Breast cancer is a disease that can be treated with surgery to remove the part of the breast that has the cancer followed by radiation treatment to the whole breast. The whole breast radiation treatment improves the chances of not having the cancer come back in that breast. However, this treatment does not completely cure everyone who has it. In some people, the breast cancer comes back, despite having this treatment. If the breast cancer comes back, most of the time it comes back in the part of the breast from which the cancer was removed. Therefore, in some people, it may be possible to give radiation to area around the removed cancer. This is called partial breast irradiation. This research project is testing if partial breast irradiation could be given using a new technique called 3-dimensional (3-D) conformal radiation therapy. In this technique, the radiation beams are planned using a CT scan that gives a 3-D view of the tumour bed. This 3-D planning allows the researchers to shape and direct the radiation more accurately to the tumour bed. The whole breast will not receive radiation. In doing so, it is possible to avoid the nearby organs such as the heart and lungs. A total of 48 women have taken part in this research project to see if partial breast irradiation can be successfully given using the new 3-D radiation technique.