Most breast cancers are removed by partial mastectomy/lumpectomy. The most important factor in ensuring that the cancer does not return in the breast is to ensure that it is completely removed during surgery. Complete removal is dependent on having a rim of normal tissue (clear margin) surrounding the cancer. If the margin is positive, or the cancer recurs, more surgery or mastectomy is required. Approximately one third of breast cancers are detected by mammograms or ultrasounds and cannot be felt by patients or physicians. Accordingly, a localization technique is required to help the surgeon to find and remove the cancer. The current technique (needle localization) has a higher chance of having cancer cells at the margin. This is a study about a surgical technique. The objective of this comparison study is to determine whether a new technique (radioguided seed localization) is a better way to remove nonpalpable breast cancers. The main objective of this study is to determine if the new technique generates fewer positive margins compared to the standard technique. An improved technique would benefit thousands of women every year.