When most people hear the words “breast cancer,” they think it’s one single disease. In reality, there are different types of breast cancer, each with its own characteristics and treatment options. Whether you’re looking to better understand a recent diagnosis or simply learn more about your breast health, Breast Surgical Oncologist Dr. Lindsey Fauveau explains what breast cancer is, how it develops and spreads, and ways to reduce your risk.

What Is Breast Cancer? 

Breast cancer begins when abnormal cells in the breast grow and multiply uncontrollably. Under normal circumstances, your immune system would identify and eliminate these cells before they can cause harm. However, some of these cells can hide, which allows them to survive, multiply, and eventually form cancer. 

Although breasts are mainly made up of fatty tissue, most breast cancers begin in the ducts (tube-like structures that bring the milk to the nipple). They can also begin in the lobules (grape-like clusters that produce milk). In very rare cases, breast cancer may first be detected in a lymph node under the arm. This is known as occult breast cancer. 

Positive vs. Negative Cancer 

When breast cancer is described as “positive” or “negative,” it doesn’t mean the cancer is better or worse. Instead, it refers to whether the cancer cells have certain proteins or hormone receptors on their surface. Knowing this will help your healthcare team predict how the cancer is likely to behave and determine which treatments are most likely to work. 

HER2 (Human Epidermal Growth Factor Receptor 2) 

HER2 is a protein found on the surface of breast cells that helps control normal cell growth. 

• HER2-Positive: These cells have too much HER2, causing the cancer to grow more quickly. Therapies that specifically block the HER2 protein can often treat these cancers. 

• HER2-Negative: These cells have a normal amount of HER2. Although targeted HER2 therapies won’t work for these cancers, the cells may respond to other treatments. 

Hormone Receptors (HR) 

Hormone receptors include estrogen receptors (ER) and progesterone receptors (PR). Some breast cancers use these hormones as fuel to grow. 

• HR-Positive: These breast cancer cells have receptors for estrogen, progesterone, or both. Because the cancer depends on these hormones, hormone therapy can often slow or stop its growth by lowering hormone levels or blocking the receptors. 

• HR-Negative: These cancer cells don’t rely on estrogen or progesterone to grow, so hormone therapy isn’t effective. Treatment for these cancers usually involves chemotherapy, targeted therapy, or immunotherapy. 

You may also hear the term “triple-negative breast cancer.” This means the cancer is ER-negative, PR-negative, and HER2-negative. Because these cancers lack all three receptors, hormone therapy and HER2-targeted treatments aren’t effective. Depending on the tumor and the stage of the cancer, treatment can include chemotherapy, immunotherapy, or other targeted therapies. 

Invasive vs. Noninvasive Cancer 

In addition to being classified by their hormone receptor and HER2 status, breast cancers are also categorized by whether they have spread. This distinction helps guide treatment decisions. 

• Noninvasive breast cancer, also known as ductal carcinoma in situ (DCIS), remains confined within the milk ducts. Because it’s contained, it usually doesn’t have access to the lymphatic system, which is one way cancer can spread outside the breast. Noninvasive breast cancer often has a great prognosis when caught and successfully treated with surgery. 

• Invasive breast cancer has broken through the walls of the ducts or lobules and entered the surrounding breast tissue. Once cancer becomes invasive, it has the potential to spread to nearby lymph nodes or other parts of the body. Thankfully, advances in breast imaging have made it possible to detect many breast cancers before they become invasive.  

Why Mammograms Matter 

Regular mammograms are one of the most effective tools for early breast cancer detection, especially because different types of breast cancer can grow in different ways. For example, ductal cancers often form a more defined mass that’s usually easier to see in mammograms and ultrasounds. On the other hand, lobular cancers tend to spread through the breast tissue in thin strands, making it more difficult to detect with traditional imaging. 

Most women should begin routine mammogram screenings at age 40. Women with a higher risk of developing breast cancer may benefit from earlier or additional screening beyond routine mammograms; this could include more frequent clinical breast exams, a whole breast MRI, or supplemental ultrasound screenings. 

Although getting your routine mammograms might be scary, Dr. Fauveau points out that she would be “more scared of getting a diagnosis that isn’t caught.” 

Reducing Your Risk and Staying Proactive 

Inherited gene mutations (such as BRCA1 and BRCA2) account for only about 5% to 10% of breast cancer cases. Most breast cancers develop due to a combination of family history, environmental factors, and sporadic causes. 

Although some risk factors can’t be changed, healthy lifestyle choices can play an important role in overall breast health. Smoking, excessive alcohol use, and obesity have all been linked to chronic inflammation, which may increase your risk of breast cancer. Staying active, maintaining a healthy weight, and eating a balanced diet can help support your overall breast health. 

Just as important as reducing your risk is learning what your breasts normally look like and how they feel. Talk to your healthcare provider if you notice a new lump, persistent pain, skin changes, or changes in your breasts’ shape or size. If you’re unsure whether or not a change is significant, err on the side of caution and schedule an appointment. 

“Live your life the best you can,” says Dr. Fauveau. “Go to the doctor, take care of yourself and make yourself a priority – that’s all you can do.” 

Understanding Your Diagnosis 

If you’ve been diagnosed with breast cancer, don’t be afraid to ask questions if something is unclear. Whether you communicate directly with your physician, work with a nurse navigator, or seek support from a breast cancer community, it’s important to feel informed and confident in your care. You deserve to fully understand your options so you can make choices that feel right for you, especially because treatment decisions can have a lasting impact.