Many people are surprised to learn that breast cancer is not a singular disease. In fact, there are many different types of breast cancer—each with its own unique molecular features, prognosis, treatments, and origins in the breast.
Our understanding of breast cancer today was informed by decades of research and technological advancements that have made it possible to study breast cancer at its most basic level. Notably, in 2001, Dr. Charles Perou (a BCRF investigator since 2003) first classified breast cancer into subtypes based on genomic patterns—discoveries that transformed our understanding of the disease and how it’s treated.
BCRF-funded projects have further informed the classification of different types of breast cancer and deepened our understanding of the disease. Our researchers are seeking ways to advance the development of targeted treatments and personalized care to benefit patients with all types of breast cancer.
And thanks to that research, the field of breast cancer care has long been at the forefront of personalized medicine. Today, patients diagnosed with the disease are not given the same treatments; their tumors are profiled and staged to inform the exact right therapies and care plans that best suit their type of breast cancer. Still, because breast cancer is such a complex collection of diseases, there is much more to uncover through further research.
Here, BCRF outlines the most common types of breast cancer, along with rarer forms and an overview of metastatic breast cancer, a major BCRF research priority.
There are two broad types of breast cancer: non-invasive breast cancers and invasive breast cancers. Under these umbrellas, there are breast cancers that are more aggressive than others, breast cancers that have different molecular features, breast cancers that originate in certain areas of the breast, and much more.
Non-invasive breast cancers are also known as stage 0 breast cancers or carcinomas in situ (Latin for “in the original place”). They’re thought to be early precursors to breast cancer, hence the stage 0 designation.
Essentially, non-invasive breast cancers are abnormal cells that stay in the area of the breast where they are first formed. These non-invasive breast cancers are not generally life-threatening, but they can become invasive breast cancers if left untreated.
The most common non-invasive breast cancer, ductal carcinoma in situ (DCIS), starts in the cells lining the milk ducts of the breast. About 51,400 women will be diagnosed with DCIS in 2023.
Lobular carcinoma in situ (LCIS), a much rarer non-invasive breast cancer, starts in the cells lining the breast lobules, the glands that make milk. Unlike DCIS, LCIS is not thought to generally progress to invasive breast cancer, but instead they are believed to increase a person’s risk of breast cancer in the future. Some refer to LCIS as lobular neoplasia instead, as neoplasia refers generally to an abnormal growth of cells that can be benign or malignant.
Read more about DCIS and LCIS and BCRF research on non-invasive breast cancers
Unlike non-invasive breast cancers, invasive breast cancers do not stay put; these breast cancers leave their original sites in the ducts or breast lobules and invade nearby breast tissue, lymph nodes, and even distant organs (at which point it is metastatic; see metastatic breast cancer below).
Originating in the ducts of the breast, invasive ductal carcinoma (IDC) is by far the most common type of breast cancer with 70 to 80 percent of women being diagnosed with this form each year. Typically, IDC tumors form a hard mass or lump in the breast. Prognosis and treatment depend on the stage/grade at which it was diagnosed, the molecular characteristics of the tumor, and whether it has spread.
The second most common type of invasive breast cancer, invasive lobular carcinoma (ILC, also more simply known as lobular breast cancer), accounts for 10 to 15 percent of diagnoses. This type originates in the milk-producing glands of the breast called lobules. Though it is often treated like invasive ductal breast cancer, ILC has a different biology and presents differently on a mammogram (the tumors more commonly grow in lines in the breast rather than lumps). Thanks to research, our understanding of ILC is rapidly expanding.
Read more about invasive lobular carcinoma and BCRF research
Invasive breast cancers can be further broken down into subtypes based on certain characteristics of the tumors that can then inform how those tumors are treated and what medications will benefit patients.
The four main subtypes are:
Generally, these classifications above are only widely referenced in research. Patients most commonly learn instead whether their breast cancer is being fueled by estrogen, progesterone, or HER2 receptors—or none of these three main drivers of breast cancer growth. In other words, they learn about their estrogen receptor, progesterone receptor, and HER2 status.
When a patient is diagnosed with breast cancer that is classified as estrogen receptor (ER)–positive and/or progesterone receptor (PR or PgR)–positive—based on the presence of estrogen and progesterone receptors in the tumor cell—this is also called hormone receptor (HR)–positive breast cancer.
HR-positive breast cancer is the most common form of invasive breast cancer and has the best outcome when diagnosed early. It can be treated in several ways, including with hormone therapies that either suppress estrogen levels in the body or prevent estrogen from attaching to receptors. In both cases, HR-positive breast cancers are cut off from hormones needed to grow.
In normal breast cells, the human epidermal growth factor receptor 2 (HER2) protein helps those cells grow and repair. In the 1980s, researchers discovered that mutations in the HER2 gene cause an overproduction of HER2 protein, which drives excessive growth of breast cancer cells.
Today, breast tumors are routinely tested for an overexpression of the HER2 protein, and, if found, they are then classified as HER2-positive breast cancers. An estimated 25 to 30 percent of diagnoses are HER2-positive breast cancer.
