Overview of Breast Cancer
The cells of our bodies normally grow and divide in an orderly, highly controlled way governed by multiple genes contained within each cell. Cancer occurs when mutations, or abnormal changes, occur in the genes which control cell growth and division. Mutated cells grow and multiply in an uncontrolled disorganized way, forming a tumor. Although breast cancer is always caused by genetic abnormalities, only 5-10% of these genetic alterations are inherited from one’s mother or father. 90-95% of breast cancers are due to genetic abnormalities that happen as a result of hormonal, environmental and/or other acquired factors. Multiple “genetic hits” must occur for a cancer cell to develop.
Breast tissue is composed of glandular tissue, the ducts and lobules, which are surrounded by stroma (supporting tissue) comprised of connective and fatty tissue. The ducts are tubular, branching structures which transport milk from the lobules to the nipple. The lobules are glandular tissue capable of producing milk. The stroma contains blood vessels and lymphatic channels. Breast cancer most commonly originates in the cells lining the breast ducts. Less commonly, breast cancer can arise from the cells lining the lobules. When the cancer cells are confined to the ducts, the cancer is ductal carcinoma in-situ. These in-situ or non-invasive cancers have a < 2% chance of metastasizing (spreading) to other parts of the body. Some cancers develop the ability to grow through the membrane of the ducts or lobules and microscopically invade into the stroma. These are invasive or infiltrating carcinoma. Once the breast cancer cells invade the stroma where lymphatic channels and blood vessels are, the cancer has the potential to spread (metastasize) to the lymph nodes in the under- arm (via the lymphatic channels) or to other organs (via the blood vessels). The most common sites for distant metastasis are bone, liver, lung or brain. Breast cancer can also spread by growing directly into surrounding structures such as the skin, chest wall, or muscles. Most breast cancers are detected at early stages when they are confined to the breast with or without involvement of the lymph nodes in the underarm (axillary lymph nodes).
(Based on American Cancer Society estimates)
Breast cancer is the most common form of cancer in women in the United States except for skin cancer. Slightly less than 30% of cancers in women are breast cancers.
Fortunately, breast cancer incidence has been steadily decreasing from its peak in 1999. Breast cancer mortality, the death rate, has been dropping by 2% per year since 1990. The 5 year survival for breast cancer has increased to 90%, and is 98% for women with cancer only in the breast and not the lymph nodes. Based on the most recent data, the 5–year, 10–year and 15–year survival rates for women diagnosed with breast cancer are 91%, 86% and 80% respectively. The overall 5-year relative survival rate is 99% for localized disease, 85% for regional disease, and 27% for distant-stage disease.
The lifetime risk for breast cancer in US women is 1 in 8 (about 12.4%). In 2019, it is estimated that 268,600 new cases of invasive breast cancer and 62,930 new cases of non-invasive (in situ) breast cancer will be diagnosed in US women.
From 2000 to 2009, the incidence of breast cancer decreased by 0.9% per year among US women. The decrease was seen only in women > 50, at least in part because fewer women were using hormone replacement therapy (HRT) after the Women’s Health Initiative study, published in 2002, suggested a link between HRT and breast cancer risk. In recent years the incidence has been increasing by 0.4% per year.
Breast cancer is second only to lung cancer in cancer-related deaths (mortality) in US women. Approximately 41,760 women in the U.S. will die from breast cancer in 2019. Breast cancer mortality has been decreasing since 1990. The breast cancer death rate decreased by 40% from 1989 to 2016. From 2000 to 2009, breast cancer deaths decreased by 2.1% per year in US women. Since 2007 breast cancer mortality in women under 50 has been stable, but it has continued to decrease in women over 50. These decreases are thought to be the result of more effective treatments, earlier detection through screening, and increased awareness.
In women under 45, breast cancer is more common in African-American women than white women. Overall, African-American women are more likely to die of breast cancer. For Asian, Hispanic, and Native-American women, the risk of developing and dying from breast cancer is lower.
A US man’s lifetime risk for breast cancer is about 1 in 1,000. About 2,550 new cases of invasive breast cancer are expected in men in 2018 and 480 male breast cancer deaths.
About 5-10% of breast cancers can be linked to gene mutations inherited from one’s mother or father, such as BRCA1 or BRCA2 mutations. Only 1-20% of women with breast cancer have a family history of breast cancer.
In 2018, there were more than 3.1 million breast cancer survivors in the United States.