What you need to know.

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According to the American Cancer Society, breast cancer is the second leading cause of cancer death in women. It's true that breast cancer can be dangerous, but the severity really depends on what type of breast cancer you have and in what stage you're in, because not all breast cancer is the same. 

"Breast cancer describes breast cells that are acting abnormally, including uncontrolled cell growth, which can cause the cells to form a tumor," says Anne Peled, MD, a dual-trained breast and plastic surgeon and co-director of the Sutter Health CPMC Breast Cancer Program in San Francisco, CA. "Breast cancer can start in different cell types in the breast and can potentially spread out of the breast to other parts of the body through channels called lymphatics."

In situ vs. invasive breast cancer—what's the difference?

If you're someone who has been diagnosed with breast cancer, your doctor has either described it as in situ or an invasive breast cancer. "In situ breast cancer describes abnormal breast cells that have not yet spread out of the breast tissue type that they started in (i.e. in ductal carcinoma in situ, or DCIS, there is not any sign that the cells have spread out of the ducts)," explains Peled. "Invasive breast cancer means that the cells have spread out of the breast tissue type they started in, but does not necessarily imply that the cancer has spread out of the breast or into the lymphatic channels."

What are the different types of breast cancer?

According to Heather Richardson, MD, FACS, a breast surgeon at the Bedford Breast Center in Beverly Hills, CA, any cell that originates from breast tissue can become cancerous. "The most common types of breast cancer start from the milk ducts (ductal carcinoma) or the milk production glands (lobular carcinoma)," says Richardson. Both ductal and lobular breast cancers can be in situ or invasive, and all types of breast cells can form cancer with different features that can make some more aggressive and some less aggressive.

Ductal carcinoma in situ - This form of breast cancer is a non-invasive, Stage 0 cancer where cancer cells or non-cancerous abnormal cells have been found in the lining of the breast milk duct, but haven't spread outside the ducts into the surrounding tissue. Although ductal carcinoma in situ is an early-stage cancer that's highly treatable, it can spread into the surrounding breast tissue if left untreated or undetected.

Invasive ductal carcinoma - At Stage 1, invasive ductal carcinoma occurs when the abnormal cancer cells that formed in the milk ducts have now spread beyond the lining of the breast milk duct, into other parts of the breast tissue. Invasive ductal carcinoma is the most common type of breast cancer, with nearly 70-80% of all breast cancer diagnoses. 

Triple-negative breast cancer - Typically found at Stage 2, this type of breast cancer happens when the cancer tests negative for estrogen receptors, progesterone receptors, and HER2 protein. Basically, the cancer growth isn't fueled by the estrogen, progesterone, or HER 2 protein, and can't be treated with hormonal therapy or medicines that target HER2 protein receptors. About 10-20% of breast cancers are triple-negative breast cancers, and are usually successfully treated with other forms of medicines. 

Inflammatory breast cancer - Considered Stage 3, inflammatory breast cancer is a very rare, fast-growing, and aggressive form of cancer where cancer cells infiltrate the skin and lymph vessels of the breast. About 1-5% of breast cancer diagnoses are inflammatory breast cancer. 

Metastatic breast cancer - Also known as Stage 4 breast cancer, metastatic breast cancer is a type of cancer that has spread beyond the breast and lymph nodes, into other parts of the body such as the liver, lungs, bones, or even brain). 

Which types of breast cancers are more common vs. less common?

Infiltrating ductal carcinoma is the most common, making up around 80% of breast cancers, while infiltrating lobular carcinoma is the second most common, making up about 10% of cancers. "Some will have features of both ducts and glands, and are typically called mammary or mixed carcinomas—these are about 8% of breast cancers," says Richardson.

There are also some less common types of breast cancers that account for only a small percentage of invasive breast cancers, including mucinous, tubular, papillary, metaplastic, and mixed ductal-lobular. "Some of these tend to be less aggressive (like mucinous, tubular, and papillary), while others can be more aggressive (like metaplastic)," says Peled.

Can you identify which breast cancer type you might have just based on a breast self-exam?

The short answer? No. "You won't be able to tell what cell types are comprising cancer just by feel," says Richardson. "Your doctor may have a prediction of what type of cancer might be present based on patterns they see on a mammogram, ultrasound, or MRI imaging, but ultimately, a tissue biopsy with an examination of the cells under a microscope is needed to know exactly what the origin cells are and what features are present. This vital information allows your doctor to guide you to the most effective treatments."