Breast cancer is one of the most prevalent cancers impacting women worldwide, affecting approximately 1 in 8 women during their lifetime.[1] Breast cancer can also occur in men, albeit less frequently. Breast cancer originates in the cells of the breast, most commonly in the lining of the milk ducts (ductal carcinoma) or the lobules that supply milk (lobular carcinoma). It can spread to other parts of the body through the blood and lymphatic systems.[2]
Breast cancer can present with various subtypes, each characterized by distinct features and biological behaviors:
Ductal Carcinoma In Situ (DCIS): A non-invasive cancer where abnormal cells are found in the lining of a breast duct but have not spread outside the duct, DCIS is considered the earliest form of breast cancer and is highly treatable.
Invasive Ductal Carcinoma (IDC): The most common type of breast cancer, IDC starts in the milk ducts and then invades nearby tissue in the breast. From there, it can spread to other parts of the body.
Invasive Lobular Carcinoma (ILC): The second most common type of breast cancer, ILC begins in the milk-producing lobules and can spread to nearby tissues and other parts of the body.
Triple-Negative Breast Cancer: This type of breast cancer does not have estrogen receptors, progesterone receptors, or excess HER2 protein. It is more common in younger women and African American women, and is often more aggressive and harder to treat because it doesn’t respond to hormonal therapy or therapies targeting HER2 receptors.
HER2-Positive Breast Cancer: This type of breast cancer has high levels of the HER2 protein, which promotes the growth of cancer cells. It tends to be more aggressive, but targeted therapies can be very effective.
Hormone Receptor-Positive Breast Cancer: These cancers grow in response to estrogen (ER-positive), progesterone (PR-positive), or both. They are often treated with hormone-blocking therapies, which can be very effective.
Inflammatory Breast Cancer (IBC): IBC is a rare and aggressive form of breast cancer where cancer cells block lymph vessels in the skin of the breast, causing it to look red and swollen. It tends to be diagnosed at a more advanced stage and requires aggressive treatment.
Paget's Disease of the Breast: This rare type of breast cancer starts in the ducts of the nipple and then extends to the nipple surface and the areola.
Phyllodes Tumors: These rare tumors develop in the connective tissue of the breast and can be benign or malignant. They require surgical removal, and malignant phyllodes tumors may require additional treatment.
Angiosarcoma: This rare form of breast cancer starts in the cells lining blood or lymph vessels in the breast. It tends to be very aggressive and requires specialized treatment.
Breast cancer symptoms can vary widely, and may not be immediately obvious. The most common symptoms include swelling or a lump in the breast or underarm area, breast or nipple pain, nipple discharge, or changes to the appearance of the nipple or breast.[3] The National Cancer Institute’s Surveillance, Epidemiology, and End Results (SEER) database tracks breast cancer stages based on how far the cancer has spread:
Localized: There is no sign that the cancer has spread outside of the breast.
Regional: The cancer has spread outside the breast to nearby structures or lymph nodes.
Distant: The cancer has spread to distant parts of the body such as the lungs, liver or bones
The 5-year survival rate at each stage is shown below
Incidence
Below is the overall incidence of breast cancer by age group:
Non-hispanic white women have the highest overall incidence rates of breast cancer, particularly hormone receptor-positive types. Although African American women have slightly lower incidence rates compared to non-Hispanic white women, they tend to be diagnosed with breast cancer at younger ages and are more likely to have aggressive forms of the disease, such as triple-negative breast cancer. Hispanic women have lower incidence rates of breast cancer compared to non-Hispanic white women, however they are often diagnosed at later stages. Asian women also have lower incidence rates of breast cancer, but incidence is increasing in this group. The risk varies among different subgroups within the Asian American and Pacific Islander population.
Risk Factors
There are several confirmed risk factors that contribute to an individual’s risk for breast cancer.
Below are some of the factors that increase an individual’s breast cancer risk[4]:
Alcohol consumption: Alcohol consumption has been linked to an increased risk of several types of cancer, including breast cancer. When the body metabolizes alcohol, it produces acetaldehyde, a toxic substance that can damage DNA and proteins. Alcohol can also influence hormone levels and impair the absorption of certain nutrients. The risk associated with alcohol is dose-dependent, meaning it tends to increase with the amount of alcohol consumed.
BRCA genetic mutations: BRCA is a tumor suppressor gene, meaning its normal function is to help prevent the development of cancer by repairing damaged DNA and regulating cell growth. Mutations in the BRCA gene can interfere with its ability to carry out these functions effectively. Approximately 1 in 400 people carry mutated BRCA1 or BRCA2 genes.
Body mass index (BMI): Numerous studies have shown a link between higher BMI and an increased risk of various types of cancer. Excess body fat can lead to higher levels of estrogen and insulin, as well as increased production of insulin-like growth factors. These hormones can promote the development and growth of certain cancers. Higher BMI is also associated with chronic low-level inflammation, which can cause DNA damage over time and contribute to the development of cancer. Furthermore, high BMI can impair immune function, reducing the body's ability to fight off cancer cells.
