Although lactation does not affect the incidence of breast cancer, women who were never pregnant have a higher risk of breast cancer than those who are multiparous. Women who give birth to their first child later in life have a higher incidence of breast cancer than do younger primigravida women (12).
A historic well-controlled study from the Centers for Disease Control and Prevention showed that oral contraceptive use does not increase the risk of breast cancer, regardless of duration of use, family history, or coexistence of benign breast disease (13). A pooled analysis from 54 epidemiologic studies showed current users of oral contraceptives had a small but significant increased risk when compared with nonusers. Ten years after discontinuation, the risk of past users declined to that of the normal population (14).
It was reported that short-term estrogen treatment for menopausal symptoms did not increase the risk of breast cancer, but this belief was refuted by the results of the Women’s Health Initiative randomized trial. This prospective trial, involving 16,000 postmenopausal women randomly assigned to receive estrogen plus progesterone or placebo, revealed an association between hormone therapy use and the development of breast cancer. When invasive breast cancer developed, it was diagnosed at a more advanced stage compared with tumors that developed among placebo users. Based on interim analysis, the trial was stopped early and the investigators concluded that even relatively short-term use of combined estrogen–progesterone therapy increases the development of invasive breast cancer (15). The risk demonstrated by this study must be considered when postmenopausal hormone therapy is used to treat conditions such as hot ﬂashes and osteoporosis.
History of Cancer Women with a history of breast cancer have a 50% risk of developing microscopic cancer and a 20% to 25% risk of developing clinically apparent cancer in the contralateral breast, which occurs at a rate of 1% to 2% per year (16). Lobular carcinoma has a higher incidence of bilaterality than does ductal carcinoma. A history of endometrial, ovarian, or colon cancer is associated with an increased risk of subsequent breast cancer, as is a history of radiation therapy for Hodgkin’s lymphoma, even if the patient is BRCA1 and BRCA2 negative.
Breast cancer commonly arises in the upper outer quadrant, where there is proportionally more breast tissue. Masses are often discovered by the patient and less frequently by the physician during routine breast examination. The increasing use of screening mammography has enhanced the ability to detect nonpalpable breast abnormalities. Metastatic breast cancer is found as an axillary mass without obvious malignancy in less the 1% of cases.