In most cases, treatment for breast cancer begins with surgical excision of the tumor. Modern treatment attempts to preserve as much tissue as possible for both functional and cosmetic reasons. This may mean a lumpectomy (simple excision of only the cancerous tumor) or mastectomy (excision of part or all of the breast tissue, sometimes with adjacent muscle). The lymph nodes under the arm are often excised in a procedure known as an axillary dissection if a sentinel node (one of the first nodes to filter fluid from the portion of the breast with the cancer) shows evidence of cancer. In some cases, chemotherapy and external beam radiation therapy or radioactive isotopes implanted directly into the area of the cancer, are used in addition to or instead of surgery. Hormone therapy in the form of ovary removal or drugs such as tamoxifen and selective estrogen receptor modulators or anastrozole and other aromatase inhibitors may be used to slow the growth of or prevent recurrence of hormonally sensitive tumors; tamoxifen is also used to control the growth of metastatic breast cancer. Breast cancer that tests positive for human epidermal growth factor receptor 2 (HER2-positive breast cancer) is less responsive to hormone treatment, but may be treated with HER2-specific drugs such as trastuzumab (Herceptin) and lapatinib (Tykerb). Bone marrow transplantation is sometimes used when bone marrow that has been destroyed by large doses of chemotherapy or radiation therapy needs to be replaced.
Many women who have had a mastectomy decide to have breast reconstruction surgery. This reconstruction is done with breast implants or the patient's own tissue. Due to the controversy over silicone implants, saline-filled implants were used from 1992 to 1998, but either type may be used now. Women who have had an axillary dissection often experience chronic, progressive pain, numbness, and weakness in the affected arm. Lymphedema, painful swelling of the arm, can occur after node dissection or radiation treatment of the lymph nodes. Following surgery, chemotherapy, and radiation, women who had estrogen-sensitive tumors are given tamoxifen or, if they are postmenopausal, anastrozole or another aromatase inihibitor to help prevent a recurrence.