Height: Research has consistently shown a correlation between height and an increased risk of several types of cancer. Taller individuals tend to have higher levels of IGF-1, a hormone that plays a critical role in growth and development. IGF-1 can promote cell proliferation and inhibit cell death, which may contribute to the development and progression of cancer. Taller individuals also have more cells in their body simply due to their size, which leads to a higher probability of cells mutating.
Smoking & secondhand smoke exposure: Smoking and secondhand smoke are some of the most significant risk factors for cancer, contributing to various types of cancer across the body including breast. Tobacco smoke contains numerous carcinogens that can lead to genetic mutations and uncontrolled cell growth, ultimately resulting in cancer. Quitting smoking at any age can significantly reduce the risk of cancer and other smoking-related diseases. The body begins to repair itself almost immediately after quitting, with the risk of cancer decreasing over time. Even individuals who have smoked for many years can benefit from quitting, as the risk decreases with each year of abstinence.
Radiation exposure: Radiation impacts cancer risk by damaging DNA in cells, which can lead to mutations and ultimately the development of cancer. Certain occupations, such as flight attendant, pilot, baggage screener, construction, or working in the military may increase the likelihood of prolonged exposure to radiation. The dose, duration, and rate of exposure can affect the risk that radiation poses. (For more information on the dangers of radiation, see our article here.)
Benign breast disease: Benign breast disease with proliferative lesions refers to non-cancerous abnormalities in breast tissue that show increased cell proliferation. While most benign breast diseases are not associated with an increased risk of breast cancer, certain proliferative lesions may elevate the risk.
Dense breast tissue: Dense breast tissue has a higher proportion of glandular and fibrous tissue compared to fatty tissue. Women with dense breast tissue have a slightly higher risk of developing breast cancer compared to women with less dense breast tissue. The exact mechanism is not fully understood, but it is thought to be related to the increased amount of glandular tissue and higher cell density in dense breasts, which may create a more conducive environment for the development of cancerous changes. Furthermore, dense breast tissue can make it more challenging to detect breast cancer on mammograms, often leading to delayed detection or diagnosis.
Red meat consumption[5]: Red meat consumption has been associated with several cancers, including breast. Red meat contains compounds such as heme iron and saturated fats, which have been hypothesized to contribute to cancer risk through mechanisms like oxidative stress and inflammation. The risk may be amplified depending on how red meat is cooked. When cooked at high temperatures or over an open flame (such as through grilling), carcinogenic compounds known as heterocylic amines (HCAs) and polycyclic aromatic hydrocarbons (PAHs) are formed.
Endometriosis: Endometriosis is a condition in which tissue similar to the lining of the uterus grows outside the uterus, commonly on the ovaries, fallopian tubes, and the tissues lining the pelvis. Endometriosis causes chronic inflammation in the pelvic cavity, as the presence of endometrial-like tissue outside the uterus can cause irritation, scarring, and the formation of adhesions. Women with endometriosis may also have higher levels of circulating estrogen and other hormonal imbalances, which can influence cancer risk. Prolonged exposure to estrogen has been linked to an increased risk of certain cancers, including breast and uterine cancer.
Family history: Family history plays a significant role in determining cancer risk. Studies have shown that having a first degree relative who has been diagnosed with breast cancer increases your personal breast cancer risk by anywhere from 8 - 15%, depending on whether you have gone through menopause. Individuals with a more extensive family history are recommended to get screened for BRCA genetic mutations.
Oral contraceptive use: Taking birth control pills influences cancer risk in both positive and negative ways, and can depend on the duration of use. Studies have shown that use of oral contraceptives may slightly increase risk of melanoma, breast, and liver cancer. However, they simultaneously decrease the risk of ovarian and uterine cancer.
Hormone Replacement Therapy: Hormone replacement therapy (HRT), used to relieve symptoms of menopause by replacing hormones that are at lower levels, can impact cancer risk in various ways. Multiple studies have shown that HRT increases the risk of breast cancer and ovarian cancer. The risk increases with the duration of use, and decreases after stopping HRT.
Age of first menstrual period: Women who experience their first period at a younger age have a longer lifetime exposure to estrogen and progesterone. These hormones influence the development of hormone-sensitive tissues, increasing the likelihood of cellular changes that can lead to cancer.
Blood type A: Individuals with blood type A have been shown to be at a 12% increased risk of developing breast cancer. While the exact mechanism underlying this risk is unknown, some studies have shown that individuals with blood type A have higher levels of certain inflammatory markers, which might contribute to cancer development. Blood type can also influence the immune system's ability to recognize and fight off cancer cells. For example, people with blood type O are thought to have a more robust immune response, which could help them better recognize and eliminate cancer cells.
