Lumpectomy

A lumpectomy or tylectomy is a surgical procedure used to treat malignant breast cancer. Unlike a mastectomy, which removes most of the breast tissue, a lumpectomy limits removal of tissue to the tumor and the immediately surrounding tissue. A lumpectomy is always followed with a course of chemotherapy and/or radiation therapy.
The theory of mastectomy was that the malignant cancer would spread outwards from the tumor, so the surgeon would remove as much surrounding tissue as possible. However, modern theories of oncology hold that malignancies are cells that break off from the main tumor and circulate in the bloodstream before becoming tumors elsewhere. As such, removing the main body will remove most of the cancerous cells, leaving the others to be dealt with by other therapies.
Clinical studies have shown that lumpectomies followed by other treatment are at least as effective as mastectomy in the treatment of breast cancer. The obvious advantage is that the procedure is less invasive and leads to much easier reconstruction of the breast tissue if the patient wishes. 
Lumpectomy at Wikipedia