional radiotherapy in this setting delivers radiation to the entire breast, with the consequence that the radiotherapy has to be given over a period of 3 - 6 weeks. However, the majority (70 - 80%) of local recurrences after breast conserving surgery develop in the vicinity of the primary tumor. Partial breast irradiation with the radiation target volume limited to the surgical cavity (SC) in selected patients is therefore potentially feasible if it results in similar local control rates compared to whole breast irradiation. In addition to limiting the radiation to breast tissue most likely to harbor microscopic disease and thereby sparing the normal breast tissue, local breast irradiation offers the distinct advantage of shortening the duration of radiotherapy, thereby minimizing the impact radiotherapy has on the quality of life of breast cancer patients.