Male breast cancer (MBC) is a cancer in males that originates in their breasts. Males account for less than 1% of new breast cancers with about 20,000 new cases being diagnosed worldwide every year. Its incidence rates in males vs. females are, respectively, 0.4 and 66.7 per 100,000 person-years (person-years is the number of new cases divided by the product of the relevant population's size multiplied by the average number of years of observation, i.e. new cases ÷ [population × years]). The worldwide incidences of male as well as female breast cancers have been increasing since the late 20th century. As of 2018, one of every 800 men were estimated to develop this cancer during their lifetimes.
Because it has a far lower incidence in males and because large-scale breast cancer studies have routinely excluded males, current knowledge of male breast cancer is far less than female breast cancer and often rests on small, retrospective, single-center studies. Consequently, the majority of strategies for evaluating and treating MBC have been adopted from those used for female breast cancer. However, MBC appears to have some features that warrant clinical approaches differing from those for female breast cancer. Features of male breast cancers that differ from those in females include variations in their presentations, associations with other diseases, associations with non-medical predisposing conditions, expressions of key breast cancer-related hormones, causes (including frequency and forms of genetic alterations), tumor types, and treatments.
Presentation, diagnosis and treatment of breast cancer in males The handling of MBC has typically followed many of the same protocols established for female breast cancer. Female breast cancer most often presents as a mass found on routine screening mammography or self-examination. It may have attracted attention by its size, tenderness, painfulness, or, less commonly, nipple discharge (which may be bloody), nipple retraction, breast swelling, or appearance of a skin lesion such as an ulcer.
The physical examination of these individuals focuses on measuring the size of their lumps and checking other sites, particularly the lymph nodes in the armpit nearest the tumor, for enlargements or masses that may indicate the tumor has metastasized. At this time or later, depending on further findings such as results of a biopsy, the women may be evaluated by medical imaging techniques such as ultrasonography, mammography, CT scans, magnetic resonance imaging, positron-emission tomography, scintimammography, and/or single-photon emission computed tomography to determine the extent of the primary tumor and presence of nearby lymph node and/or distant tissue lesions that may be metastases.
Before or after these initial studies, tissues from the lump are obtained by needle biopsy (sampling with a surgical needle), incisional biopsy (surgical sampling of a part of the tumor), or excisional biopsy (surgical removal of the entire tumor). The recovered tissue is examined for its microscopic histopathology and presence of tumor cells that express the estrogen receptor, progesterone receptor, and HER2/neu receptor. The tumor may also be examined for cells that express the androgen receptor and various gene mutations or other types of gene alterations that are known to be associated with and may contributor to the development and/or progression of breast cancer. Biopsy of the sentinel lymph node (i.e. armpit lymph node closest to the tumor) or multiple other lymph nodes located in this armpit as well as any suspicious and accessible lesion may also be taken. (Sentinel lymph node biopsy is preferred over broad dissection of the armpit lymph nodes for detecting local lymph node metastases.)
Based on the findings of these examinations, the tumor, if considered cancerous, is further defined based on its: a) clinical stage typically using the TNM staging system (i.e. scoring (T)umor size, (N)umber of armpit lymph nodes near the primary tumor with metastases, and (M)etastases in distant tissues); b) grade using the Nottingham system (grades 1, 2, and 3 indicate the tumor cells look progressively less like, and therefore are potentially more aggressive than, the normal tissue cells from which they arose); and c) histopathology (i.e. either in situ or invasive, i.e. cancer cells confined to their tissue of origin or invading adjacent tissue(s), respectively). The histopathology of these tumor is also critical for classifying the breast cancer's type. Depending on their cancers' severity predicted by these analyses, female breast cancer patients are treated with surgical removal, radiotherapy, chemotherapy, hormonal therapy, and/or immunotherapy (i.e. drugs activating or suppressing an individual's immune system).
Differences between MBC and female breast cancer The following lists medically important breast cancer features that differ between males and females:
Presentation Due to men's smaller breast sizes, their breast tumors may become palpable and cause symptoms earlier than those in females. Nonetheless, males tend to lack awareness of breast cancer, may have gynecomastia masking their breast tumors, and may delay seeking medical attention. These differences appear to underlie findings that the diagnosis of breast cancer is made later in males than females (average age 67 vs. 63 years old, respectively).
Studies have reported that males more often than females present with breast cancers that have spread to nearby axillary lymph nodes and appear more aggressive based on their microscopic histopathology. However, a large study by the Surveillance, Epidemiology, and End Results program of the National Institutes of Health ranked breast cancer severity based on their TNM stage. The study reported that the percentage of cases presenting with purely local disease (i.e. no metastases) was 63.1% in males and 45.4% in females; with spread to local lymph nodes was 29.1% in males and 43.6% in females; and with distant metastases was 5.7% in males and 8.1% in females (2.1% of males and 2.9% of females were not staged).
