Approximately 1 percent of breast cancer cases occur in men, and less than 1 percent of men’s cancer diagnoses are for breast cancer (ACS, 2011b). Because it is rare, breast cancer in men has been difficult to study. Based on what is known, however, it is considered to resemble breast cancer in postmenopausal women (Korde et al., 2010).
As in women, men’s breasts respond to changes in sex hormone concentrations (both estrogens and androgens), but under normal circumstances they do not undergo the differentiation and lobular development that women’s breasts experience with puberty, pregnancy, and lactation (Johansen Taber et al., 2010). Either an excess of estrogens or deficit of androgens appears to increase risk of breast cancer in men (Korde et al., 2010). Beginning after age 20, rates rise steadily with age. Approximately 92 percent of male breast cancers are estrogen receptor positive, compared with approximately 78 percent of breast cancers in women (Anderson et al., 2010). As is the case for women, inherited mutations in BRCA1 and especially BRCA2, as well as other mutations, are associated with an increased risk of male breast cancer, but the majority of cases are not associated with a family history of the disease (Korde et al., 2010).
The Breast, Breast Development, and Breast Cancer
The development of the human female breast begins during gestation but is not complete at the time of birth. Further development and differentiation of breast tissue occurs over time and especially in response to fluctuating estrogen and other hormonal signals beginning in puberty, continuing through the reproductive years, during pregnancy and lactation, and at menopause. Monthly ovulatory cycles are accompanied by cyclical changes in the form and behavior of cells and structures in the breast, including progressive differentiation. Pregnancy and lactation trigger maximal differentiation of the breast. When pregnancy and lactation end, as well as at menopause, breast tissue regresses to a less differentiated state.
Within the breast are adipose and connective tissues that surround multiple collections of lobules in which milk is produced during lactation. Milk moves to the nipple through ductal structures. The ducts are lined by luminal epithelial cells and have an outer layer of myoepithelial cells. Popu-