Several factors have been shown to have protective effects against breast cancer:
Bilateral oophorectomy (ovary removal)[6]: Oophorectomy induces surgical menopause by eliminating ovarian hormone production, leading to a significant decrease in circulating estrogen and progesterone levels. As a result, it may reduce the risk of hormone-sensitive cancers, such as estrogen receptor-positive breast cancer.
Daily use of NSAIDs: Nonsteroidal anti-inflammatory drugs (NSAIDs) are a class of medications commonly used to relieve pain, reduce inflammation, and lower fever, and they have also been shown to reduce cancer risk with regular use. In addition to their anti-inflammatory properties, NSAIDs have been shown to inhibit cell proliferation, induce programmed cell death, suppress tumor growth, and modulate immune responses. While NSAIDs have been associated with potential benefits in cancer prevention, their use is not without risk. Long-term or high-dose NSAID use has been linked to gastrointestinal bleeding, ulcers, and perforation. Additionally, certain NSAIDs have been associated with an increased risk of cardiovascular events such as heart attack and stroke.
Physical activity: Regular exercise can reduce your risk of several types of cancer through a combination of physiological, hormonal, and immunological changes. It can reduce inflammation, improve immune function, improve hormonal regulation, and lower insulin levels. The American Cancer Society recommends engaging in at least 150 minutes of moderate activity or 75 minutes of vigorous activity each week. However, data indicates that individuals can see significant incremental benefit with additional exercise.
Breastfeeding: Research suggests that breastfeeding can reduce the risk of several cancers, including breast, ovarian, and uterine. The longer a woman breastfeeds over her lifetime, the greater the protective effect may be. This is thought to be due to the hormonal changes that occur during lactation, which may help to reduce the number of ovulatory cycles a woman experiences over her lifetime, thereby reducing her exposure to estrogen, which can promote cancer growth.
Fruit consumption: Fruits are rich in antioxidants, such as vitamins C and E, beta-carotene, and various phytochemicals. Some fruits also have anti-inflammatory effects.
Vegetable consumption: Vegetables are rich in various nutrients which have been shown to have protective effects against various cancers. Vegetables contain a variety of antioxidants, which help neutralize free radicals in the body. In addition, vegetables may help reduce inflammation in the body.
Age at first birth: The age at which a woman has her first child can significantly impact her risk for developing certain types of cancer, particularly breast and ovarian cancers. This relationship is influenced by hormonal changes, reproductive history, and other biological factors. Women who have their first child at a younger age tend to have a reduced risk of developing breast cancer. This is because a full-term pregnancy early in life leads to differentiation of breast cells, making them less susceptible to carcinogenic changes. Early full-term pregnancy also reduces the lifetime exposure to estrogen and progesterone, hormones that can promote breast cancer cell growth.
Blood type O: Individuals with blood type O have a 10% lower risk of developing breast cancer. Similar to type A above, the exact underlying mechanism of this connection is unknown, although it may be due to lower levels of inflammatory markers or a more robust immune response that helps individuals with blood type O to better recognize and eliminate cancer cells.
Screening
Early detection of breast cancer has a dramatic impact on prognosis. The 5-year survival rate for localized breast cancer is around 99%, as compared to just 31% when it has spread to distant parts of the body. Furthermore, early-stage breast cancer is more likely to be treated effectively with less aggressive treatments, potentially avoiding the need for invasive procedures such as a mastectomy.
Depending on your age and risk factors, the optimal breast cancer screening will vary:
Traditional 2D Mammogram: Mammograms are the standard screening protocol for individuals at average risk of breast cancer. Mammograms are low-dose x-rays that can help to detect abnormal areas in the breast. They can't determine whether an abnormal area is cancer, but they can help doctors determine whether additional testing such as a biopsy is needed.
3D Mammogram: 3D mammography, also known as digital breast tomosynthesis (DBT), is an advanced imaging technology that provides a three-dimensional view of breast tissue. Similar to a traditional mammogram, the breast is compressed between plates and x-ray images are taken from different angles. However unlike a traditional mammogram, DBT takes images from multiple angles in order to reconstruct a 3D image of the breast, which allows radiologists to review thin high-resolution images of the breast tissue.
3D Mammogram + Breast MRI: For individuals at high risk of breast cancer, breast MRI is recommended along with a yearly mammogram. While mammography is better at detecting microcalcifications and certain types of tumors, MRI is more sensitive for detecting invasive cancers and assessing tumor size and extent. Combining mammography and MRI can improve detection, and provide complementary information about breast tissue.
The United States Preventive Services Task Force (USPSTF) recommends that cisgender women and others assigned female at birth aged 40-74 should get a mammogram every two years. Though mammography is known to be less effective for women with dense breast tissue, the Task Force does not take a stance for or against additional screening for this cohort, citing that there is “not yet enough evidence” to provide a conclusive recommendation.
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