Development of contralateral breast cancers Men with breast cancer have an absolute risk of presenting with a second cancer in their other breast of 1.75, i.e. they have a 75% increase of developing a contralateral breast cancer over their lifetimes compared to men who develop a breast cancer without having had a prior breast cancer. Female breast cancer is likewise associated with the development of contralateral breast cancer, with one large study finding the five-year cumulative incidence of developing a contralateral breast cancer of 2.5%. The relative risk of developing a contralateral breast cancer following mastectomy of the involved breast in men versus the probability of developing breast cancer in men without a history of breast cancer is significantly higher than the relative risk in women.
In order to prevent cancer from developing later in their contralateral breasts, women have had prophylactic mastectomy of their uninvolved breasts at the time of their diseased breast's mastectomy. However, increasing evidence in some studies suggest prophylactic mastectomy provides no survival advantage and is associated with increased costs and higher complication rates. The rates of female vs. male contralateral breast cancer and the value of prophylactic mastectomy of the uninvolved breast in males, perhaps because of their rarity compared to females, is unclear. However, breast cancers associated with BRCA1 or BRCA2 gene mutations (see below section on "Gene mutations") are widely considered to be the strongest indication for contralateral prophylactic mastectomy in both male and female breast cancers.
Risk factors Radiation exposure to the chest or entire body is clearly associated with increased rates of MBC. For example, men treated with radiotherapy to the chest for thymus gland enlargement or gynecomastia have an increased rate of later developing breast cancer; men and women treated with radiotherapy for breast cancers have increased rates of developing contralateral breast cancer; and male and female survivors of the atomic bomb attacks in Japan (1945) had increased rates of developing breast cancer in proportion to their increasing levels of radiation exposure.
Men with a history of high alcohol consumption and men with occupations entailing long-term exposure to high temperatures (e.g. such as to blast furnaces, steel works, and rolling mills, i.e. mills processing metals), petrol emissions, or exhaust emissions have had, in some studies, increased risks of developing breast cancer. High alcohol consumption is also a risk factor for breast cancer in females.
Studies have reported 1) lower rates of breast cancer (i.e. by 20-25%) in men with an employment history involving high levels of physical activity and 2) higher rates of breast cancer in men with an employment history involving low levels of physical activity. Changes in the fat tissue microenvironment in the male breast as a result of physical activity may contribute to these differences. The level of physical activity has similar effects on the development of female physical activity. However, most studies show that the protective effect in female breast cancer is a 13% decreased risk in high versus low physical activity groups and is limited to postmenopausal women. It is not clear that the higher benefit of physical activity in MBC (20 to 25%) is significantly greater than the 12% benefit seen in female breast cancer.
Expression of key receptors The estrogen (ER), progesterone (PR), androgen (AR), and HER2/neu receptors are expressed by breast cancer cells and when active elicit various potentially pro-cancerous responses (e.g. excessive growth) in their parent cells. ER, PR, and AR are activated by estrogens, progesterone, and androgens, respectively, while the HER2/neu receptor has no known hormone-like activators but is active when attached to other members of the ErbB family within the same cell, when overproduced, or when containing certain mutations. The percentage of cases in which breast cancer cells express an ER is 99% of males and 77% of females, the PR is 82% of males and 64% of females, an AR is 97% of males and 77% of females; and the HER2/neu is 9% of males and 11% of females. About 0.3% of males and 11% of females have triple-negative breast cancer, i.e. do not have breast cancer cells that express ER, PR, and HER2/neu receptors and consequently are not amenable to treatment with inhibitors of these receptors. On the other hand, the extremely high rate of estrogen receptor expression in MBC has led to commonly treating these men with a selective estrogen receptor modulator, tamoxifen. Tamoxifen acts indirectly to inhibit ER signaling in breast cancer cells.
Associations with other diseases and conditions Klinefelter syndrome is a rare genetic disease in which males have inherited an extra X chromosome. Men with this disease have a 20 to 50-fold increased risk of developing MBC. It is thought that this increased risk is primarily due to their low androgen levels, high gonadotrophins levels, and consequently high estrogen levels relative to androgen levels. Cases of MBC occur in individuals with three other rare inherited genetic disorders, the Li-Fraumeni syndrome, Lynch syndrome, and Cowden syndrome, although the odds ratios (i.e. statistical strength) of these associations is not yet known.