A malignant neoplasm (usually an adenocarcinoma) of the breast; the most common malignancy of American women and the leading cause of death in American women aged 40 to 55.

INCIDENCE
In 2011 the American Cancer Society estimated that 230,000 women would be newly diagnosed with invasive breast cancer, and that more than 39,000 women would die of the disease.

CAUSES
There are several known risk factors for breast cancer. SEE TABLE: Selected Risk Factors for Breast Cancer*

SYMPTOMS AND SIGNS
Breast cancer usually presents as a dominant mass in one breast, although the malignancy may first become evident when nipple discharge, nipple retraction, skin dimpling, or asymmetrical swelling of the breast occurs. In most cases breast cancers are first identified by women performing breast self-examination. A smaller but considerable number are detected by professional examination or mammography. About 1000 men are diagnosed with breast cancer annually. Breast cancer has several pathological variants. Carcinoma in situ, the most localized form of the disease, represents a preinvasive stage confined to a duct or lobule. Other presentations include lobular carcinoma, infiltrating ductal carcinoma, inflammatory carcinoma, and Paget disease of the nipple.

DIAGNOSIS
Regular breast self-examination, professional breast examination, and mammography are the keys to screening for breast cancer. All these screenings identify many more benign lesions than malignant ones, esp. in younger patients, and none of these techniques can definitively exclude breast cancer. Many mammographically detected lesions are benign, and about 15% of the time mammography will fail to detect lesions that are truly malignant. Digital mammography provides significantly better detection in women with dense breasts, those under age 50, and those who are premenopausal or perimenopausal. If a suspicious mass is identified, fine needle aspiration, core biopsy, or excisional biopsy must be used to obtain tissue for analysis. Ultrasonography can be used before biopsy to identify solid masses and cysts. Solid breast masses have a much greater chance of being malignant than cysts. Other imaging techniques used to help identify breast cancers include magnetic resonance imaging, positron emission tomography, and ductal imaging. SEE TABLE: Common and Experimental Techniques Used in Breast Imaging
SEE: breast self-examination; SEE: double reading; SEE: mammography

STAGING
The size of tumors and their possible metastasis to the chest wall, skin, axillae, or distant sites all determine the stage of breast cancer. Lymphatic mapping during cancer surgery can be used to find metastases to sentinel lymph nodes and guide therapies. Staging provides important information about the need for particular forms of therapy and the prognosis.

A biopsy (obtained by fine needle aspiration, with a stereotactic core needle, or by surgical lumpectomy) is usually recommended for any breast mass that does not resolve spontaneously within one or two menstrual cycles and for all postmenopausal women. Negative results from mammography and ultrasonography are not always accurate enough to rule out a malignant diagnosis.