As most of the clinical evidence that has a significant impact on the care of breast cancer is based on studies conducted in Western countries, it is important to understand the differences in the natural history of the disease as well as in the patterns of care before applying that evidence to daily practice in Japan. For example, the EORTC 22881 study demonstrated significant improvement of local control after breast conserving therapy with the additional use of 16 Gy of boost irradiation to the tumor bed after 50 Gy of whole breast irradiation for margin negative patients. However, the extent of the surgery in this study was smaller than that common in Japan, and the criteria for a negative margin was less strict than that in Japan. Therefore, it may be over-treatment to give boost irradiation routinely to margin negative patients in Japan. On the other hand, as it is a universal observation that patients under the age of 40 have greater risk of local recurrence, the result of EORTC 22881 study that the benefit of boost irradiation was greatest in patients younger than age of 40 encourages us to confirm this finding in our own clinical settings. It is desirable to develop clinical evidence of high quality in Japan that are directly applicable to Japanese patients.