For a long time, HER2-positive breast cancer, which tends to be aggressive, had a poor prognosis. In the late 1990s, the FDA approved the groundbreaking monoclonal antibody trastuzumab (Herceptin®)—the first targeted therapy for breast cancer. Today, HER2-positive breast cancer has one of the best outcomes overall and a multitude of treatment options.
Until recently, HER2-targeted therapies could only work for patients who had a certain level of HER2 overexpression (3+ values on HER2 testing). Trastuzumab deruxtecan (T-DXd/Enhertu®)—a new drug in an emerging class of targeted treatments called antibody-drug conjugates—was shown to target cells that were previously classified as HER2-negative but actually have lower levels of HER2 protein (values of 1+ or 2+ on HER2 testing, or FISH-). This led to the new classification of breast cancers as HER2-low and opened an exciting avenue for more research breakthroughs.
Patients who don’t have any HER2 overexpression—those that are called HER2-0 in testing—are classified as HER2-negative; they may be HR-positive or triple-negative (see triple-negative breast cancer below). Patients who have HR-positive and HER2-positive breast cancer are sometimes referred to as having triple-positive breast cancer.
Read more about HER2-positive breast cancer, HER2-low breast cancer, and BCRF research
Triple-negative breast cancer (TNBC) is characterized by the fact that it’s “negative” for the three main receptors in breast cancer mentioned above (estrogen, progesterone, and HER2). These markers inform prognosis and treatment strategies for breast cancer, as we have many targeted therapies for these types.
TNBC accounts for approximately 15 percent of breast cancer diagnoses and is more commonly diagnosed than other forms in younger women, Black and Hispanic women, as well as women with inherited BRCA1 gene mutations. These breast cancers tend to be more aggressive than those that are HR-positive, and they also tend to be diagnosed at later stages. TNBC is sometimes referred to as basal-like breast cancer, but not all basal tumors are triple-negative.
Because TNBCs lack the common targets for treatment, they have fewer treatment options than other breast cancers. Research is beginning to identify subsets or subtypes of TNBC—and investigating targeted therapies. TNBC is typically treated with surgery, chemotherapy, and radiation, as well as drugs including PARP inhibitors, immunotherapy agents, and antibody-drug conjugates. BCRF investigators are urgently researching potential treatment targets and probing the underlying biology of TNBC to give these patients more options.
Read more about triple-negative breast cancer and BCRF research
Inflammatory breast cancer (IBC) is a rare (about 1 to 5 percent of diagnoses) and aggressive breast cancer that is characterized by its unusual symptoms. Instead of a typical lump, patients usually experience breast swelling, skin dimpling, and/or discoloration on the breast that looks like a pink or red rash. IBC is more common in younger women and Black women.
Metaplastic breast cancer is another very rare (less than 1 percent of diagnoses) and aggressive form. These tumors are very fast-growing and are more likely to metastasize. Metaplastic breast cancer’s biology has been understudied. Tumors appear to resemble invasive breast cancer tumors but under a microscope they show multiple types of cancer cells.
Though male breast cancer is not a specific type of breast cancer (most men are diagnosed with HR-positive breast cancer like women are), the disease will impact approximately 2,800 men in the U.S. in 2023 (about 1 percent of diagnoses). Men tend to be diagnosed with breast cancer at later stages and have potentially worse outcomes, likely due in large part to a lack of awareness and stigma about a diagnosis.
Read more about male breast cancer and BCRF research
Angiosarcoma is a cancer that forms in the blood vessels of the breast and lymph nodes and is extremely rare.
Paget disease of the breast (also known as Paget disease of the nipple) is a type of breast cancer in the nipple or areola. It is often present with ductal carcinoma or invasive breast cancer in a person’s breast.
As part of the breast cancer staging process, invasive breast cancers are classified as local (in the breast only), regional (in the breast and nearby lymph nodes), or distant/metastatic (spread beyond the breast and lymph nodes). Though it’s not a type of breast cancer like others named in this guide, it’s important to call attention to metastatic breast cancer (MBC).
MBC—also referred to as stage 4 breast cancer or advanced breast cancer—occurs when breast cancer cells break away from the original site in the breast or lymph nodes and travel to other areas of the body, most commonly a person’s lungs, liver, bones, and brain. Any of the types of breast cancer named here can become metastatic.
Only about 6 to 10 percent of patients are diagnosed with MBC initially (also known as de novo metastatic breast cancer); others experience a recurrence of their breast cancer in another area of the body.
MBC is an urgent research priority for BCRF because it’s the form of breast cancer that overwhelmingly takes lives. There are more treatments than ever before for MBC and the pace of research and drug development has rapidly increased in recent years—drastically improving patients’ quality of life and allowing people to live with MBC for years or even decades.
Devastatingly, we still lose more than 42,000 people to MBC each year and the hundreds of thousands of people living with the disease are waiting for breakthroughs to further extend their lives and, hopefully, cure their disease entirely.
Read more about metastatic breast cancer and BCRF’s commitment to MBC research
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